DsBinds.hs 53 KB
Newer Older
Austin Seipp's avatar
Austin Seipp committed
1 2 3 4
{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998

Simon Marlow's avatar
Simon Marlow committed
5 6

Pattern-matching bindings (HsBinds and MonoBinds)
7

8 9 10
Handles @HsBinds@; those at the top level require different handling,
in that the @Rec@/@NonRec@/etc structure is thrown away (whereas at
lower levels it is preserved with @let@/@letrec@s).
Austin Seipp's avatar
Austin Seipp committed
11
-}
12

13
{-# LANGUAGE CPP #-}
14
{-# LANGUAGE TypeFamilies #-}
15 16
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE FlexibleContexts #-}
Ian Lynagh's avatar
Ian Lynagh committed
17

18
module DsBinds ( dsTopLHsBinds, dsLHsBinds, decomposeRuleLhs, dsSpec,
19
                 dsHsWrapper, dsTcEvBinds, dsTcEvBinds_s, dsEvBinds, dsMkUserRule
20
  ) where
21

22 23
#include "HsVersions.h"

24 25
import GhcPrelude

26 27
import {-# SOURCE #-}   DsExpr( dsLExpr )
import {-# SOURCE #-}   Match( matchWrapper )
28

29
import DsMonad
Simon Marlow's avatar
Simon Marlow committed
30
import DsGRHSs
31
import DsUtils
32
import Check ( checkGuardMatches )
33

34 35
import HsSyn            -- lots of things
import CoreSyn          -- lots of things
36
import CoreOpt          ( simpleOptExpr )
37
import OccurAnal        ( occurAnalyseExpr )
38
import MkCore
Simon Marlow's avatar
Simon Marlow committed
39
import CoreUtils
40
import CoreArity ( etaExpand )
41
import CoreUnfold
42
import CoreFVs
43
import Digraph
44

45
import PrelNames
46
import TyCon
47
import TcEvidence
48
import TcType
49
import Type
50
import Coercion
Eric Seidel's avatar
Eric Seidel committed
51
import TysWiredIn ( typeNatKind, typeSymbolKind )
Simon Marlow's avatar
Simon Marlow committed
52
import Id
53
import MkId(proxyHashId)
54
import Name
55
import VarSet
Simon Marlow's avatar
Simon Marlow committed
56
import Rules
57
import VarEnv
David Eichmann's avatar
David Eichmann committed
58
import Var( EvVar )
59
import Outputable
60
import Module
Simon Marlow's avatar
Simon Marlow committed
61 62
import SrcLoc
import Maybes
63
import OrdList
Simon Marlow's avatar
Simon Marlow committed
64
import Bag
65
import BasicTypes
Ian Lynagh's avatar
Ian Lynagh committed
66
import DynFlags
Simon Marlow's avatar
Simon Marlow committed
67
import FastString
68
import Util
69
import UniqSet( nonDetEltsUniqSet )
70
import MonadUtils
71
import qualified GHC.LanguageExtensions as LangExt
72
import Control.Monad
73

74
{-**********************************************************************
Austin Seipp's avatar
Austin Seipp committed
75
*                                                                      *
76
           Desugaring a MonoBinds
Austin Seipp's avatar
Austin Seipp committed
77
*                                                                      *
78
**********************************************************************-}
79

80 81
-- | Desugar top level binds, strict binds are treated like normal
-- binds since there is no good time to force before first usage.
82
dsTopLHsBinds :: LHsBinds GhcTc -> DsM (OrdList (Id,CoreExpr))
83 84 85 86
dsTopLHsBinds binds
     -- see Note [Strict binds checks]
  | not (isEmptyBag unlifted_binds) || not (isEmptyBag bang_binds)
  = do { mapBagM_ (top_level_err "bindings for unlifted types") unlifted_binds
87
       ; mapBagM_ (top_level_err "strict bindings")             bang_binds
88
       ; return nilOL }
89

90 91 92 93 94 95
  | otherwise
  = do { (force_vars, prs) <- dsLHsBinds binds
       ; when debugIsOn $
         do { xstrict <- xoptM LangExt.Strict
            ; MASSERT2( null force_vars || xstrict, ppr binds $$ ppr force_vars ) }
              -- with -XStrict, even top-level vars are listed as force vars.
96

97 98 99 100
       ; return (toOL prs) }

  where
    unlifted_binds = filterBag (isUnliftedHsBind . unLoc) binds
101
    bang_binds     = filterBag (isBangedHsBind   . unLoc) binds
102

103
    top_level_err desc (dL->L loc bind)
104 105 106
      = putSrcSpanDs loc $
        errDs (hang (text "Top-level" <+> text desc <+> text "aren't allowed:")
                  2 (ppr bind))
107

108

109
-- | Desugar all other kind of bindings, Ids of strict binds are returned to
110
-- later be forced in the binding group body, see Note [Desugar Strict binds]
111
dsLHsBinds :: LHsBinds GhcTc -> DsM ([Id], [(Id,CoreExpr)])
112
dsLHsBinds binds
113
  = do { ds_bs <- mapBagM dsLHsBind binds
114 115 116
       ; return (foldBag (\(a, a') (b, b') -> (a ++ b, a' ++ b'))
                         id ([], []) ds_bs) }

117
------------------------
118
dsLHsBind :: LHsBind GhcTc
119
          -> DsM ([Id], [(Id,CoreExpr)])
120 121
dsLHsBind (dL->L loc bind) = do dflags <- getDynFlags
                                putSrcSpanDs loc $ dsHsBind dflags bind
122 123 124

-- | Desugar a single binding (or group of recursive binds).
dsHsBind :: DynFlags
125
         -> HsBind GhcTc
126 127 128 129 130
         -> DsM ([Id], [(Id,CoreExpr)])
         -- ^ The Ids of strict binds, to be forced in the body of the
         -- binding group see Note [Desugar Strict binds] and all
         -- bindings and their desugared right hand sides.

131 132 133
dsHsBind dflags (VarBind { var_id = var
                         , var_rhs = expr
                         , var_inline = inline_regardless })
134
  = do  { core_expr <- dsLExpr expr
135 136
                -- Dictionary bindings are always VarBinds,
                -- so we only need do this here
137
        ; let var' | inline_regardless = var `setIdUnfolding` mkCompulsoryUnfolding core_expr
138
                   | otherwise         = var
139
        ; let core_bind@(id,_) = makeCorePair dflags var' False 0 core_expr
140
              force_var = if xopt LangExt.Strict dflags
141 142 143 144
                          then [id]
                          else []
        ; return (force_var, [core_bind]) }

145 146 147 148
dsHsBind dflags b@(FunBind { fun_id = (dL->L _ fun)
                           , fun_matches = matches
                           , fun_co_fn = co_fn
                           , fun_tick = tick })
149
 = do   { (args, body) <- matchWrapper
150
                           (mkPrefixFunRhs (noLoc $ idName fun))
151
                           Nothing matches
152
        ; core_wrap <- dsHsWrapper co_fn
153
        ; let body' = mkOptTickBox tick body
154 155
              rhs   = core_wrap (mkLams args body')
              core_binds@(id,_) = makeCorePair dflags fun False 0 rhs
156 157 158 159 160
              force_var
                  -- Bindings are strict when -XStrict is enabled
                | xopt LangExt.Strict dflags
                , matchGroupArity matches == 0 -- no need to force lambdas
                = [id]
161
                | isBangedHsBind b
162 163 164
                = [id]
                | otherwise
                = []
165 166 167 168 169 170
        ; --pprTrace "dsHsBind" (vcat [ ppr fun <+> ppr (idInlinePragma fun)
          --                          , ppr (mg_alts matches)
          --                          , ppr args, ppr core_binds]) $
          return (force_var, [core_binds]) }

dsHsBind dflags (PatBind { pat_lhs = pat, pat_rhs = grhss
171
                         , pat_ext = NPatBindTc _ ty
172
                         , pat_ticks = (rhs_tick, var_ticks) })
173
  = do  { body_expr <- dsGuarded grhss ty
174
        ; checkGuardMatches PatBindGuards grhss
175
        ; let body' = mkOptTickBox rhs_tick body_expr
176
              pat'  = decideBangHood dflags pat
177
        ; (force_var,sel_binds) <- mkSelectorBinds var_ticks pat body'
178 179
          -- We silently ignore inline pragmas; no makeCorePair
          -- Not so cool, but really doesn't matter
180 181
        ; let force_var' = if isBangedLPat pat'
                           then [force_var]
182 183
                           else []
        ; return (force_var', sel_binds) }
sof's avatar
sof committed
184

185 186 187 188
dsHsBind dflags (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
                          , abs_exports = exports
                          , abs_ev_binds = ev_binds
                          , abs_binds = binds, abs_sig = has_sig })
Simon Peyton Jones's avatar
Simon Peyton Jones committed
189
  = do { ds_binds <- addDictsDs (listToBag dicts) $
190
                     dsLHsBinds binds
Simon Peyton Jones's avatar
Simon Peyton Jones committed
191 192 193
                         -- addDictsDs: push type constraints deeper
                         --             for inner pattern match check
                         -- See Check, Note [Type and Term Equality Propagation]
194 195 196 197 198 199 200

       ; ds_ev_binds <- dsTcEvBinds_s ev_binds

       -- dsAbsBinds does the hard work
       ; dsAbsBinds dflags tyvars dicts exports ds_ev_binds ds_binds has_sig }

dsHsBind _ (PatSynBind{}) = panic "dsHsBind: PatSynBind"
201
dsHsBind _ (XHsBindsLR{}) = panic "dsHsBind: XHsBindsLR"
202 203 204 205 206


-----------------------
dsAbsBinds :: DynFlags
           -> [TyVar] -> [EvVar] -> [ABExport GhcTc]
Gabor Greif's avatar
Gabor Greif committed
207
           -> [CoreBind]                -- Desugared evidence bindings
208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227
           -> ([Id], [(Id,CoreExpr)])   -- Desugared value bindings
           -> Bool                      -- Single binding with signature
           -> DsM ([Id], [(Id,CoreExpr)])

dsAbsBinds dflags tyvars dicts exports
           ds_ev_binds (force_vars, bind_prs) has_sig

    -- A very important common case: one exported variable
    -- Non-recursive bindings come through this way
    -- So do self-recursive bindings
  | [export] <- exports
  , ABE { abe_poly = global_id, abe_mono = local_id
        , abe_wrap = wrap, abe_prags = prags } <- export
  , Just force_vars' <- case force_vars of
                           []                  -> Just []
                           [v] | v == local_id -> Just [global_id]
                           _                   -> Nothing
       -- If there is a variable to force, it's just the
       -- single variable we are binding here
  = do { core_wrap <- dsHsWrapper wrap -- Usually the identity
228

229 230
       ; let rhs = core_wrap $
                   mkLams tyvars $ mkLams dicts $
231 232 233 234 235 236 237 238 239
                   mkCoreLets ds_ev_binds $
                   body

             body | has_sig
                  , [(_, lrhs)] <- bind_prs
                  = lrhs
                  | otherwise
                  = mkLetRec bind_prs (Var local_id)

240
       ; (spec_binds, rules) <- dsSpecs rhs prags
241

242 243 244 245
       ; let global_id' = addIdSpecialisations global_id rules
             main_bind  = makeCorePair dflags global_id'
                                       (isDefaultMethod prags)
                                       (dictArity dicts) rhs
246

247
       ; return (force_vars', main_bind : fromOL spec_binds) }
sof's avatar
sof committed
248

249 250 251 252 253
    -- Another common case: no tyvars, no dicts
    -- In this case we can have a much simpler desugaring
  | null tyvars, null dicts

  = do { let mk_bind (ABE { abe_wrap = wrap
254 255 256
                          , abe_poly = global
                          , abe_mono = local
                          , abe_prags = prags })
257
              = do { core_wrap <- dsHsWrapper wrap
258 259
                   ; return (makeCorePair dflags global
                                          (isDefaultMethod prags)
260
                                          0 (core_wrap (Var local))) }
261
             mk_bind (XABExport _) = panic "dsAbsBinds"
262 263
       ; main_binds <- mapM mk_bind exports

264 265 266 267 268 269
       ; return (force_vars, flattenBinds ds_ev_binds ++ bind_prs ++ main_binds) }

    -- The general case
    -- See Note [Desugaring AbsBinds]
  | otherwise
  = do { let core_bind = Rec [ makeCorePair dflags (add_inline lcl_id) False 0 rhs
270
                              | (lcl_id, rhs) <- bind_prs ]
271
                -- Monomorphic recursion possible, hence Rec
272 273 274 275 276 277 278 279 280 281 282
             new_force_vars = get_new_force_vars force_vars
             locals       = map abe_mono exports
             all_locals   = locals ++ new_force_vars
             tup_expr     = mkBigCoreVarTup all_locals
             tup_ty       = exprType tup_expr
       ; let poly_tup_rhs = mkLams tyvars $ mkLams dicts $
                            mkCoreLets ds_ev_binds $
                            mkLet core_bind $
                            tup_expr

       ; poly_tup_id <- newSysLocalDs (exprType poly_tup_rhs)
283

284 285 286
        -- Find corresponding global or make up a new one: sometimes
        -- we need to make new export to desugar strict binds, see
        -- Note [Desugar Strict binds]
287
       ; (exported_force_vars, extra_exports) <- get_exports force_vars
288

289 290 291 292
       ; let mk_bind (ABE { abe_wrap = wrap
                          , abe_poly = global
                          , abe_mono = local, abe_prags = spec_prags })
                          -- See Note [AbsBinds wrappers] in HsBinds
293
                = do { tup_id  <- newSysLocalDs tup_ty
294 295 296 297 298
                     ; core_wrap <- dsHsWrapper wrap
                     ; let rhs = core_wrap $ mkLams tyvars $ mkLams dicts $
                                 mkTupleSelector all_locals local tup_id $
                                 mkVarApps (Var poly_tup_id) (tyvars ++ dicts)
                           rhs_for_spec = Let (NonRec poly_tup_id poly_tup_rhs) rhs
299 300
                     ; (spec_binds, rules) <- dsSpecs rhs_for_spec spec_prags
                     ; let global' = (global `setInlinePragma` defaultInlinePragma)
301 302 303
                                             `addIdSpecialisations` rules
                           -- Kill the INLINE pragma because it applies to
                           -- the user written (local) function.  The global
304
                           -- Id is just the selector.  Hmm.
305
                     ; return ((global', rhs) : fromOL spec_binds) }
306
             mk_bind (XABExport _) = panic "dsAbsBinds"
307

308
       ; export_binds_s <- mapM mk_bind (exports ++ extra_exports)
309

310 311
       ; return ( exported_force_vars
                , (poly_tup_id, poly_tup_rhs) :
312
                   concat export_binds_s) }
313
  where
314
    inline_env :: IdEnv Id -- Maps a monomorphic local Id to one with
315 316 317
                             -- the inline pragma from the source
                             -- The type checker put the inline pragma
                             -- on the *global* Id, so we need to transfer it
318 319 320 321
    inline_env
      = mkVarEnv [ (lcl_id, setInlinePragma lcl_id prag)
                 | ABE { abe_mono = lcl_id, abe_poly = gbl_id } <- exports
                 , let prag = idInlinePragma gbl_id ]
322 323

    add_inline :: Id -> Id    -- tran
324 325
    add_inline lcl_id = lookupVarEnv inline_env lcl_id
                        `orElse` lcl_id
326

327 328 329 330 331 332 333 334 335 336 337 338 339 340
    global_env :: IdEnv Id -- Maps local Id to its global exported Id
    global_env =
      mkVarEnv [ (local, global)
               | ABE { abe_mono = local, abe_poly = global } <- exports
               ]

    -- find variables that are not exported
    get_new_force_vars lcls =
      foldr (\lcl acc -> case lookupVarEnv global_env lcl of
                           Just _ -> acc
                           Nothing -> lcl:acc)
            [] lcls

    -- find exports or make up new exports for force variables
341
    get_exports :: [Id] -> DsM ([Id], [ABExport GhcTc])
342 343 344 345 346 347 348 349 350 351 352 353
    get_exports lcls =
      foldM (\(glbls, exports) lcl ->
              case lookupVarEnv global_env lcl of
                Just glbl -> return (glbl:glbls, exports)
                Nothing   -> do export <- mk_export lcl
                                let glbl = abe_poly export
                                return (glbl:glbls, export:exports))
            ([],[]) lcls

    mk_export local =
      do global <- newSysLocalDs
                     (exprType (mkLams tyvars (mkLams dicts (Var local))))
354 355
         return (ABE { abe_ext   = noExt
                     , abe_poly  = global
356 357 358
                     , abe_mono  = local
                     , abe_wrap  = WpHole
                     , abe_prags = SpecPrags [] })
359 360

-- | This is where we apply INLINE and INLINABLE pragmas. All we need to
361 362 363 364 365 366
-- do is to attach the unfolding information to the Id.
--
-- Other decisions about whether to inline are made in
-- `calcUnfoldingGuidance` but the decision about whether to then expose
-- the unfolding in the interface file is made in `TidyPgm.addExternal`
-- using this information.
367
------------------------
368 369
makeCorePair :: DynFlags -> Id -> Bool -> Arity -> CoreExpr
             -> (Id, CoreExpr)
370
makeCorePair dflags gbl_id is_default_method dict_arity rhs
Simon Peyton Jones's avatar
Simon Peyton Jones committed
371 372
  | is_default_method    -- Default methods are *always* inlined
                         -- See Note [INLINE and default methods] in TcInstDcls
373 374
  = (gbl_id `setIdUnfolding` mkCompulsoryUnfolding rhs, rhs)

375 376
  | otherwise
  = case inlinePragmaSpec inline_prag of
377 378 379 380
          NoUserInline -> (gbl_id, rhs)
          NoInline     -> (gbl_id, rhs)
          Inlinable    -> (gbl_id `setIdUnfolding` inlinable_unf, rhs)
          Inline       -> inline_pair
381

382 383
  where
    inline_prag   = idInlinePragma gbl_id
384
    inlinable_unf = mkInlinableUnfolding dflags rhs
385 386
    inline_pair
       | Just arity <- inlinePragmaSat inline_prag
387 388
        -- Add an Unfolding for an INLINE (but not for NOINLINE)
        -- And eta-expand the RHS; see Note [Eta-expanding INLINE things]
389
       , let real_arity = dict_arity + arity
390
        -- NB: The arity in the InlineRule takes account of the dictionaries
391
       = ( gbl_id `setIdUnfolding` mkInlineUnfoldingWithArity real_arity rhs
392 393 394 395
         , etaExpand real_arity rhs)

       | otherwise
       = pprTrace "makeCorePair: arity missing" (ppr gbl_id) $
396
         (gbl_id `setIdUnfolding` mkInlineUnfolding rhs, rhs)
397 398 399 400

dictArity :: [Var] -> Arity
-- Don't count coercion variables in arity
dictArity dicts = count isId dicts
401

Austin Seipp's avatar
Austin Seipp committed
402
{-
403 404
Note [Desugaring AbsBinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~
405 406 407 408 409 410 411 412
In the general AbsBinds case we desugar the binding to this:

       tup a (d:Num a) = let fm = ...gm...
                             gm = ...fm...
                         in (fm,gm)
       f a d = case tup a d of { (fm,gm) -> fm }
       g a d = case tup a d of { (fm,gm) -> fm }

413 414 415 416
Note [Rules and inlining]
~~~~~~~~~~~~~~~~~~~~~~~~~
Common special case: no type or dictionary abstraction
This is a bit less trivial than you might suppose
417
The naive way would be to desugar to something like
418 419
        f_lcl = ...f_lcl...     -- The "binds" from AbsBinds
        M.f = f_lcl             -- Generated from "exports"
420
But we don't want that, because if M.f isn't exported,
421 422
it'll be inlined unconditionally at every call site (its rhs is
trivial).  That would be ok unless it has RULES, which would
423 424 425
thereby be completely lost.  Bad, bad, bad.

Instead we want to generate
426 427 428
        M.f = ...f_lcl...
        f_lcl = M.f
Now all is cool. The RULES are attached to M.f (by SimplCore),
429 430 431 432
and f_lcl is rapidly inlined away.

This does not happen in the same way to polymorphic binds,
because they desugar to
433
        M.f = /\a. let f_lcl = ...f_lcl... in f_lcl
434
Although I'm a bit worried about whether full laziness might
435
float the f_lcl binding out and then inline M.f at its call site
436 437 438 439 440 441 442 443 444 445 446 447 448 449 450

Note [Specialising in no-dict case]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Even if there are no tyvars or dicts, we may have specialisation pragmas.
Class methods can generate
      AbsBinds [] [] [( ... spec-prag]
         { AbsBinds [tvs] [dicts] ...blah }
So the overloading is in the nested AbsBinds. A good example is in GHC.Float:

  class  (Real a, Fractional a) => RealFrac a  where
    round :: (Integral b) => a -> b

  instance  RealFrac Float  where
    {-# SPECIALIZE round :: Float -> Int #-}

451
The top-level AbsBinds for $cround has no tyvars or dicts (because the
452 453 454 455 456 457 458
instance does not).  But the method is locally overloaded!

Note [Abstracting over tyvars only]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When abstracting over type variable only (not dictionaries), we don't really need to
built a tuple and select from it, as we do in the general case. Instead we can take

459 460 461 462 463
        AbsBinds [a,b] [ ([a,b], fg, fl, _),
                         ([b],   gg, gl, _) ]
                { fl = e1
                  gl = e2
                   h = e3 }
464 465 466

and desugar it to

467 468 469
        fg = /\ab. let B in e1
        gg = /\b. let a = () in let B in S(e2)
        h  = /\ab. let B in e3
470 471

where B is the *non-recursive* binding
472 473 474
        fl = fg a b
        gl = gg b
        h  = h a b    -- See (b); note shadowing!
475 476

Notice (a) g has a different number of type variables to f, so we must
477 478
             use the mkArbitraryType thing to fill in the gaps.
             We use a type-let to do that.
479

480 481 482 483
         (b) The local variable h isn't in the exports, and rather than
             clone a fresh copy we simply replace h by (h a b), where
             the two h's have different types!  Shadowing happens here,
             which looks confusing but works fine.
484

485 486 487 488
         (c) The result is *still* quadratic-sized if there are a lot of
             small bindings.  So if there are more than some small
             number (10), we filter the binding set B by the free
             variables of the particular RHS.  Tiresome.
489 490

Why got to this trouble?  It's a common case, and it removes the
491
quadratic-sized tuple desugaring.  Less clutter, hopefully faster
492 493 494 495
compilation, especially in a case where there are a *lot* of
bindings.


496 497 498 499 500 501 502 503
Note [Eta-expanding INLINE things]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   foo :: Eq a => a -> a
   {-# INLINE foo #-}
   foo x = ...

If (foo d) ever gets floated out as a common sub-expression (which can
504
happen as a result of method sharing), there's a danger that we never
505 506 507 508
get to do the inlining, which is a Terribly Bad thing given that the
user said "inline"!

To avoid this we pre-emptively eta-expand the definition, so that foo
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
509
has the arity with which it is declared in the source code.  In this
510
example it has arity 2 (one for the Eq and one for x). Doing this
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
511
should mean that (foo d) is a PAP and we don't share it.
512 513 514

Note [Nested arities]
~~~~~~~~~~~~~~~~~~~~~
515 516 517 518 519 520 521 522 523 524 525 526 527 528
For reasons that are not entirely clear, method bindings come out looking like
this:

  AbsBinds [] [] [$cfromT <= [] fromT]
    $cfromT [InlPrag=INLINE] :: T Bool -> Bool
    { AbsBinds [] [] [fromT <= [] fromT_1]
        fromT :: T Bool -> Bool
        { fromT_1 ((TBool b)) = not b } } }

Note the nested AbsBind.  The arity for the InlineRule on $cfromT should be
gotten from the binding for fromT_1.

It might be better to have just one level of AbsBinds, but that requires more
thought!
529 530 531 532


Note [Desugar Strict binds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
533
See https://gitlab.haskell.org/ghc/ghc/wikis/strict-pragma
534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585

Desugaring strict variable bindings looks as follows (core below ==>)

  let !x = rhs
  in  body
==>
  let x = rhs
  in x `seq` body -- seq the variable

and if it is a pattern binding the desugaring looks like

  let !pat = rhs
  in body
==>
  let x = rhs -- bind the rhs to a new variable
      pat = x
  in x `seq` body -- seq the new variable

if there is no variable in the pattern desugaring looks like

  let False = rhs
  in body
==>
  let x = case rhs of {False -> (); _ -> error "Match failed"}
  in x `seq` body

In order to force the Ids in the binding group they are passed around
in the dsHsBind family of functions, and later seq'ed in DsExpr.ds_val_bind.

Consider a recursive group like this

  letrec
     f : g = rhs[f,g]
  in <body>

Without `Strict`, we get a translation like this:

  let t = /\a. letrec tm = rhs[fm,gm]
                      fm = case t of fm:_ -> fm
                      gm = case t of _:gm -> gm
                in
                (fm,gm)

  in let f = /\a. case t a of (fm,_) -> fm
  in let g = /\a. case t a of (_,gm) -> gm
  in <body>

Here `tm` is the monomorphic binding for `rhs`.

With `Strict`, we want to force `tm`, but NOT `fm` or `gm`.
Alas, `tm` isn't in scope in the `in <body>` part.

Gabor Greif's avatar
Gabor Greif committed
586
The simplest thing is to return it in the polymorphic
587 588 589 590 591 592 593 594 595 596 597 598 599 600
tuple `t`, thus:

  let t = /\a. letrec tm = rhs[fm,gm]
                      fm = case t of fm:_ -> fm
                      gm = case t of _:gm -> gm
                in
                (tm, fm, gm)

  in let f = /\a. case t a of (_,fm,_) -> fm
  in let g = /\a. case t a of (_,_,gm) -> gm
  in let tm = /\a. case t a of (tm,_,_) -> tm
  in tm `seq` <body>


601
See https://gitlab.haskell.org/ghc/ghc/wikis/strict-pragma for a more
602 603
detailed explanation of the desugaring of strict bindings.

604 605 606 607 608 609 610 611 612 613 614 615 616 617 618
Note [Strict binds checks]
~~~~~~~~~~~~~~~~~~~~~~~~~~
There are several checks around properly formed strict bindings. They
all link to this Note. These checks must be here in the desugarer because
we cannot know whether or not a type is unlifted until after zonking, due
to levity polymorphism. These checks all used to be handled in the typechecker
in checkStrictBinds (before Jan '17).

We define an "unlifted bind" to be any bind that binds an unlifted id. Note that

  x :: Char
  (# True, x #) = blah

is *not* an unlifted bind. Unlifted binds are detected by HsUtils.isUnliftedHsBind.

619
Define a "banged bind" to have a top-level bang. Detected by HsPat.isBangedHsBind.
620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635
Define a "strict bind" to be either an unlifted bind or a banged bind.

The restrictions are:
  1. Strict binds may not be top-level. Checked in dsTopLHsBinds.

  2. Unlifted binds must also be banged. (There is no trouble to compile an unbanged
     unlifted bind, but an unbanged bind looks lazy, and we don't want users to be
     surprised by the strictness of an unlifted bind.) Checked in first clause
     of DsExpr.ds_val_bind.

  3. Unlifted binds may not have polymorphism (#6078). (That is, no quantified type
     variables or constraints.) Checked in first clause
     of DsExpr.ds_val_bind.

  4. Unlifted binds may not be recursive. Checked in second clause of ds_val_bind.

Austin Seipp's avatar
Austin Seipp committed
636
-}
637

638
------------------------
639
dsSpecs :: CoreExpr     -- Its rhs
640
        -> TcSpecPrags
641 642
        -> DsM ( OrdList (Id,CoreExpr)  -- Binding for specialised Ids
               , [CoreRule] )           -- Rules for the Global Ids
643
-- See Note [Handling SPECIALISE pragmas] in TcBinds
644 645 646 647 648 649
dsSpecs _ IsDefaultMethod = return (nilOL, [])
dsSpecs poly_rhs (SpecPrags sps)
  = do { pairs <- mapMaybeM (dsSpec (Just poly_rhs)) sps
       ; let (spec_binds_s, rules) = unzip pairs
       ; return (concatOL spec_binds_s, rules) }

650 651 652
dsSpec :: Maybe CoreExpr        -- Just rhs => RULE is for a local binding
                                -- Nothing => RULE is for an imported Id
                                --            rhs is in the Id's unfolding
653 654
       -> Located TcSpecPrag
       -> DsM (Maybe (OrdList (Id,CoreExpr), CoreRule))
655
dsSpec mb_poly_rhs (dL->L loc (SpecPrag poly_id spec_co spec_inl))
656
  | isJust (isClassOpId_maybe poly_id)
657
  = putSrcSpanDs loc $
658 659
    do { warnDs NoReason (text "Ignoring useless SPECIALISE pragma for class method selector"
                          <+> quotes (ppr poly_id))
660
       ; return Nothing  }  -- There is no point in trying to specialise a class op
661 662
                            -- Moreover, classops don't (currently) have an inl_sat arity set
                            -- (it would be Just 0) and that in turn makes makeCorePair bleat
663

664 665
  | no_act_spec && isNeverActive rule_act
  = putSrcSpanDs loc $
666 667
    do { warnDs NoReason (text "Ignoring useless SPECIALISE pragma for NOINLINE function:"
                          <+> quotes (ppr poly_id))
Gabor Greif's avatar
Gabor Greif committed
668
       ; return Nothing  }  -- Function is NOINLINE, and the specialisation inherits that
669
                            -- See Note [Activation pragmas for SPECIALISE]
670

671
  | otherwise
672
  = putSrcSpanDs loc $
673 674
    do { uniq <- newUnique
       ; let poly_name = idName poly_id
675 676
             spec_occ  = mkSpecOcc (getOccName poly_name)
             spec_name = mkInternalName uniq spec_occ (getSrcSpan poly_name)
677 678 679 680 681 682 683 684 685 686
             (spec_bndrs, spec_app) = collectHsWrapBinders spec_co
               -- spec_co looks like
               --         \spec_bndrs. [] spec_args
               -- perhaps with the body of the lambda wrapped in some WpLets
               -- E.g. /\a \(d:Eq a). let d2 = $df d in [] (Maybe a) d2

       ; core_app <- dsHsWrapper spec_app

       ; let ds_lhs  = core_app (Var poly_id)
             spec_ty = mkLamTypes spec_bndrs (exprType ds_lhs)
687 688 689
       ; -- pprTrace "dsRule" (vcat [ text "Id:" <+> ppr poly_id
         --                         , text "spec_co:" <+> ppr spec_co
         --                         , text "ds_rhs:" <+> ppr ds_lhs ]) $
690 691
         dflags <- getDynFlags
       ; case decomposeRuleLhs dflags spec_bndrs ds_lhs of {
692
           Left msg -> do { warnDs NoReason msg; return Nothing } ;
693
           Right (rule_bndrs, _fn, args) -> do
694

695
       { this_mod <- getModule
Simon Peyton Jones's avatar
Simon Peyton Jones committed
696
       ; let fn_unf    = realIdUnfolding poly_id
697
             spec_unf  = specUnfolding dflags spec_bndrs core_app arity_decrease fn_unf
698 699 700
             spec_id   = mkLocalId spec_name spec_ty
                            `setInlinePragma` inl_prag
                            `setIdUnfolding`  spec_unf
701 702
             arity_decrease = count isValArg args - count isId spec_bndrs

703
       ; rule <- dsMkUserRule this_mod is_local_id
Ian Lynagh's avatar
Ian Lynagh committed
704
                        (mkFastString ("SPEC " ++ showPpr dflags poly_name))
705 706
                        rule_act poly_name
                        rule_bndrs args
707
                        (mkVarApps (Var spec_id) spec_bndrs)
708

709
       ; let spec_rhs = mkLams spec_bndrs (core_app poly_rhs)
710

711 712
-- Commented out: see Note [SPECIALISE on INLINE functions]
--       ; when (isInlinePragma id_inl)
713
--              (warnDs $ text "SPECIALISE pragma on INLINE function probably won't fire:"
714
--                        <+> quotes (ppr poly_name))
Simon Peyton Jones's avatar
Simon Peyton Jones committed
715 716 717 718 719

       ; return (Just (unitOL (spec_id, spec_rhs), rule))
            -- NB: do *not* use makeCorePair on (spec_id,spec_rhs), because
            --     makeCorePair overwrites the unfolding, which we have
            --     just created using specUnfolding
720 721 722 723
       } } }
  where
    is_local_id = isJust mb_poly_rhs
    poly_rhs | Just rhs <-  mb_poly_rhs
724
             = rhs          -- Local Id; this is its rhs
725 726
             | Just unfolding <- maybeUnfoldingTemplate (realIdUnfolding poly_id)
             = unfolding    -- Imported Id; this is its unfolding
727 728 729
                            -- Use realIdUnfolding so we get the unfolding
                            -- even when it is a loop breaker.
                            -- We want to specialise recursive functions!
730
             | otherwise = pprPanic "dsImpSpecs" (ppr poly_id)
731
                            -- The type checker has checked that it *has* an unfolding
732

733 734 735 736 737
    id_inl = idInlinePragma poly_id

    -- See Note [Activation pragmas for SPECIALISE]
    inl_prag | not (isDefaultInlinePragma spec_inl)    = spec_inl
             | not is_local_id  -- See Note [Specialising imported functions]
738
                                 -- in OccurAnal
739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755
             , isStrongLoopBreaker (idOccInfo poly_id) = neverInlinePragma
             | otherwise                               = id_inl
     -- Get the INLINE pragma from SPECIALISE declaration, or,
     -- failing that, from the original Id

    spec_prag_act = inlinePragmaActivation spec_inl

    -- See Note [Activation pragmas for SPECIALISE]
    -- no_act_spec is True if the user didn't write an explicit
    -- phase specification in the SPECIALISE pragma
    no_act_spec = case inlinePragmaSpec spec_inl of
                    NoInline -> isNeverActive  spec_prag_act
                    _        -> isAlwaysActive spec_prag_act
    rule_act | no_act_spec = inlinePragmaActivation id_inl   -- Inherit
             | otherwise   = spec_prag_act                   -- Specified by user


756 757 758 759 760 761
dsMkUserRule :: Module -> Bool -> RuleName -> Activation
       -> Name -> [CoreBndr] -> [CoreExpr] -> CoreExpr -> DsM CoreRule
dsMkUserRule this_mod is_local name act fn bndrs args rhs = do
    let rule = mkRule this_mod False is_local name act fn bndrs args rhs
    dflags <- getDynFlags
    when (isOrphan (ru_orphan rule) && wopt Opt_WarnOrphans dflags) $
762
        warnDs (Reason Opt_WarnOrphans) (ruleOrphWarn rule)
763 764 765
    return rule

ruleOrphWarn :: CoreRule -> SDoc
766
ruleOrphWarn rule = text "Orphan rule:" <+> ppr rule
767

768 769 770 771 772 773 774 775 776 777 778
{- Note [SPECIALISE on INLINE functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We used to warn that using SPECIALISE for a function marked INLINE
would be a no-op; but it isn't!  Especially with worker/wrapper split
we might have
   {-# INLINE f #-}
   f :: Ord a => Int -> a -> ...
   f d x y = case x of I# x' -> $wf d x' y

We might want to specialise 'f' so that we in turn specialise '$wf'.
We can't even /name/ '$wf' in the source code, so we can't specialise
779
it even if we wanted to.  #10721 is a case in point.
780

781 782 783 784 785 786 787 788
Note [Activation pragmas for SPECIALISE]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
From a user SPECIALISE pragma for f, we generate
  a) A top-level binding    spec_fn = rhs
  b) A RULE                 f dOrd = spec_fn

We need two pragma-like things:

789
* spec_fn's inline pragma: inherited from f's inline pragma (ignoring
790 791 792 793 794
                           activation on SPEC), unless overriden by SPEC INLINE

* Activation of RULE: from SPECIALISE pragma (if activation given)
                      otherwise from f's inline pragma

795
This is not obvious (see #5237)!
796 797 798 799 800 801 802 803 804 805 806 807 808 809 810

Examples      Rule activation   Inline prag on spec'd fn
---------------------------------------------------------------------
SPEC [n] f :: ty            [n]   Always, or NOINLINE [n]
                                  copy f's prag

NOINLINE f
SPEC [n] f :: ty            [n]   NOINLINE
                                  copy f's prag

NOINLINE [k] f
SPEC [n] f :: ty            [n]   NOINLINE [k]
                                  copy f's prag

INLINE [k] f
811
SPEC [n] f :: ty            [n]   INLINE [k]
812 813 814 815 816 817 818 819 820 821
                                  copy f's prag

SPEC INLINE [n] f :: ty     [n]   INLINE [n]
                                  (ignore INLINE prag on f,
                                  same activation for rule and spec'd fn)

NOINLINE [k] f
SPEC f :: ty                [n]   INLINE [k]


Austin Seipp's avatar
Austin Seipp committed
822 823
************************************************************************
*                                                                      *
824
\subsection{Adding inline pragmas}
Austin Seipp's avatar
Austin Seipp committed
825 826 827
*                                                                      *
************************************************************************
-}
828

829 830
decomposeRuleLhs :: DynFlags -> [Var] -> CoreExpr
                 -> Either SDoc ([Var], Id, [CoreExpr])
unknown's avatar
unknown committed
831 832
-- (decomposeRuleLhs bndrs lhs) takes apart the LHS of a RULE,
-- The 'bndrs' are the quantified binders of the rules, but decomposeRuleLhs
833
-- may add some extra dictionary binders (see Note [Free dictionaries])
unknown's avatar
unknown committed
834
--
835
-- Returns an error message if the LHS isn't of the expected shape
836
-- Note [Decomposing the left-hand side of a RULE]
837
decomposeRuleLhs dflags orig_bndrs orig_lhs
838 839 840
  | not (null unbound)    -- Check for things unbound on LHS
                          -- See Note [Unused spec binders]
  = Left (vcat (map dead_msg unbound))
841 842 843
  | Var funId <- fun2
  , Just con <- isDataConId_maybe funId
  = Left (constructor_msg con) -- See Note [No RULES on datacons]
844
  | Just (fn_id, args) <- decompose fun2 args2
845
  , let extra_bndrs = mk_extra_bndrs fn_id args
846 847 848 849 850 851
  = -- pprTrace "decmposeRuleLhs" (vcat [ text "orig_bndrs:" <+> ppr orig_bndrs
    --                                  , text "orig_lhs:" <+> ppr orig_lhs
    --                                  , text "lhs1:"     <+> ppr lhs1
    --                                  , text "extra_dict_bndrs:" <+> ppr extra_dict_bndrs
    --                                  , text "fn_id:" <+> ppr fn_id
    --                                  , text "args:"   <+> ppr args]) $
852
    Right (orig_bndrs ++ extra_bndrs, fn_id, args)
853

854
  | otherwise
855
  = Left bad_shape_msg
856
 where
857
   lhs1         = drop_dicts orig_lhs
858
   lhs2         = simpleOptExpr dflags lhs1  -- See Note [Simplify rule LHS]
859 860
   (fun2,args2) = collectArgs lhs2

861 862
   lhs_fvs    = exprFreeVars lhs2
   unbound    = filterOut (`elemVarSet` lhs_fvs) orig_bndrs
863

864
   orig_bndr_set = mkVarSet orig_bndrs
865

866 867 868
        -- Add extra tyvar binders: Note [Free tyvars in rule LHS]
        -- and extra dict binders: Note [Free dictionaries in rule LHS]
   mk_extra_bndrs fn_id args
Tobias Dammers's avatar
Tobias Dammers committed
869
     = scopedSort unbound_tvs ++ unbound_dicts
870 871 872 873 874 875 876 877
     where
       unbound_tvs   = [ v | v <- unbound_vars, isTyVar v ]
       unbound_dicts = [ mkLocalId (localiseName (idName d)) (idType d)
                       | d <- unbound_vars, isDictId d ]
       unbound_vars  = [ v | v <- exprsFreeVarsList args
                           , not (v `elemVarSet` orig_bndr_set)
                           , not (v == fn_id) ]
         -- fn_id: do not quantify over the function itself, which may
878
         -- itself be a dictionary (in pathological cases, #10251)
879 880 881 882 883 884

   decompose (Var fn_id) args
      | not (fn_id `elemVarSet` orig_bndr_set)
      = Just (fn_id, args)

   decompose _ _ = Nothing
885

886
   bad_shape_msg = hang (text "RULE left-hand side too complicated to desugar")
887 888
                      2 (vcat [ text "Optimised lhs:" <+> ppr lhs2
                              , text "Orig lhs:" <+> ppr orig_lhs])
889 890
   dead_msg bndr = hang (sep [ text "Forall'd" <+> pp_bndr bndr
                             , text "is not bound in RULE lhs"])
891 892 893
                      2 (vcat [ text "Orig bndrs:" <+> ppr orig_bndrs
                              , text "Orig lhs:" <+> ppr orig_lhs
                              , text "optimised lhs:" <+> ppr lhs2 ])
894
   pp_bndr bndr
895 896 897
    | isTyVar bndr = text "type variable" <+> quotes (ppr bndr)
    | isEvVar bndr = text "constraint"    <+> quotes (ppr (varType bndr))
    | otherwise    = text "variable"      <+> quotes (ppr bndr)
898

899 900 901 902 903
   constructor_msg con = vcat
     [ text "A constructor," <+> ppr con <>
         text ", appears as outermost match in RULE lhs."
     , text "This rule will be ignored." ]

904
   drop_dicts :: CoreExpr -> CoreExpr
905
   drop_dicts e
906 907 908
       = wrap_lets needed bnds body
     where
       needed = orig_bndr_set `minusVarSet` exprFreeVars body
909
       (bnds, body) = split_lets (occurAnalyseExpr e)
910
           -- The occurAnalyseExpr drops dead bindings which is
911 912
           -- crucial to ensure that every binding is used later;
           -- which in turn makes wrap_lets work right
913 914

   split_lets :: CoreExpr -> ([(DictId,CoreExpr)], CoreExpr)
915 916
   split_lets (Let (NonRec d r) body)
     | isDictId d
917
     = ((d,r):bs, body')
918 919 920 921 922 923 924 925 926
     where (bs, body') = split_lets body

    -- handle "unlifted lets" too, needed for "map/coerce"
   split_lets (Case r d _ [(DEFAULT, _, body)])
     | isCoVar d
     = ((d,r):bs, body')
     where (bs, body') = split_lets body

   split_lets e = ([], e)
927 928 929 930

   wrap_lets :: VarSet -> [(DictId,CoreExpr)] -> CoreExpr -> CoreExpr
   wrap_lets _ [] body = body
   wrap_lets needed ((d, r) : bs) body
931
     | rhs_fvs `intersectsVarSet` needed = mkCoreLet (NonRec d r) (wrap_lets needed' bs body)
932 933 934 935
     | otherwise                         = wrap_lets needed bs body
     where
       rhs_fvs = exprFreeVars r
       needed' = (needed `minusVarSet` rhs_fvs) `extendVarSet` d
936

Austin Seipp's avatar
Austin Seipp committed
937
{-
938
Note [Decomposing the left-hand side of a RULE]
939
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
940
There are several things going on here.
941 942
* drop_dicts: see Note [Drop dictionary bindings on rule LHS]
* simpleOptExpr: see Note [Simplify rule LHS]
943
* extra_dict_bndrs: see Note [Free dictionaries]
944

945 946 947 948 949 950 951 952 953 954 955 956 957
Note [Free tyvars on rule LHS]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
  data T a = C

  foo :: T a -> Int
  foo C = 1

  {-# RULES "myrule"  foo C = 1 #-}

After type checking the LHS becomes (foo alpha (C alpha)), where alpha
is an unbound meta-tyvar.  The zonker in TcHsSyn is careful not to
turn the free alpha into Any (as it usually does).  Instead it turns it
958
into a TyVar 'a'.  See TcHsSyn Note [Zonking the LHS of a RULE].
959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992

Now we must quantify over that 'a'.  It's /really/ inconvenient to do that
in the zonker, because the HsExpr data type is very large.  But it's /easy/
to do it here in the desugarer.

Moreover, we have to do something rather similar for dictionaries;
see Note [Free dictionaries on rule LHS].   So that's why we look for
type variables free on the LHS, and quantify over them.

Note [Free dictionaries on rule LHS]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When the LHS of a specialisation rule, (/\as\ds. f es) has a free dict,
which is presumably in scope at the function definition site, we can quantify
over it too.  *Any* dict with that type will do.

So for example when you have
        f :: Eq a => a -> a
        f = <rhs>
        ... SPECIALISE f :: Int -> Int ...

Then we get the SpecPrag
        SpecPrag (f Int dInt)

And from that we want the rule

        RULE forall dInt. f Int dInt = f_spec
        f_spec = let f = <rhs> in f Int dInt

But be careful!  That dInt might be GHC.Base.$fOrdInt, which is an External
Name, and you can't bind them in a lambda or forall without getting things
confused.   Likewise it might have an InlineRule or something, which would be
utterly bogus. So we really make a fresh Id, with the same unique and type
as the old one, but with an Internal name and no IdInfo.

993 994
Note [Drop dictionary bindings on rule LHS]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
995
drop_dicts drops dictionary bindings on the LHS where possible.
996 997
   E.g.  let d:Eq [Int] = $fEqList $fEqInt in f d
     --> f d
998
   Reasoning here is that there is only one d:Eq [Int], and so we can
999 1000 1001 1002
   quantify over it. That makes 'd' free in the LHS, but that is later
   picked up by extra_dict_bndrs (Note [Dead spec binders]).

   NB 1: We can only drop the binding if the RHS doesn't bind
1003
         one of the orig_bndrs, which we assume occur on RHS.
1004 1005 1006 1007 1008 1009
         Example
            f :: (Eq a) => b -> a -> a
            {-# SPECIALISE f :: Eq a => b -> [a] -> [a] #-}
         Here we want to end up with
            RULE forall d:Eq a.  f ($dfEqList d) = f_spec d
         Of course, the ($dfEqlist d) in the pattern makes it less likely
1010
         to match, but there is no other way to get d:Eq a
1011

1012
   NB 2: We do drop_dicts *before* simplOptEpxr, so that we expect all
1013 1014 1015 1016 1017 1018
         the evidence bindings to be wrapped around the outside of the
         LHS.  (After simplOptExpr they'll usually have been inlined.)
         dsHsWrapper does dependency analysis, so that civilised ones
         will be simple NonRec bindings.  We don't handle recursive
         dictionaries!

Gabor Greif's avatar
Gabor Greif committed
1019
    NB3: In the common case of a non-overloaded, but perhaps-polymorphic
1020
         specialisation, we don't need to bind *any* dictionaries for use
1021
         in the RHS. For example (#8331)
1022 1023 1024 1025
             {-# SPECIALIZE INLINE useAbstractMonad :: ReaderST s Int #-}
             useAbstractMonad :: MonadAbstractIOST m => m Int
         Here, deriving (MonadAbstractIOST (ReaderST s)) is a lot of code
         but the RHS uses no dictionaries, so we want to end up with
1026
             RULE forall s (d :: MonadAbstractIOST (ReaderT s)).
1027 1028
                useAbstractMonad (ReaderT s) d = $suseAbstractMonad s

1029
   #8848 is a good example of where there are some interesting
1030 1031
   dictionary bindings to discard.

1032 1033 1034 1035 1036 1037 1038 1039 1040 1041
The drop_dicts algorithm is based on these observations:

  * Given (let d = rhs in e) where d is a DictId,
    matching 'e' will bind e's free variables.

  * So we want to keep the binding if one of the needed variables (for
    which we need a binding) is in fv(rhs) but not already in fv(e).

  * The "needed variables" are simply the orig_bndrs.  Consider
       f :: (Eq a, Show b) => a -> b -> String
Austin Seipp's avatar
Austin Seipp committed
1042
       ... SPECIALISE f :: (Show b) => Int -> b -> String ...
1043 1044 1045 1046 1047 1048
    Then orig_bndrs includes the *quantified* dictionaries of the type
    namely (dsb::Show b), but not the one for Eq Int

So we work inside out, applying the above criterion at each step.


1049 1050 1051 1052
Note [Simplify rule LHS]
~~~~~~~~~~~~~~~~~~~~~~~~
simplOptExpr occurrence-analyses and simplifies the LHS:

1053
   (a) Inline any remaining dictionary bindings (which hopefully
1054 1055
       occur just once)

1056
   (b) Substitute trivial lets, so that they don't get in the way.
1057
       Note that we substitute the function too; we might
1058 1059
       have this as a LHS:  let f71 = M.f Int in f71

1060
   (c) Do eta reduction.  To see why, consider the fold/build rule,
1061 1062 1063 1064
       which without simplification looked like:
          fold k z (build (/\a. g a))  ==>  ...
       This doesn't match unless you do eta reduction on the build argument.
       Similarly for a LHS like
1065
         augment g (build h)
1066
       we do not want to get
1067
         augment (\a. g a) (build h)
1068 1069
       otherwise we don't match when given an argument like
          augment (\a. h a a) (build h)
1070

1071
Note [Matching seqId]
1072 1073
~~~~~~~~~~~~~~~~~~~
The desugarer turns (seq e r) into (case e of _ -> r), via a special-case hack
1074
and this code turns it back into an application of seq!
1075 1076
See Note [Rules for seq] in MkId for the details.

1077 1078 1079
Note [Unused spec binders]
~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
1080
        f :: a -> a
Austin Seipp's avatar
Austin Seipp committed
1081
        ... SPECIALISE f :: Eq a => a -> a ...
1082 1083
It's true that this *is* a more specialised type, but the rule
we get is something like this:
1084 1085
        f_spec d = f
        RULE: f = f_spec d
Gabor Greif's avatar
typos  
Gabor Greif committed
1086 1087
Note that the rule is bogus, because it mentions a 'd' that is
not bound on the LHS!  But it's a silly specialisation anyway, because
1088 1089 1090 1091
the constraint is unused.  We could bind 'd' to (error "unused")
but it seems better to reject the program because it's almost certainly
a mistake.  That's what the isDeadBinder call detects.

1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107
Note [No RULES on datacons]
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Previously, `RULES` like

    "JustNothing" forall x . Just x = Nothing

were allowed. Simon Peyton Jones says this seems to have been a
mistake, that such rules have never been supported intentionally,
and that he doesn't know if they can break in horrible ways.
Furthermore, Ben Gamari and Reid Barton are considering trying to
detect the presence of "static data" that the simplifier doesn't
need to traverse at all. Such rules do not play well with that.
So for now, we ban them altogether as requested by #13290. See also #7398.


Austin Seipp's avatar
Austin Seipp committed
1108 1109
************************************************************************
*                                                                      *
1110
                Desugaring evidence
Austin Seipp's avatar
Austin Seipp committed
1111 1112
*                                                                      *
************************************************************************
1113

Austin Seipp's avatar
Austin Seipp committed
1114
-}
1115

1116 1117 1118 1119 1120 1121 1122 1123 1124 1125
dsHsWrapper :: HsWrapper -> DsM (CoreExpr -> CoreExpr)
dsHsWrapper WpHole            = return $ \e -> e
dsHsWrapper (WpTyApp ty)      = return $ \e -> App e (Type ty)
dsHsWrapper (WpEvLam ev)      = return $ Lam ev
dsHsWrapper (WpTyLam tv)      = return $ Lam tv
dsHsWrapper (WpLet ev_binds)  = do { bs <- dsTcEvBinds ev_binds
                                   ; return (mkCoreLets bs) }
dsHsWrapper (WpCompose c1 c2) = do { w1 <- dsHsWrapper c1
                                   ; w2 <- dsHsWrapper c2
                                   ; return (w1 . w2) }
1126 1127 1128 1129
 -- See comments on WpFun in TcEvidence for an explanation of what
 -- the specification of this clause is
dsHsWrapper (WpFun c1 c2 t1 doc)
                              = do { x  <- newSysLocalDsNoLP t1
1130 1131 1132
                                   ; w1 <- dsHsWrapper c1
                                   ; w2 <- dsHsWrapper c2
                                   ; let app f a = mkCoreAppDs (text "dsHsWrapper") f a
1133
                                         arg     = w1 (Var x)
1134 1135 1136 1137
                                   ; (_, ok) <- askNoErrsDs $ dsNoLevPolyExpr arg doc
                                   ; if ok
                                     then return (\e -> (Lam x (w2 (app e arg))))
                                     else return id }  -- this return is irrelevant
1138 1139 1140 1141
dsHsWrapper (WpCast co)       = ASSERT(coercionRole co == Representational)
                                return $ \e -> mkCastDs e co
dsHsWrapper (WpEvApp tm)      = do { core_tm <- dsEvTerm tm
                                   ; return (\e -> App e core_tm) }
1142 1143

--------------------------------------
1144 1145 1146 1147 1148
dsTcEvBinds_s :: [TcEvBinds] -> DsM [CoreBind]
dsTcEvBinds_s []       = return []
dsTcEvBinds_s (b:rest) = ASSERT( null rest )  -- Zonker ensures null
                         dsTcEvBinds b

1149
dsTcEvBinds :: TcEvBinds -> DsM [CoreBind]