DsBinds.hs 53.2 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 #-}
Ian Lynagh's avatar
Ian Lynagh committed
14

15
module DsBinds ( dsTopLHsBinds, dsLHsBinds, decomposeRuleLhs, dsSpec,
16
                 dsHsWrapper, dsTcEvBinds, dsTcEvBinds_s, dsEvBinds, dsMkUserRule
17
  ) where
18

19
20
#include "HsVersions.h"

21
22
import {-# SOURCE #-}   DsExpr( dsLExpr )
import {-# SOURCE #-}   Match( matchWrapper )
23

24
import DsMonad
Simon Marlow's avatar
Simon Marlow committed
25
import DsGRHSs
26
import DsUtils
27

28
29
import HsSyn            -- lots of things
import CoreSyn          -- lots of things
30
import Literal          ( Literal(MachStr) )
31
import CoreSubst
32
import OccurAnal        ( occurAnalyseExpr )
33
import MkCore
Simon Marlow's avatar
Simon Marlow committed
34
import CoreUtils
35
import CoreArity ( etaExpand )
36
import CoreUnfold
37
import CoreFVs
38
import Digraph
39

40
import PrelNames
41
import TysPrim ( mkProxyPrimTy )
42
import TyCon
43
import TcEvidence
44
import TcType
45
import Type
46
import Coercion
Eric Seidel's avatar
Eric Seidel committed
47
import TysWiredIn ( typeNatKind, typeSymbolKind )
Simon Marlow's avatar
Simon Marlow committed
48
import Id
49
import MkId(proxyHashId)
50
import Class
51
import Name
52
import VarSet
Simon Marlow's avatar
Simon Marlow committed
53
import Rules
54
import VarEnv
55
import Outputable
56
import Module
Simon Marlow's avatar
Simon Marlow committed
57
58
import SrcLoc
import Maybes
59
import OrdList
Simon Marlow's avatar
Simon Marlow committed
60
import Bag
Richard Eisenberg's avatar
Richard Eisenberg committed
61
import BasicTypes
Ian Lynagh's avatar
Ian Lynagh committed
62
import DynFlags
Simon Marlow's avatar
Simon Marlow committed
63
import FastString
64
import Util
65
import MonadUtils
66
import qualified GHC.LanguageExtensions as LangExt
67
import Control.Monad
68

69
{-**********************************************************************
Austin Seipp's avatar
Austin Seipp committed
70
*                                                                      *
71
           Desugaring a MonoBinds
Austin Seipp's avatar
Austin Seipp committed
72
*                                                                      *
73
**********************************************************************-}
74

ase's avatar
ase committed
75
76
-- | Desugar top level binds, strict binds are treated like normal
-- binds since there is no good time to force before first usage.
77
dsTopLHsBinds :: LHsBinds Id -> DsM (OrdList (Id,CoreExpr))
Richard Eisenberg's avatar
Richard Eisenberg committed
78
79
80
81
82
83
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
       ; mapBagM_ (top_level_err "strict pattern bindings")    bang_binds
       ; return nilOL }
84

Richard Eisenberg's avatar
Richard Eisenberg committed
85
86
87
88
89
90
  | 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.
91

Richard Eisenberg's avatar
Richard Eisenberg committed
92
93
94
95
96
97
98
99
100
101
       ; return (toOL prs) }

  where
    unlifted_binds = filterBag (isUnliftedHsBind . unLoc) binds
    bang_binds     = filterBag (isBangedPatBind  . unLoc) binds

    top_level_err desc (L loc bind)
      = putSrcSpanDs loc $
        errDs (hang (text "Top-level" <+> text desc <+> text "aren't allowed:")
                  2 (ppr bind))
102

ase's avatar
ase committed
103

Richard Eisenberg's avatar
Richard Eisenberg committed
104
-- | Desugar all other kind of bindings, Ids of strict binds are returned to
105
-- later be forced in the binding group body, see Note [Desugar Strict binds]
Richard Eisenberg's avatar
Richard Eisenberg committed
106
107
108
109
dsLHsBinds :: LHsBinds Id -> DsM ([Id], [(Id,CoreExpr)])
dsLHsBinds binds
  = do { MASSERT( allBag (not . isUnliftedHsBind . unLoc) binds )
       ; ds_bs <- mapBagM dsLHsBind binds
ase's avatar
ase committed
110
111
112
       ; return (foldBag (\(a, a') (b, b') -> (a ++ b, a' ++ b'))
                         id ([], []) ds_bs) }

Richard Eisenberg's avatar
Richard Eisenberg committed
113
------------------------
ase's avatar
ase committed
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
dsLHsBind :: LHsBind Id
          -> DsM ([Id], [(Id,CoreExpr)])
dsLHsBind (L loc bind) = do dflags <- getDynFlags
                            putSrcSpanDs loc $ dsHsBind dflags bind

-- | Desugar a single binding (or group of recursive binds).
dsHsBind :: DynFlags
         -> HsBind Id
         -> 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.

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

dsHsBind dflags
         (FunBind { fun_id = L _ fun, fun_matches = matches
144
                  , fun_co_fn = co_fn, fun_tick = tick })
145
146
147
 = do   { (args, body) <- matchWrapper
                           (FunRhs (noLoc $ idName fun) Prefix)
                           Nothing matches
Simon Peyton Jones's avatar
Simon Peyton Jones committed
148
        ; core_wrap <- dsHsWrapper co_fn
149
        ; let body' = mkOptTickBox tick body
Simon Peyton Jones's avatar
Simon Peyton Jones committed
150
151
              rhs   = core_wrap (mkLams args body')
              core_binds@(id,_) = makeCorePair dflags fun False 0 rhs
ase's avatar
ase committed
152
              force_var =
153
                if xopt LangExt.Strict dflags
ase's avatar
ase committed
154
155
156
                   && matchGroupArity matches == 0 -- no need to force lambdas
                then [id]
                else []
157
        ; {- pprTrace "dsHsBind" (ppr fun <+> ppr (idInlinePragma fun)) $ -}
ase's avatar
ase committed
158
           return (force_var, [core_binds]) }
159

ase's avatar
ase committed
160
161
dsHsBind dflags
         (PatBind { pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty
162
                  , pat_ticks = (rhs_tick, var_ticks) })
163
  = do  { body_expr <- dsGuarded grhss ty
164
        ; let body' = mkOptTickBox rhs_tick body_expr
165
              pat'  = decideBangHood dflags pat
166
        ; (force_var,sel_binds) <- mkSelectorBinds var_ticks pat body'
167
168
          -- We silently ignore inline pragmas; no makeCorePair
          -- Not so cool, but really doesn't matter
169
170
        ; let force_var' = if isBangedLPat pat'
                           then [force_var]
ase's avatar
ase committed
171
172
                           else []
        ; return (force_var', sel_binds) }
sof's avatar
sof committed
173

ase's avatar
ase committed
174
        -- A common case: one exported variable, only non-strict binds
175
        -- Non-recursive bindings come through this way
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
176
177
        -- So do self-recursive bindings
        -- Bindings with complete signatures are AbsBindsSigs, below
ase's avatar
ase committed
178
179
dsHsBind dflags
         (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
180
181
                   , abs_exports = [export]
                   , abs_ev_binds = ev_binds, abs_binds = binds })
182
  | ABE { abe_wrap = wrap, abe_poly = global
183
        , abe_mono = local, abe_prags = prags } <- export
184
185
186
  , not (xopt LangExt.Strict dflags)             -- Handle strict binds
  , not (anyBag (isBangedPatBind . unLoc) binds) --        in the next case
  = -- See Note [AbsBinds wrappers] in HsBinds
187
    addDictsDs (toTcTypeBag (listToBag dicts)) $
188
         -- addDictsDs: push type constraints deeper for pattern match check
Richard Eisenberg's avatar
Richard Eisenberg committed
189
    do { (_, bind_prs) <- dsLHsBinds binds
190
191
       ; let core_bind = Rec bind_prs
       ; ds_binds <- dsTcEvBinds_s ev_binds
Simon Peyton Jones's avatar
Simon Peyton Jones committed
192
       ; core_wrap <- dsHsWrapper wrap -- Usually the identity
193

Simon Peyton Jones's avatar
Simon Peyton Jones committed
194
195
196
197
198
       ; let rhs = core_wrap $
                   mkLams tyvars $ mkLams dicts $
                   mkCoreLets ds_binds $
                   Let core_bind $
                   Var local
199
       ; (spec_binds, rules) <- dsSpecs rhs prags
200

201
202
203
       ; let   global'  = addIdSpecialisations global rules
               main_bind = makeCorePair dflags global' (isDefaultMethod prags)
                                        (dictArity dicts) rhs
204

205
       ; return ([], main_bind : fromOL spec_binds) }
sof's avatar
sof committed
206

207
208
209
210
211
212
        -- Another common case: no tyvars, no dicts
        -- In this case we can have a much simpler desugaring
dsHsBind dflags
         (AbsBinds { abs_tvs = [], abs_ev_vars = []
                   , abs_exports = exports
                   , abs_ev_binds = ev_binds, abs_binds = binds })
Richard Eisenberg's avatar
Richard Eisenberg committed
213
  = do { (force_vars, bind_prs) <- dsLHsBinds binds
214
215
216
217
       ; let mk_bind (ABE { abe_wrap = wrap
                          , abe_poly = global
                          , abe_mono = local
                          , abe_prags = prags })
Simon Peyton Jones's avatar
Simon Peyton Jones committed
218
              = do { core_wrap <- dsHsWrapper wrap
219
220
                   ; return (makeCorePair dflags global
                                          (isDefaultMethod prags)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
221
                                          0 (core_wrap (Var local))) }
222
223
224
225
226
       ; main_binds <- mapM mk_bind exports

       ; ds_binds <- dsTcEvBinds_s ev_binds
       ; return (force_vars, flattenBinds ds_binds ++ bind_prs ++ main_binds) }

ase's avatar
ase committed
227
228
dsHsBind dflags
         (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
229
230
                   , abs_exports = exports, abs_ev_binds = ev_binds
                   , abs_binds = binds })
231
         -- See Note [Desugaring AbsBinds]
232
233
  = addDictsDs (toTcTypeBag (listToBag dicts)) $
         -- addDictsDs: push type constraints deeper for pattern match check
Richard Eisenberg's avatar
Richard Eisenberg committed
234
     do { (local_force_vars, bind_prs) <- dsLHsBinds binds
235
        ; let core_bind = Rec [ makeCorePair dflags (add_inline lcl_id) False 0 rhs
ase's avatar
ase committed
236
                              | (lcl_id, rhs) <- bind_prs ]
237
                -- Monomorphic recursion possible, hence Rec
ase's avatar
ase committed
238
              new_force_vars = get_new_force_vars local_force_vars
239
              locals       = map abe_mono exports
ase's avatar
ase committed
240
241
              all_locals   = locals ++ new_force_vars
              tup_expr     = mkBigCoreVarTup all_locals
242
              tup_ty       = exprType tup_expr
243
        ; ds_binds <- dsTcEvBinds_s ev_binds
244
245
246
247
        ; let poly_tup_rhs = mkLams tyvars $ mkLams dicts $
                             mkCoreLets ds_binds $
                             Let core_bind $
                             tup_expr
248

249
        ; poly_tup_id <- newSysLocalDs (exprType poly_tup_rhs)
250

ase's avatar
ase committed
251
252
253
254
255
        -- 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]
        ; (exported_force_vars, extra_exports) <- get_exports local_force_vars

256
        ; let mk_bind (ABE { abe_wrap = wrap
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
257
                           , abe_poly = global
258
                           , abe_mono = local, abe_prags = spec_prags })
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
259
                         -- See Note [AbsBinds wrappers] in HsBinds
260
                = do { tup_id  <- newSysLocalDs tup_ty
Simon Peyton Jones's avatar
Simon Peyton Jones committed
261
262
263
264
265
                     ; 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
266
267
                     ; (spec_binds, rules) <- dsSpecs rhs_for_spec spec_prags
                     ; let global' = (global `setInlinePragma` defaultInlinePragma)
268
269
270
                                             `addIdSpecialisations` rules
                           -- Kill the INLINE pragma because it applies to
                           -- the user written (local) function.  The global
271
                           -- Id is just the selector.  Hmm.
ase's avatar
ase committed
272
                     ; return ((global', rhs) : fromOL spec_binds) }
273

ase's avatar
ase committed
274
        ; export_binds_s <- mapM mk_bind (exports ++ extra_exports)
275

ase's avatar
ase committed
276
277
278
        ; return (exported_force_vars
                 ,(poly_tup_id, poly_tup_rhs) :
                   concat export_binds_s) }
279
280
281
282
283
  where
    inline_env :: IdEnv Id   -- Maps a monomorphic local Id to one with
                             -- the inline pragma from the source
                             -- The type checker put the inline pragma
                             -- on the *global* Id, so we need to transfer it
284
285
286
287
    inline_env
      = mkVarEnv [ (lcl_id, setInlinePragma lcl_id prag)
                 | ABE { abe_mono = lcl_id, abe_poly = gbl_id } <- exports
                 , let prag = idInlinePragma gbl_id ]
288
289

    add_inline :: Id -> Id    -- tran
290
291
    add_inline lcl_id = lookupVarEnv inline_env lcl_id
                        `orElse` lcl_id
292

ase's avatar
ase committed
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
    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
    get_exports :: [Id] -> DsM ([Id], [ABExport Id])
    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))))
         return (ABE {abe_poly = global
                     ,abe_mono = local
                     ,abe_wrap = WpHole
                     ,abe_prags = SpecPrags []})

325
-- AbsBindsSig is a combination of AbsBinds and FunBind
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
326
327
328
329
330
331
332
333
334
335
dsHsBind dflags (AbsBindsSig { abs_tvs = tyvars, abs_ev_vars = dicts
                             , abs_sig_export  = global
                             , abs_sig_prags   = prags
                             , abs_sig_ev_bind = ev_bind
                             , abs_sig_bind    = bind })
  | L bind_loc FunBind { fun_matches = matches
                       , fun_co_fn   = co_fn
                       , fun_tick    = tick } <- bind
  = putSrcSpanDs bind_loc $
    addDictsDs (toTcTypeBag (listToBag dicts)) $
336
             -- addDictsDs: push type constraints deeper for pattern match check
337
338
339
    do { (args, body) <- matchWrapper
                           (FunRhs (noLoc $ idName global) Prefix)
                           Nothing matches
Simon Peyton Jones's avatar
Simon Peyton Jones committed
340
341
342
343
       ; core_wrap <- dsHsWrapper co_fn
       ; let body'   = mkOptTickBox tick body
             fun_rhs = core_wrap (mkLams args body')
             force_vars
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
               | xopt LangExt.Strict dflags
               , matchGroupArity matches == 0 -- no need to force lambdas
               = [global]
               | otherwise
               = []

       ; ds_binds <- dsTcEvBinds ev_bind
       ; let rhs = mkLams tyvars $
                   mkLams dicts $
                   mkCoreLets ds_binds $
                   fun_rhs

       ; (spec_binds, rules) <- dsSpecs rhs prags
       ; let global' = addIdSpecialisations global rules
             main_bind = makeCorePair dflags global' (isDefaultMethod prags)
                                      (dictArity dicts) rhs

       ; return (force_vars, main_bind : fromOL spec_binds) }

  | otherwise
  = pprPanic "dsHsBind: AbsBindsSig" (ppr bind)

ase's avatar
ase committed
366
367
dsHsBind _ (PatSynBind{}) = panic "dsHsBind: PatSynBind"

Gergő Érdi's avatar
Gergő Érdi committed
368

369
370

-- | This is where we apply INLINE and INLINABLE pragmas. All we need to
371
372
373
374
375
376
-- 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.
377
------------------------
378
379
makeCorePair :: DynFlags -> Id -> Bool -> Arity -> CoreExpr -> (Id, CoreExpr)
makeCorePair dflags gbl_id is_default_method dict_arity rhs
380
  | is_default_method                 -- Default methods are *always* inlined
381
382
  = (gbl_id `setIdUnfolding` mkCompulsoryUnfolding rhs, rhs)

383
384
  | otherwise
  = case inlinePragmaSpec inline_prag of
385
386
387
          EmptyInlineSpec -> (gbl_id, rhs)
          NoInline        -> (gbl_id, rhs)
          Inlinable       -> (gbl_id `setIdUnfolding` inlinable_unf, rhs)
388
          Inline          -> inline_pair
389

390
391
  where
    inline_prag   = idInlinePragma gbl_id
392
    inlinable_unf = mkInlinableUnfolding dflags rhs
393
394
    inline_pair
       | Just arity <- inlinePragmaSat inline_prag
395
396
        -- Add an Unfolding for an INLINE (but not for NOINLINE)
        -- And eta-expand the RHS; see Note [Eta-expanding INLINE things]
397
       , let real_arity = dict_arity + arity
398
        -- NB: The arity in the InlineRule takes account of the dictionaries
399
       = ( gbl_id `setIdUnfolding` mkInlineUnfoldingWithArity real_arity rhs
400
401
402
403
         , etaExpand real_arity rhs)

       | otherwise
       = pprTrace "makeCorePair: arity missing" (ppr gbl_id) $
404
         (gbl_id `setIdUnfolding` mkInlineUnfolding rhs, rhs)
405
406
407
408

dictArity :: [Var] -> Arity
-- Don't count coercion variables in arity
dictArity dicts = count isId dicts
409

Austin Seipp's avatar
Austin Seipp committed
410
{-
ase's avatar
ase committed
411
412
Note [Desugaring AbsBinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~
413
414
415
416
417
418
419
420
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 }

421
422
423
424
425
Note [Rules and inlining]
~~~~~~~~~~~~~~~~~~~~~~~~~
Common special case: no type or dictionary abstraction
This is a bit less trivial than you might suppose
The naive way woudl be to desguar to something like
426
427
        f_lcl = ...f_lcl...     -- The "binds" from AbsBinds
        M.f = f_lcl             -- Generated from "exports"
428
But we don't want that, because if M.f isn't exported,
429
430
it'll be inlined unconditionally at every call site (its rhs is
trivial).  That would be ok unless it has RULES, which would
431
432
433
thereby be completely lost.  Bad, bad, bad.

Instead we want to generate
434
435
436
        M.f = ...f_lcl...
        f_lcl = M.f
Now all is cool. The RULES are attached to M.f (by SimplCore),
437
438
439
440
and f_lcl is rapidly inlined away.

This does not happen in the same way to polymorphic binds,
because they desugar to
441
        M.f = /\a. let f_lcl = ...f_lcl... in f_lcl
442
Although I'm a bit worried about whether full laziness might
443
float the f_lcl binding out and then inline M.f at its call site
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458

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 #-}

459
The top-level AbsBinds for $cround has no tyvars or dicts (because the
460
461
462
463
464
465
466
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

467
468
469
470
471
        AbsBinds [a,b] [ ([a,b], fg, fl, _),
                         ([b],   gg, gl, _) ]
                { fl = e1
                  gl = e2
                   h = e3 }
472
473
474

and desugar it to

475
476
477
        fg = /\ab. let B in e1
        gg = /\b. let a = () in let B in S(e2)
        h  = /\ab. let B in e3
478
479

where B is the *non-recursive* binding
480
481
482
        fl = fg a b
        gl = gg b
        h  = h a b    -- See (b); note shadowing!
483
484

Notice (a) g has a different number of type variables to f, so we must
485
486
             use the mkArbitraryType thing to fill in the gaps.
             We use a type-let to do that.
487

488
489
490
491
         (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.
492

493
494
495
496
         (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.
497
498

Why got to this trouble?  It's a common case, and it removes the
499
quadratic-sized tuple desugaring.  Less clutter, hopefully faster
500
501
502
503
compilation, especially in a case where there are a *lot* of
bindings.


504
505
506
507
508
509
510
511
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
512
happen as a result of method sharing), there's a danger that we never
513
514
515
516
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
517
has the arity with which it is declared in the source code.  In this
518
example it has arity 2 (one for the Eq and one for x). Doing this
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
519
should mean that (foo d) is a PAP and we don't share it.
520
521
522

Note [Nested arities]
~~~~~~~~~~~~~~~~~~~~~
523
524
525
526
527
528
529
530
531
532
533
534
535
536
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!
ase's avatar
ase committed
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
586
587
588
589
590
591
592


Note [Desugar Strict binds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~

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
593
The simplest thing is to return it in the polymorphic
ase's avatar
ase committed
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
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>


See https://ghc.haskell.org/trac/ghc/wiki/StrictPragma for a more
detailed explanation of the desugaring of strict bindings.

Richard Eisenberg's avatar
Richard Eisenberg committed
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
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.

Define a "banged bind" to have a top-level bang. Detected by HsPat.isBangedPatBind.
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
643
-}
644

645
------------------------
646
dsSpecs :: CoreExpr     -- Its rhs
647
        -> TcSpecPrags
648
649
        -> DsM ( OrdList (Id,CoreExpr)  -- Binding for specialised Ids
               , [CoreRule] )           -- Rules for the Global Ids
650
-- See Note [Handling SPECIALISE pragmas] in TcBinds
651
652
653
654
655
656
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) }

657
658
659
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
660
661
662
       -> Located TcSpecPrag
       -> DsM (Maybe (OrdList (Id,CoreExpr), CoreRule))
dsSpec mb_poly_rhs (L loc (SpecPrag poly_id spec_co spec_inl))
663
  | isJust (isClassOpId_maybe poly_id)
664
  = putSrcSpanDs loc $
665
666
    do { warnDs NoReason (text "Ignoring useless SPECIALISE pragma for class method selector"
                          <+> quotes (ppr poly_id))
667
       ; return Nothing  }  -- There is no point in trying to specialise a class op
668
669
                            -- Moreover, classops don't (currently) have an inl_sat arity set
                            -- (it would be Just 0) and that in turn makes makeCorePair bleat
670

671
672
  | no_act_spec && isNeverActive rule_act
  = putSrcSpanDs loc $
673
674
    do { warnDs NoReason (text "Ignoring useless SPECIALISE pragma for NOINLINE function:"
                          <+> quotes (ppr poly_id))
675
       ; return Nothing  }  -- Function is NOINLINE, and the specialiation inherits that
676
                            -- See Note [Activation pragmas for SPECIALISE]
677

678
  | otherwise
679
  = putSrcSpanDs loc $
680
681
    do { uniq <- newUnique
       ; let poly_name = idName poly_id
682
683
             spec_occ  = mkSpecOcc (getOccName poly_name)
             spec_name = mkInternalName uniq spec_occ (getSrcSpan poly_name)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
684
685
686
687
688
689
690
691
692
693
             (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)
694
695
696
       ; -- pprTrace "dsRule" (vcat [ text "Id:" <+> ppr poly_id
         --                         , text "spec_co:" <+> ppr spec_co
         --                         , text "ds_rhs:" <+> ppr ds_lhs ]) $
Simon Peyton Jones's avatar
Simon Peyton Jones committed
697
         case decomposeRuleLhs spec_bndrs ds_lhs of {
698
           Left msg -> do { warnDs NoReason msg; return Nothing } ;
699
           Right (rule_bndrs, _fn, args) -> do
700

701
       { dflags <- getDynFlags
702
       ; this_mod <- getModule
Simon Peyton Jones's avatar
Simon Peyton Jones committed
703
       ; let fn_unf    = realIdUnfolding poly_id
Simon Peyton Jones's avatar
Simon Peyton Jones committed
704
             spec_unf  = specUnfolding spec_bndrs core_app arity_decrease fn_unf
705
706
707
             spec_id   = mkLocalId spec_name spec_ty
                            `setInlinePragma` inl_prag
                            `setIdUnfolding`  spec_unf
Simon Peyton Jones's avatar
Simon Peyton Jones committed
708
709
             arity_decrease = count isValArg args - count isId spec_bndrs

710
       ; rule <- dsMkUserRule this_mod is_local_id
Ian Lynagh's avatar
Ian Lynagh committed
711
                        (mkFastString ("SPEC " ++ showPpr dflags poly_name))
712
713
                        rule_act poly_name
                        rule_bndrs args
Simon Peyton Jones's avatar
Simon Peyton Jones committed
714
                        (mkVarApps (Var spec_id) spec_bndrs)
715

Simon Peyton Jones's avatar
Simon Peyton Jones committed
716
       ; let spec_rhs = mkLams spec_bndrs (core_app poly_rhs)
717

718
719
-- Commented out: see Note [SPECIALISE on INLINE functions]
--       ; when (isInlinePragma id_inl)
720
--              (warnDs $ text "SPECIALISE pragma on INLINE function probably won't fire:"
721
--                        <+> quotes (ppr poly_name))
Simon Peyton Jones's avatar
Simon Peyton Jones committed
722
723
724
725
726

       ; 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
727
728
729
730
       } } }
  where
    is_local_id = isJust mb_poly_rhs
    poly_rhs | Just rhs <-  mb_poly_rhs
731
             = rhs          -- Local Id; this is its rhs
732
733
             | Just unfolding <- maybeUnfoldingTemplate (realIdUnfolding poly_id)
             = unfolding    -- Imported Id; this is its unfolding
734
735
736
                            -- Use realIdUnfolding so we get the unfolding
                            -- even when it is a loop breaker.
                            -- We want to specialise recursive functions!
737
             | otherwise = pprPanic "dsImpSpecs" (ppr poly_id)
738
                            -- The type checker has checked that it *has* an unfolding
739

740
741
742
743
744
    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]
745
                                 -- in OccurAnal
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
             , 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


763
764
765
766
767
768
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) $
769
        warnDs (Reason Opt_WarnOrphans) (ruleOrphWarn rule)
770
771
772
    return rule

ruleOrphWarn :: CoreRule -> SDoc
773
ruleOrphWarn rule = text "Orphan rule:" <+> ppr rule
774

775
776
777
778
779
780
781
782
783
784
785
786
787
{- 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
it even if we wanted to.  Trac #10721 is a case in point.

788
789
790
791
792
793
794
795
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:

796
* spec_fn's inline pragma: inherited from f's inline pragma (ignoring
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
                           activation on SPEC), unless overriden by SPEC INLINE

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

This is not obvious (see Trac #5237)!

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
818
SPEC [n] f :: ty            [n]   INLINE [k]
819
820
821
822
823
824
825
826
827
828
                                  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
829
830
************************************************************************
*                                                                      *
831
\subsection{Adding inline pragmas}
Austin Seipp's avatar
Austin Seipp committed
832
833
834
*                                                                      *
************************************************************************
-}
835

836
decomposeRuleLhs :: [Var] -> CoreExpr -> Either SDoc ([Var], Id, [CoreExpr])
unknown's avatar
unknown committed
837
838
-- (decomposeRuleLhs bndrs lhs) takes apart the LHS of a RULE,
-- The 'bndrs' are the quantified binders of the rules, but decomposeRuleLhs
839
-- may add some extra dictionary binders (see Note [Free dictionaries])
unknown's avatar
unknown committed
840
--
841
-- Returns Nothing if the LHS isn't of the expected shape
842
843
844
845
846
847
-- Note [Decomposing the left-hand side of a RULE]
decomposeRuleLhs orig_bndrs orig_lhs
  | not (null unbound)    -- Check for things unbound on LHS
                          -- See Note [Unused spec binders]
  = Left (vcat (map dead_msg unbound))

848
  | Just (fn_id, args) <- decompose fun2 args2
849
  , let extra_bndrs = mk_extra_bndrs fn_id args
850
851
852
853
854
855
  = -- 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]) $
856
    Right (orig_bndrs ++ extra_bndrs, fn_id, args)
857

858
  | otherwise
859
  = Left bad_shape_msg
860
 where
861
862
863
864
   lhs1         = drop_dicts orig_lhs
   lhs2         = simpleOptExpr lhs1  -- See Note [Simplify rule LHS]
   (fun2,args2) = collectArgs lhs2

865
866
   lhs_fvs    = exprFreeVars lhs2
   unbound    = filterOut (`elemVarSet` lhs_fvs) orig_bndrs
867

868
   orig_bndr_set = mkVarSet orig_bndrs
869

870
871
872
873
874
875
876
877
878
879
880
881
882
        -- 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
     = toposortTyVars unbound_tvs ++ unbound_dicts
     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
         -- itself be a dictionary (in pathological cases, Trac #10251)
883
884
885
886
887
888

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

   decompose _ _ = Nothing
889

890
   bad_shape_msg = hang (text "RULE left-hand side too complicated to desugar")
891
892
                      2 (vcat [ text "Optimised lhs:" <+> ppr lhs2
                              , text "Orig lhs:" <+> ppr orig_lhs])
893
894
   dead_msg bndr = hang (sep [ text "Forall'd" <+> pp_bndr bndr
                             , text "is not bound in RULE lhs"])
895
896
897
                      2 (vcat [ text "Orig bndrs:" <+> ppr orig_bndrs
                              , text "Orig lhs:" <+> ppr orig_lhs
                              , text "optimised lhs:" <+> ppr lhs2 ])
898
   pp_bndr bndr
899
900
901
    | isTyVar bndr                      = text "type variable" <+> quotes (ppr bndr)
    | Just pred <- evVarPred_maybe bndr = text "constraint" <+> quotes (ppr pred)
    | otherwise                         = text "variable" <+> quotes (ppr bndr)
902
903

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

   split_lets :: CoreExpr -> ([(DictId,CoreExpr)], CoreExpr)
914
915
   split_lets (Let (NonRec d r) body)
     | isDictId d
916
     = ((d,r):bs, body')
917
918
919
920
921
922
923
924
925
     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)
926
927
928
929

   wrap_lets :: VarSet -> [(DictId,CoreExpr)] -> CoreExpr -> CoreExpr
   wrap_lets _ [] body = body
   wrap_lets needed ((d, r) : bs) body
930
     | rhs_fvs `intersectsVarSet` needed = mkCoreLet (NonRec d r) (wrap_lets needed' bs body)
931
932
933
934
     | otherwise                         = wrap_lets needed bs body
     where
       rhs_fvs = exprFreeVars r
       needed' = (needed `minusVarSet` rhs_fvs) `extendVarSet` d
935

Austin Seipp's avatar
Austin Seipp committed
936
{-
937
Note [Decomposing the left-hand side of a RULE]
938
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
939
There are several things going on here.
940
941
* drop_dicts: see Note [Drop dictionary bindings on rule LHS]
* simpleOptExpr: see Note [Simplify rule LHS]
942
* extra_dict_bndrs: see Note [Free dictionaries]
943

944
945
946
947
948
949
950
951
952
953
954
955
956
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
957
into a TyVar 'a'.  See TcHsSyn Note [Zonking the LHS of a RULE].
958
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

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.

992
993
Note [Drop dictionary bindings on rule LHS]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
994
drop_dicts drops dictionary bindings on the LHS where possible.
995
996
   E.g.  let d:Eq [Int] = $fEqList $fEqInt in f d
     --> f d
997
   Reasoning here is that there is only one d:Eq [Int], and so we can
998
999
1000
1001
   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
1002
         one of the orig_bndrs, which we assume occur on RHS.
1003
1004
1005
1006
1007
1008
         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
1009
         to match, but there is no other way to get d:Eq a
1010

1011
   NB 2: We do drop_dicts *before* simplOptEpxr, so that we expect all
1012
1013
1014
1015
1016
1017
         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
1018
    NB3: In the common case of a non-overloaded, but perhaps-polymorphic
1019
1020
1021
1022
1023
1024
         specialisation, we don't need to bind *any* dictionaries for use
         in the RHS. For example (Trac #8331)
             {-# 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
1025
             RULE forall s (d :: MonadAbstractIOST (ReaderT s)).
1026
1027
                useAbstractMonad (ReaderT s) d = $suseAbstractMonad s

1028
1029
1030
   Trac #8848 is a good example of where there are some intersting
   dictionary bindings to discard.

1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
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
1041
       ... SPECIALISE f :: (Show b) => Int -> b -> String ...
1042
1043
1044
1045
1046
1047
    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.


1048
1049
1050
1051
Note [Simplify rule LHS]
~~~~~~~~~~~~~~~~~~~~~~~~
simplOptExpr occurrence-analyses and simplifies the LHS:

1052
   (a) Inline any remaining dictionary bindings (which hopefully
1053
1054
       occur just once)

1055
   (b) Substitute trivial lets, so that they don't get in the way.
1056
       Note that we substitute the function too; we might
1057
1058
       have this as a LHS:  let f71 = M.f Int in f71

1059
   (c) Do eta reduction.  To see why, consider the fold/build rule,
1060
1061
1062
1063
       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
1064
         augment g (build h)
1065
       we do not want to get
1066
         augment (\a. g a) (build h)
1067
1068
       otherwise we don't match when given an argument like
          augment (\a. h a a) (build h)
1069

1070
Note [Matching seqId]
1071
1072
~~~~~~~~~~~~~~~~~~~
The desugarer turns (seq e r) into (case e of _ -> r), via a special-case hack
1073
and this code turns it back into an application of seq!
1074
1075
See Note [Rules for seq] in MkId for the details.

1076
1077
1078
Note [Unused spec binders]
~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
1079
        f :: a -> a
Austin Seipp's avatar
Austin Seipp committed
1080
        ... SPECIALISE f :: Eq a => a -> a ...
1081
1082
It's true that this *is* a more specialised type, but the rule
we get is something like this:
1083
1084
        f_spec d = f
        RULE: f = f_spec d
Gabor Greif's avatar
typos    
Gabor Greif committed
1085
1086
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
1087
1088
1089
1090
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.

Austin Seipp's avatar
Austin Seipp committed
1091
1092
************************************************************************
*                                                                      *
1093
                Desugaring evidence
Austin Seipp's avatar
Austin Seipp committed
1094
1095
*                                                                      *
************************************************************************
1096

Austin Seipp's avatar
Austin Seipp committed
1097
-}
1098