TcSimplify.lhs 64.5 KB
Newer Older
1
%
2
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
3
4
5
%
\section[TcSimplify]{TcSimplify}

6
7
8


\begin{code}
9
module TcSimplify (
10
	tcSimplifyInfer, tcSimplifyInferCheck,
11
	tcSimplifyCheck, tcSimplifyRestricted,
12
	tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop,
13

14
	tcSimplifyDeriv, tcSimplifyDefault,
15
	bindInstsOfLocalFuns
16
17
    ) where

18
#include "HsVersions.h"
19

20
21
import {-# SOURCE #-} TcUnify( unifyTauTy )

22
import HsSyn		( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
23
import TcHsSyn		( TcExpr, TcId,
24
25
			  TcMonoBinds, TcDictBinds
			)
26

27
import TcMonad
28
import Inst		( lookupInst, lookupSimpleInst, LookupInstResult(..),
29
			  tyVarsOfInst, predsOfInsts, predsOfInst, newDicts,
30
31
32
			  isDict, isClassDict, isLinearInst, linearInstType,
			  isStdClassTyVarDict, isMethodFor, isMethod,
			  instToId, tyVarsOfInsts,  cloneDict,
33
			  ipNamesOfInsts, ipNamesOfInst, dictPred,
34
			  instBindingRequired, instCanBeGeneralised,
35
			  newDictsFromOld, newMethodAtLoc,
36
			  getDictClassTys, isTyVarDict,
37
			  instLoc, pprInst, zonkInst, tidyInsts, tidyMoreInsts,
38
			  Inst, LIE, pprInsts, pprInstsInFull,
39
			  mkLIE, lieToList
sof's avatar
sof committed
40
			)
41
import TcEnv		( tcGetGlobalTyVars, tcGetInstEnv, tcLookupGlobalId )
42
import InstEnv		( lookupInstEnv, classInstEnv, InstLookupResult(..) )
43
import TcMType		( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
44
import TcType		( TcTyVar, TcTyVarSet, ThetaType, PredType, 
45
			  mkClassPred, isOverloadedTy, mkTyConApp,
46
			  mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
47
48
			  tyVarsOfPred, getClassPredTys_maybe, isClassPred, isIPPred,
			  inheritablePred, predHasFDs )
49
import Id		( idType, mkUserLocal )
50
import Var		( TyVar )
51
import Name		( getOccName, getSrcLoc )
52
import NameSet		( NameSet, mkNameSet, elemNameSet )
53
import Class		( classBigSig )
54
import FunDeps		( oclose, grow, improve, pprEquationDoc )
55
56
import PrelInfo		( isNumericClass, isCreturnableClass, isCcallishClass, 
			  splitIdName, fstIdName, sndIdName )
57

58
import Subst		( mkTopTyVarSubst, substTheta, substTy )
59
import TysWiredIn	( unitTy, pairTyCon )
60
import VarSet
61
62
import FiniteMap
import Outputable
63
import ListSetOps	( equivClasses )
64
import Util		( zipEqual )
65
import List		( partition )
66
import CmdLineOpts
67
68
69
70
71
\end{code}


%************************************************************************
%*									*
72
\subsection{NOTES}
73
74
75
%*									*
%************************************************************************

76
	--------------------------------------
77
		Notes on quantification
78
	--------------------------------------
79
80
81
82
83
84

Suppose we are about to do a generalisation step.
We have in our hand

	G	the environment
	T	the type of the RHS
85
	C	the constraints from that RHS
86
87
88
89
90
91
92
93
94
95
96

The game is to figure out

	Q	the set of type variables over which to quantify
	Ct	the constraints we will *not* quantify over
	Cq	the constraints we will quantify over

So we're going to infer the type

	forall Q. Cq => T

97
and float the constraints Ct further outwards.
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135

Here are the things that *must* be true:

 (A)	Q intersect fv(G) = EMPTY			limits how big Q can be
 (B)	Q superset fv(Cq union T) \ oclose(fv(G),C)	limits how small Q can be

(A) says we can't quantify over a variable that's free in the
environment.  (B) says we must quantify over all the truly free
variables in T, else we won't get a sufficiently general type.  We do
not *need* to quantify over any variable that is fixed by the free
vars of the environment G.

	BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!

Example:	class H x y | x->y where ...

	fv(G) = {a}	C = {H a b, H c d}
			T = c -> b

	(A)  Q intersect {a} is empty
	(B)  Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}

	So Q can be {c,d}, {b,c,d}

Other things being equal, however, we'd like to quantify over as few
variables as possible: smaller types, fewer type applications, more
constraints can get into Ct instead of Cq.


-----------------------------------------
We will make use of

  fv(T)	 	the free type vars of T

  oclose(vs,C)	The result of extending the set of tyvars vs
		using the functional dependencies from C

  grow(vs,C)	The result of extend the set of tyvars vs
136
		using all conceivable links from C.
137
138
139
140
141
142
143

		E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
		Then grow(vs,C) = {a,b,c}

		Note that grow(vs,C) `superset` grow(vs,simplify(C))
		That is, simplfication can only shrink the result of grow.

144
Notice that
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
   oclose is conservative one way:      v `elem` oclose(vs,C) => v is definitely fixed by vs
   grow is conservative the other way:  if v might be fixed by vs => v `elem` grow(vs,C)


-----------------------------------------

Choosing Q
~~~~~~~~~~
Here's a good way to choose Q:

	Q = grow( fv(T), C ) \ oclose( fv(G), C )

That is, quantify over all variable that that MIGHT be fixed by the
call site (which influences T), but which aren't DEFINITELY fixed by
G.  This choice definitely quantifies over enough type variables,
albeit perhaps too many.

Why grow( fv(T), C ) rather than fv(T)?  Consider

	class H x y | x->y where ...
165

166
167
168
169
170
171
172
	T = c->c
	C = (H c d)

  If we used fv(T) = {c} we'd get the type

	forall c. H c d => c -> b

173
  And then if the fn was called at several different c's, each of
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
  which fixed d differently, we'd get a unification error, because
  d isn't quantified.  Solution: quantify d.  So we must quantify
  everything that might be influenced by c.

Why not oclose( fv(T), C )?  Because we might not be able to see
all the functional dependencies yet:

	class H x y | x->y where ...
	instance H x y => Eq (T x y) where ...

	T = c->c
	C = (Eq (T c d))

  Now oclose(fv(T),C) = {c}, because the functional dependency isn't
  apparent yet, and that's wrong.  We must really quantify over d too.


There really isn't any point in quantifying over any more than
grow( fv(T), C ), because the call sites can't possibly influence
any other type variables.



197
198
199
	--------------------------------------
		Notes on ambiguity
	--------------------------------------
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238

It's very hard to be certain when a type is ambiguous.  Consider

	class K x
	class H x y | x -> y
	instance H x y => K (x,y)

Is this type ambiguous?
	forall a b. (K (a,b), Eq b) => a -> a

Looks like it!  But if we simplify (K (a,b)) we get (H a b) and
now we see that a fixes b.  So we can't tell about ambiguity for sure
without doing a full simplification.  And even that isn't possible if
the context has some free vars that may get unified.  Urgle!

Here's another example: is this ambiguous?
	forall a b. Eq (T b) => a -> a
Not if there's an insance decl (with no context)
	instance Eq (T b) where ...

You may say of this example that we should use the instance decl right
away, but you can't always do that:

	class J a b where ...
	instance J Int b where ...

	f :: forall a b. J a b => a -> a

(Notice: no functional dependency in J's class decl.)
Here f's type is perfectly fine, provided f is only called at Int.
It's premature to complain when meeting f's signature, or even
when inferring a type for f.



However, we don't *need* to report ambiguity right away.  It'll always
show up at the call site.... and eventually at main, which needs special
treatment.  Nevertheless, reporting ambiguity promptly is an excellent thing.

239
So here's the plan.  We WARN about probable ambiguity if
240
241
242
243
244

	fv(Cq) is not a subset of  oclose(fv(T) union fv(G), C)

(all tested before quantification).
That is, all the type variables in Cq must be fixed by the the variables
245
in the environment, or by the variables in the type.
246
247
248

Notice that we union before calling oclose.  Here's an example:

249
	class J a b c | a b -> c
250
251
252
	fv(G) = {a}

Is this ambiguous?
253
	forall b c. (J a b c) => b -> b
254
255

Only if we union {a} from G with {b} from T before using oclose,
256
do we see that c is fixed.
257

258
It's a bit vague exactly which C we should use for this oclose call.  If we
259
260
261
262
263
264
265
266
267
don't fix enough variables we might complain when we shouldn't (see
the above nasty example).  Nothing will be perfect.  That's why we can
only issue a warning.


Can we ever be *certain* about ambiguity?  Yes: if there's a constraint

	c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY

268
then c is a "bubble"; there's no way it can ever improve, and it's
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
certainly ambiguous.  UNLESS it is a constant (sigh).  And what about
the nasty example?

	class K x
	class H x y | x -> y
	instance H x y => K (x,y)

Is this type ambiguous?
	forall a b. (K (a,b), Eq b) => a -> a

Urk.  The (Eq b) looks "definitely ambiguous" but it isn't.  What we are after
is a "bubble" that's a set of constraints

	Cq = Ca union Cq'  st  fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY

Hence another idea.  To decide Q start with fv(T) and grow it
by transitive closure in Cq (no functional dependencies involved).
Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
287
The definitely-ambiguous can then float out, and get smashed at top level
288
289
290
(which squashes out the constants, like Eq (T a) above)


291
	--------------------------------------
292
		Notes on principal types
293
	--------------------------------------
294
295
296

    class C a where
      op :: a -> a
297

298
299
300
301
302
303
304
    f x = let g y = op (y::Int) in True

Here the principal type of f is (forall a. a->a)
but we'll produce the non-principal type
    f :: forall a. C Int => a -> a


305
	--------------------------------------
306
		Notes on implicit parameters
307
	--------------------------------------
308

309
310
311
312
313
Question 1: can we "inherit" implicit parameters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this:

	f x = (x::Int) + ?y
314

315
316
317
where f is *not* a top-level binding.
From the RHS of f we'll get the constraint (?y::Int).
There are two types we might infer for f:
318

319
320
321
	f :: Int -> Int

(so we get ?y from the context of f's definition), or
322
323
324

	f :: (?y::Int) => Int -> Int

325
326
327
328
329
330
At first you might think the first was better, becuase then
?y behaves like a free variable of the definition, rather than
having to be passed at each call site.  But of course, the WHOLE
IDEA is that ?y should be passed at each call site (that's what
dynamic binding means) so we'd better infer the second.

331
332
BOTTOM LINE: when *inferring types* you *must* quantify 
over implicit parameters. See the predicate isFreeWhenInferring.
333

334
335
336
337
338

Question 2: type signatures
~~~~~~~~~~~~~~~~~~~~~~~~~~~
BUT WATCH OUT: When you supply a type signature, we can't force you
to quantify over implicit parameters.  For example:
339
340
341
342
343
344
345
346
347
348
349

	(?x + 1) :: Int

This is perfectly reasonable.  We do not want to insist on

	(?x + 1) :: (?x::Int => Int)

That would be silly.  Here, the definition site *is* the occurrence site,
so the above strictures don't apply.  Hence the difference between
tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
and tcSimplifyCheckBind (which does not).
350

351
352
353
What about when you supply a type signature for a binding?
Is it legal to give the following explicit, user type 
signature to f, thus:
354

355
	f :: Int -> Int
356
	f x = (x::Int) + ?y
357

358
At first sight this seems reasonable, but it has the nasty property
359
that adding a type signature changes the dynamic semantics.
360
Consider this:
361

362
	(let f x = (x::Int) + ?y
363
364
365
366
 	 in (f 3, f 3 with ?y=5))  with ?y = 6

		returns (3+6, 3+5)
vs
367
	(let f :: Int -> Int
368
	     f x = x + ?y
369
370
371
372
	 in (f 3, f 3 with ?y=5))  with ?y = 6

		returns (3+6, 3+6)

373
374
Indeed, simply inlining f (at the Haskell source level) would change the
dynamic semantics.
375

376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
Nevertheless, as Launchbury says (email Oct 01) we can't really give the
semantics for a Haskell program without knowing its typing, so if you 
change the typing you may change the semantics.

To make things consistent in all cases where we are *checking* against
a supplied signature (as opposed to inferring a type), we adopt the
rule: 

	a signature does not need to quantify over implicit params.

[This represents a (rather marginal) change of policy since GHC 5.02,
which *required* an explicit signature to quantify over all implicit
params for the reasons mentioned above.]

But that raises a new question.  Consider 

	Given (signature)	?x::Int
	Wanted (inferred)	?x::Int, ?y::Bool

Clearly we want to discharge the ?x and float the ?y out.  But
what is the criterion that distinguishes them?  Clearly it isn't
what free type variables they have.  The Right Thing seems to be
to float a constraint that
	neither mentions any of the quantified type variables
	nor any of the quantified implicit parameters

See the predicate isFreeWhenChecking.
403

404

405
406
407
Question 3: monomorphism
~~~~~~~~~~~~~~~~~~~~~~~~
There's a nasty corner case when the monomorphism restriction bites:
408

409
410
	z = (x::Int) + ?y

411
412
The argument above suggests that we *must* generalise
over the ?y parameter, to get
413
414
	z :: (?y::Int) => Int,
but the monomorphism restriction says that we *must not*, giving
415
	z :: Int.
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
Why does the momomorphism restriction say this?  Because if you have

	let z = x + ?y in z+z

you might not expect the addition to be done twice --- but it will if
we follow the argument of Question 2 and generalise over ?y.



Possible choices
~~~~~~~~~~~~~~~~
(A) Always generalise over implicit parameters
    Bindings that fall under the monomorphism restriction can't
	be generalised

    Consequences:
432
	* Inlining remains valid
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
	* No unexpected loss of sharing
	* But simple bindings like
		z = ?y + 1
	  will be rejected, unless you add an explicit type signature
	  (to avoid the monomorphism restriction)
		z :: (?y::Int) => Int
		z = ?y + 1
	  This seems unacceptable

(B) Monomorphism restriction "wins"
    Bindings that fall under the monomorphism restriction can't
	be generalised
    Always generalise over implicit parameters *except* for bindings
	that fall under the monomorphism restriction

    Consequences
	* Inlining isn't valid in general
	* No unexpected loss of sharing
	* Simple bindings like
		z = ?y + 1
	  accepted (get value of ?y from binding site)

(C) Always generalise over implicit parameters
    Bindings that fall under the monomorphism restriction can't
	be generalised, EXCEPT for implicit parameters
    Consequences
	* Inlining remains valid
	* Unexpected loss of sharing (from the extra generalisation)
	* Simple bindings like
		z = ?y + 1
	  accepted (get value of ?y from occurrence sites)


Discussion
~~~~~~~~~~
None of these choices seems very satisfactory.  But at least we should
decide which we want to do.
470

471
It's really not clear what is the Right Thing To Do.  If you see
472

473
	z = (x::Int) + ?y
474

475
476
477
478
479
480
would you expect the value of ?y to be got from the *occurrence sites*
of 'z', or from the valuue of ?y at the *definition* of 'z'?  In the
case of function definitions, the answer is clearly the former, but
less so in the case of non-fucntion definitions.   On the other hand,
if we say that we get the value of ?y from the definition site of 'z',
then inlining 'z' might change the semantics of the program.
481

482
Choice (C) really says "the monomorphism restriction doesn't apply
483
to implicit parameters".  Which is fine, but remember that every
484
485
486
innocent binding 'x = ...' that mentions an implicit parameter in
the RHS becomes a *function* of that parameter, called at each
use of 'x'.  Now, the chances are that there are no intervening 'with'
487
clauses that bind ?y, so a decent compiler should common up all
488
489
490
those function calls.  So I think I strongly favour (C).  Indeed,
one could make a similar argument for abolishing the monomorphism
restriction altogether.
491

492
BOTTOM LINE: we choose (B) at present.  See tcSimplifyRestricted
493

494

495

496
497
498
499
500
501
502
503
504
%************************************************************************
%*									*
\subsection{tcSimplifyInfer}
%*									*
%************************************************************************

tcSimplify is called when we *inferring* a type.  Here's the overall game plan:

    1. Compute Q = grow( fvs(T), C )
505
506

    2. Partition C based on Q into Ct and Cq.  Notice that ambiguous
507
       predicates will end up in Ct; we deal with them at the top level
508

509
    3. Try improvement, using functional dependencies
510

511
512
513
514
    4. If Step 3 did any unification, repeat from step 1
       (Unification can change the result of 'grow'.)

Note: we don't reduce dictionaries in step 2.  For example, if we have
515
Eq (a,b), we don't simplify to (Eq a, Eq b).  So Q won't be different
516
517
518
519
520
521
522
after step 2.  However note that we may therefore quantify over more
type variables than we absolutely have to.

For the guts, we need a loop, that alternates context reduction and
improvement with unification.  E.g. Suppose we have

	class C x y | x->y where ...
523

524
525
526
527
528
529
and tcSimplify is called with:
	(C Int a, C Int b)
Then improvement unifies a with b, giving
	(C Int a, C Int a)

If we need to unify anything, we rattle round the whole thing all over
530
again.
531

532
533

\begin{code}
534
tcSimplifyInfer
535
536
	:: SDoc
	-> TcTyVarSet		-- fv(T); type vars
537
538
539
540
541
542
	-> LIE			-- Wanted
	-> TcM ([TcTyVar],	-- Tyvars to quantify (zonked)
		LIE,		-- Free
		TcDictBinds,	-- Bindings
		[TcId])		-- Dict Ids that must be bound here (zonked)
\end{code}
543

544
545
546

\begin{code}
tcSimplifyInfer doc tau_tvs wanted_lie
547
  = inferLoop doc (varSetElems tau_tvs)
548
	      (lieToList wanted_lie)	`thenTc` \ (qtvs, frees, binds, irreds) ->
549
550

	-- Check for non-generalisable insts
551
552
    mapTc_ addCantGenErr (filter (not . instCanBeGeneralised) irreds)	`thenTc_`

553
    returnTc (qtvs, mkLIE frees, binds, map instToId irreds)
554
555
556
557
558
559

inferLoop doc tau_tvs wanteds
  =   	-- Step 1
    zonkTcTyVarsAndFV tau_tvs		`thenNF_Tc` \ tau_tvs' ->
    mapNF_Tc zonkInst wanteds		`thenNF_Tc` \ wanteds' ->
    tcGetGlobalTyVars			`thenNF_Tc` \ gbl_tvs ->
560
    let
561
562
 	preds = predsOfInsts wanteds'
	qtvs  = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
563
564

	try_me inst
565
566
567
	  | isFreeWhenInferring qtvs inst = Free
	  | isClassDict inst 		  = DontReduceUnlessConstant	-- Dicts
	  | otherwise	    		  = ReduceMe 			-- Lits and Methods
568
    in
569
570
		-- Step 2
    reduceContext doc try_me [] wanteds'    `thenTc` \ (no_improvement, frees, binds, irreds) ->
571

572
573
		-- Step 3
    if no_improvement then
574
	returnTc (varSetElems qtvs, frees, binds, irreds)
575
    else
576
577
578
579
580
581
582
583
584
585
586
587
	-- If improvement did some unification, we go round again.  There
	-- are two subtleties:
	--   a) We start again with irreds, not wanteds
	-- 	Using an instance decl might have introduced a fresh type variable
	--	which might have been unified, so we'd get an infinite loop
	--	if we started again with wanteds!  See example [LOOP]
	--
	--   b) It's also essential to re-process frees, because unification
	--      might mean that a type variable that looked free isn't now.
	--
	-- Hence the (irreds ++ frees)

588
589
590
	-- However, NOTICE that when we are done, we might have some bindings, but
	-- the final qtvs might be empty.  See [NO TYVARS] below.
				
591
592
	inferLoop doc tau_tvs (irreds ++ frees)	`thenTc` \ (qtvs1, frees1, binds1, irreds1) ->
	returnTc (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
593
\end{code}
594

595
596
597
598
599
600
601
602
603
604
605
606
607
Example [LOOP]

	class If b t e r | b t e -> r
	instance If T t e t
	instance If F t e e
	class Lte a b c | a b -> c where lte :: a -> b -> c
	instance Lte Z b T
	instance (Lte a b l,If l b a c) => Max a b c

Wanted:	Max Z (S x) y

Then we'll reduce using the Max instance to:
	(Lte Z (S x) l, If l (S x) Z y)
608
and improve by binding l->T, after which we can do some reduction
609
610
611
on both the Lte and If constraints.  What we *can't* do is start again
with (Max Z (S x) y)!

612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
[NO TYVARS]

	class Y a b | a -> b where
	    y :: a -> X b
	
	instance Y [[a]] a where
	    y ((x:_):_) = X x
	
	k :: X a -> X a -> X a

	g :: Num a => [X a] -> [X a]
	g xs = h xs
	    where
	    h ys = ys ++ map (k (y [[0]])) xs

The excitement comes when simplifying the bindings for h.  Initially
try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
From this we get t1:=:t2, but also various bindings.  We can't forget
the bindings (because of [LOOP]), but in fact t1 is what g is
631
632
633
polymorphic in.  

The net effect of [NO TYVARS] 
634

635
\begin{code}
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
isFreeWhenInferring :: TyVarSet -> Inst	-> Bool
isFreeWhenInferring qtvs inst
  =  isFreeWrtTyVars qtvs inst			-- Constrains no quantified vars
  && all inheritablePred (predsOfInst inst)	-- And no implicit parameter involved
						-- (see "Notes on implicit parameters")

isFreeWhenChecking :: TyVarSet	-- Quantified tyvars
	 	   -> NameSet	-- Quantified implicit parameters
		   -> Inst -> Bool
isFreeWhenChecking qtvs ips inst
  =  isFreeWrtTyVars qtvs inst
  && isFreeWrtIPs    ips inst

isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
isFreeWrtIPs     ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
651
\end{code}
652

653

654
655
656
657
658
%************************************************************************
%*									*
\subsection{tcSimplifyCheck}
%*									*
%************************************************************************
659

660
@tcSimplifyCheck@ is used when we know exactly the set of variables
661
we are going to quantify over.  For example, a class or instance declaration.
662
663

\begin{code}
664
tcSimplifyCheck
665
	 :: SDoc
666
667
	 -> [TcTyVar]		-- Quantify over these
	 -> [Inst]		-- Given
668
	 -> LIE			-- Wanted
669
	 -> TcM (LIE,		-- Free
670
		 TcDictBinds)	-- Bindings
671

672
-- tcSimplifyCheck is used when checking expression type signatures,
673
674
675
676
-- class decls, instance decls etc.
-- Note that we psss isFree (not isFreeAndInheritable) to tcSimplCheck
-- It's important that we can float out non-inheritable predicates
-- Example:		(?x :: Int) is ok!
677
tcSimplifyCheck doc qtvs givens wanted_lie
678
  = tcSimplCheck doc get_qtvs
679
680
681
682
683
684
685
686
687
688
		 givens wanted_lie	`thenTc` \ (qtvs', frees, binds) ->
    returnTc (frees, binds)
  where
    get_qtvs = zonkTcTyVarsAndFV qtvs


-- tcSimplifyInferCheck is used when we know the constraints we are to simplify
-- against, but we don't know the type variables over which we are going to quantify.
-- This happens when we have a type signature for a mutually recursive group
tcSimplifyInferCheck
689
	 :: SDoc
690
691
692
693
694
695
696
697
	 -> TcTyVarSet		-- fv(T)
	 -> [Inst]		-- Given
	 -> LIE			-- Wanted
	 -> TcM ([TcTyVar],	-- Variables over which to quantify
		 LIE,		-- Free
		 TcDictBinds)	-- Bindings

tcSimplifyInferCheck doc tau_tvs givens wanted_lie
698
  = tcSimplCheck doc get_qtvs givens wanted_lie
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
  where
	-- Figure out which type variables to quantify over
	-- You might think it should just be the signature tyvars,
	-- but in bizarre cases you can get extra ones
	-- 	f :: forall a. Num a => a -> a
	--	f x = fst (g (x, head [])) + 1
	--	g a b = (b,a)
	-- Here we infer g :: forall a b. a -> b -> (b,a)
	-- We don't want g to be monomorphic in b just because
	-- f isn't quantified over b.
    all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)

    get_qtvs = zonkTcTyVarsAndFV all_tvs	`thenNF_Tc` \ all_tvs' ->
	       tcGetGlobalTyVars		`thenNF_Tc` \ gbl_tvs ->
	       let
	          qtvs = all_tvs' `minusVarSet` gbl_tvs
			-- We could close gbl_tvs, but its not necessary for
716
			-- soundness, and it'll only affect which tyvars, not which
717
718
719
720
721
722
723
			-- dictionaries, we quantify over
	       in
	       returnNF_Tc qtvs
\end{code}

Here is the workhorse function for all three wrappers.

724
\begin{code}
725
tcSimplCheck doc get_qtvs givens wanted_lie
726
  = check_loop givens (lieToList wanted_lie)	`thenTc` \ (qtvs, frees, binds, irreds) ->
727

728
	-- Complain about any irreducible ones
729
    complainCheck doc givens irreds		`thenNF_Tc_`
730

731
	-- Done
732
    returnTc (qtvs, mkLIE frees, binds)
733

734
  where
735
736
    ip_set = mkNameSet (ipNamesOfInsts givens)

737
738
739
740
    check_loop givens wanteds
      =		-- Step 1
    	mapNF_Tc zonkInst givens	`thenNF_Tc` \ givens' ->
    	mapNF_Tc zonkInst wanteds	`thenNF_Tc` \ wanteds' ->
741
742
    	get_qtvs 			`thenNF_Tc` \ qtvs' ->

743
744
745
746
 		    -- Step 2
    	let
 	    -- When checking against a given signature we always reduce
 	    -- until we find a match against something given, or can't reduce
747
748
 	    try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
 			| otherwise  			       = ReduceMe
749
750
    	in
    	reduceContext doc try_me givens' wanteds'	`thenTc` \ (no_improvement, frees, binds, irreds) ->
751

752
753
754
755
756
757
 		    -- Step 3
    	if no_improvement then
 	    returnTc (varSetElems qtvs', frees, binds, irreds)
    	else
 	    check_loop givens' (irreds ++ frees) 	`thenTc` \ (qtvs', frees1, binds1, irreds1) ->
 	    returnTc (qtvs', frees1, binds `AndMonoBinds` binds1, irreds1)
758
759
760
\end{code}


761
762
763
764
765
766
767
768
%************************************************************************
%*									*
\subsection{tcSimplifyRestricted}
%*									*
%************************************************************************

\begin{code}
tcSimplifyRestricted 	-- Used for restricted binding groups
769
			-- i.e. ones subject to the monomorphism restriction
770
	:: SDoc
771
	-> TcTyVarSet		-- Free in the type of the RHSs
772
773
774
775
776
777
778
	-> LIE			-- Free in the RHSs
	-> TcM ([TcTyVar],	-- Tyvars to quantify (zonked)
		LIE,		-- Free
		TcDictBinds)	-- Bindings

tcSimplifyRestricted doc tau_tvs wanted_lie
  = 	-- First squash out all methods, to find the constrained tyvars
779
   	-- We can't just take the free vars of wanted_lie because that'll
780
781
782
783
784
785
786
	-- have methods that may incidentally mention entirely unconstrained variables
	--  	e.g. a call to 	f :: Eq a => a -> b -> b
	-- Here, b is unconstrained.  A good example would be
	--	foo = f (3::Int)
	-- We want to infer the polymorphic type
	--	foo :: forall b. b -> b
    let
787
788
789
790
791
792
	wanteds = lieToList wanted_lie
	try_me inst = ReduceMe		-- Reduce as far as we can.  Don't stop at
					-- dicts; the idea is to get rid of as many type
					-- variables as possible, and we don't want to stop
					-- at (say) Monad (ST s), because that reduces
					-- immediately, with no constraint on s.
793
    in
794
    simpleReduceLoop doc try_me wanteds		`thenTc` \ (_, _, constrained_dicts) ->
795
796

	-- Next, figure out the tyvars we will quantify over
797
798
    zonkTcTyVarsAndFV (varSetElems tau_tvs)	`thenNF_Tc` \ tau_tvs' ->
    tcGetGlobalTyVars				`thenNF_Tc` \ gbl_tvs ->
799
    let
800
801
	constrained_tvs = tyVarsOfInsts constrained_dicts
	qtvs = (tau_tvs' `minusVarSet` oclose (predsOfInsts constrained_dicts) gbl_tvs)
802
803
804
805
806
807
808
			 `minusVarSet` constrained_tvs
    in

	-- The first step may have squashed more methods than
	-- necessary, so try again, this time knowing the exact
	-- set of type variables to quantify over.
	--
809
	-- We quantify only over constraints that are captured by qtvs;
810
	-- these will just be a subset of non-dicts.  This in contrast
811
	-- to normal inference (using isFreeWhenInferring) in which we quantify over
812
	-- all *non-inheritable* constraints too.  This implements choice
813
	-- (B) under "implicit parameter and monomorphism" above.
814
815
816
817
	--
	-- Remember that we may need to do *some* simplification, to
	-- (for example) squash {Monad (ST s)} into {}.  It's not enough
	-- just to float all constraints
818
819
    mapNF_Tc zonkInst (lieToList wanted_lie)	`thenNF_Tc` \ wanteds' ->
    let
820
821
        try_me inst | isFreeWrtTyVars qtvs inst = Free
	            | otherwise                 = ReduceMe
822
823
824
825
    in
    reduceContext doc try_me [] wanteds'	`thenTc` \ (no_improvement, frees, binds, irreds) ->
    ASSERT( no_improvement )
    ASSERT( null irreds )
826
	-- No need to loop because simpleReduceLoop will have
827
828
829
830
831
	-- already done any improvement necessary

    returnTc (varSetElems qtvs, mkLIE frees, binds)
\end{code}

832
833
834
835
836
837
838

%************************************************************************
%*									*
\subsection{tcSimplifyToDicts}
%*									*
%************************************************************************

839
840
841
842
On the LHS of transformation rules we only simplify methods and constants,
getting dictionaries.  We want to keep all of them unsimplified, to serve
as the available stuff for the RHS of the rule.

843
The same thing is used for specialise pragmas. Consider
844

845
846
847
848
849
850
851
852
853
854
855
856
	f :: Num a => a -> a
	{-# SPECIALISE f :: Int -> Int #-}
	f = ...

The type checker generates a binding like:

	f_spec = (f :: Int -> Int)

and we want to end up with

	f_spec = _inline_me_ (f Int dNumInt)

857
But that means that we must simplify the Method for f to (f Int dNumInt)!
858
859
So tcSimplifyToDicts squeezes out all Methods.

860
861
862
863
864
IMPORTANT NOTE:  we *don't* want to do superclass commoning up.  Consider

	fromIntegral :: (Integral a, Num b) => a -> b
	{-# RULES "foo"  fromIntegral = id :: Int -> Int #-}

865
Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
866
867
868
869
870
871
872
873
874
875
876
877
want to get

	forall dIntegralInt.
	fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int

because the scsel will mess up matching.  Instead we want

	forall dIntegralInt, dNumInt.
	fromIntegral Int Int dIntegralInt dNumInt = id Int

Hence "DontReduce NoSCs"

878
\begin{code}
879
tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds)
880
tcSimplifyToDicts wanted_lie
881
  = simpleReduceLoop doc try_me wanteds		`thenTc` \ (frees, binds, irreds) ->
882
	-- Since try_me doesn't look at types, we don't need to
883
	-- do any zonking, so it's safe to call reduceContext directly
884
    ASSERT( null frees )
885
886
    returnTc (irreds, binds)

887
  where
888
    doc = text "tcSimplifyToDicts"
889
    wanteds = lieToList wanted_lie
890
891

	-- Reduce methods and lits only; stop as soon as we get a dictionary
892
893
    try_me inst	| isDict inst = DontReduce NoSCs
		| otherwise   = ReduceMe
894
895
\end{code}

896

897
898
899
900
901
902
903
904
905
906
%************************************************************************
%*									*
\subsection{Filtering at a dynamic binding}
%*									*
%************************************************************************

When we have
	let ?x = R in B

we must discharge all the ?x constraints from B.  We also do an improvement
907
step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
908
909
910
911
912
913

Actually, the constraints from B might improve the types in ?x. For example

	f :: (?x::Int) => Char -> Char
	let ?x = 3 in f 'c'

914
then the constraint (?x::Int) arising from the call to f will
915
force the binding for ?x to be of type Int.
916
917

\begin{code}
918
tcSimplifyIPs :: [Inst]		-- The implicit parameters bound here
919
920
	      -> LIE
	      -> TcM (LIE, TcDictBinds)
921
922
tcSimplifyIPs given_ips wanted_lie
  = simpl_loop given_ips wanteds	`thenTc` \ (frees, binds) ->
923
    returnTc (mkLIE frees, binds)
924
  where
925
    doc	     = text "tcSimplifyIPs" <+> ppr given_ips
926
    wanteds  = lieToList wanted_lie
927
    ip_set   = mkNameSet (ipNamesOfInsts given_ips)
928

929
	-- Simplify any methods that mention the implicit parameter
930
931
    try_me inst | isFreeWrtIPs ip_set inst = Free
		| otherwise		   = ReduceMe
932
933
934
935

    simpl_loop givens wanteds
      = mapNF_Tc zonkInst givens		`thenNF_Tc` \ givens' ->
        mapNF_Tc zonkInst wanteds		`thenNF_Tc` \ wanteds' ->
936

937
938
939
940
941
942
943
944
        reduceContext doc try_me givens' wanteds'    `thenTc` \ (no_improvement, frees, binds, irreds) ->

        if no_improvement then
	    ASSERT( null irreds )
	    returnTc (frees, binds)
	else
	    simpl_loop givens' (irreds ++ frees)	`thenTc` \ (frees1, binds1) ->
	    returnTc (frees1, binds `AndMonoBinds` binds1)
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
\end{code}


%************************************************************************
%*									*
\subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
%*									*
%************************************************************************

When doing a binding group, we may have @Insts@ of local functions.
For example, we might have...
\begin{verbatim}
let f x = x + 1	    -- orig local function (overloaded)
    f.1 = f Int	    -- two instances of f
    f.2 = f Float
 in
    (f.1 5, f.2 6.7)
\end{verbatim}
The point is: we must drop the bindings for @f.1@ and @f.2@ here,
where @f@ is in scope; those @Insts@ must certainly not be passed
upwards towards the top-level.	If the @Insts@ were binding-ified up
there, they would have unresolvable references to @f@.

We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
For each method @Inst@ in the @init_lie@ that mentions one of the
@Ids@, we create a binding.  We return the remaining @Insts@ (in an
@LIE@), as well as the @HsBinds@ generated.

\begin{code}
bindInstsOfLocalFuns ::	LIE -> [TcId] -> TcM (LIE, TcMonoBinds)

bindInstsOfLocalFuns init_lie local_ids
977
  | null overloaded_ids
978
979
980
981
	-- Common case
  = returnTc (init_lie, EmptyMonoBinds)

  | otherwise
982
  = simpleReduceLoop doc try_me wanteds		`thenTc` \ (frees, binds, irreds) ->
983
    ASSERT( null irreds )
984
    returnTc (mkLIE frees, binds)
985
986
987
988
  where
    doc		     = text "bindInsts" <+> ppr local_ids
    wanteds	     = lieToList init_lie
    overloaded_ids   = filter is_overloaded local_ids
989
    is_overloaded id = isOverloadedTy (idType id)
990
991

    overloaded_set = mkVarSet overloaded_ids	-- There can occasionally be a lot of them
992
						-- so it's worth building a set, so that
993
994
						-- lookup (in isMethodFor) is faster

995
    try_me inst | isMethodFor overloaded_set inst = ReduceMe
996
		| otherwise		          = Free
997
\end{code}
998

999

1000
1001
%************************************************************************
%*									*
1002
\subsection{Data types for the reduction mechanism}
1003
1004
1005
%*									*
%************************************************************************

1006
1007
The main control over context reduction is here

1008
\begin{code}
1009
data WhatToDo
1010
1011
 = ReduceMe		-- Try to reduce this
			-- If there's no instance, behave exactly like
1012
1013
			-- DontReduce: add the inst to
			-- the irreductible ones, but don't
1014
1015
			-- produce an error message of any kind.
			-- It might be quite legitimate such as (Eq a)!
1016

1017
 | DontReduce WantSCs		-- Return as irreducible
1018
1019
1020

 | DontReduceUnlessConstant	-- Return as irreducible unless it can
				-- be reduced to a constant in one step
1021

1022
 | Free			  -- Return as free
1023

1024
1025
1026
reduceMe :: Inst -> WhatToDo
reduceMe inst = ReduceMe

1027
1028
data WantSCs = NoSCs | AddSCs	-- Tells whether we should add the superclasses
				-- of a predicate when adding it to the avails
1029
\end{code}
1030
1031
1032
1033



\begin{code}
1034
type Avails = FiniteMap Inst Avail
1035

1036
data Avail
1037
1038
  = IsFree		-- Used for free Insts
  | Irred		-- Used for irreducible dictionaries,
1039
1040
			-- which are going to be lambda bound

1041
  | Given TcId 		-- Used for dictionaries for which we have a binding
1042
			-- e.g. those "given" in a signature
1043
	  Bool		-- True <=> actually consumed (splittable IPs only)
1044
1045

  | NoRhs 		-- Used for Insts like (CCallable f)
1046
1047
			-- where no witness is required.

1048
  | Rhs 		-- Used when there is a RHS
1049
1050
	TcExpr	 	-- The RHS
	[Inst]		-- Insts free in the RHS; we need these too
1051

1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
  | Linear 		-- Splittable Insts only.
	Int		-- The Int is always 2 or more; indicates how
			-- many copies are required
	Inst 		-- The splitter
	Avail		-- Where the "master copy" is

  | LinRhss		-- Splittable Insts only; this is used only internally
			-- 	by extractResults, where a Linear 
			--	is turned into an LinRhss
	[TcExpr]	-- A supply of suitable RHSs

1063
1064
pprAvails avails = vcat [ppr inst <+> equals <+> pprAvail avail
			| (inst,avail) <- fmToList avails ]
1065
1066
1067
1068

instance Outputable Avail where
    ppr = pprAvail

1069
1070
1071
1072
1073
1074
1075
1076
pprAvail NoRhs	       	= text "<no rhs>"
pprAvail IsFree	       	= text "Free"
pprAvail Irred	       	= text "Irred"
pprAvail (Given x b)   	= text "Given" <+> ppr x <+> 
		 	  if b then text "(used)" else empty
pprAvail (Rhs rhs bs)   = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1077
1078
1079
1080
1081
1082
1083
1084
1085
\end{code}

Extracting the bindings from a bunch of Avails.
The bindings do *not* come back sorted in dependency order.
We assume that they'll be wrapped in a big Rec, so that the
dependency analyser can sort them out later

The loop startes
\begin{code}
1086
extractResults :: Avails
1087
	       -> [Inst]		-- Wanted
1088
1089
1090
	       -> NF_TcM (TcDictBinds, 	-- Bindings
			  [Inst],	-- Irreducible ones
			  [Inst])	-- Free ones
1091

1092
1093
extractResults avails wanteds
  = go avails EmptyMonoBinds [] [] wanteds
1094
  where
1095
1096
    go avails binds irreds frees [] 
      = returnNF_Tc (binds, irreds, frees)
1097

1098
    go avails binds irreds frees (w:ws)
1099
      = case lookupFM avails w of
1100
1101
	  Nothing    -> pprTrace "Urk: extractResults" (ppr w) $
			go avails binds irreds frees ws
1102

1103
1104
1105
	  Just NoRhs  -> go avails		 binds irreds     frees     ws
	  Just IsFree -> go (add_free avails w)  binds irreds     (w:frees) ws
	  Just Irred  -> go (add_given avails w) binds (w:irreds) frees     ws
1106

1107
	  Just (Given id _) -> go avails new_binds irreds frees ws
1108
			    where
1109
1110
1111
1112
			       new_binds | id == instToId w = binds
					 | otherwise        = addBind binds w (HsVar id)
		-- The sought Id can be one of the givens, via a superclass chain
		-- and then we definitely don't want to generate an x=x binding!
1113

1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
	  Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
			     where
				new_binds = addBind binds w rhs

	  Just (LinRhss (rhs:rhss))	-- Consume one of the Rhss
		-> go new_avails new_binds irreds frees ws
		where		
		   new_binds  = addBind binds w rhs
		   new_avails = addToFM avails w (LinRhss rhss)

	  Just (Linear n split_inst avail)
	    -> split n (instToId split_inst) avail w	`thenNF_Tc` \ (binds', (rhs:rhss), irreds') ->
	       go (addToFM avails w (LinRhss rhss))
		  (binds `AndMonoBinds` addBind binds' w rhs)
		  (irreds' ++ irreds) frees (split_inst:ws)


    add_given avails w 
	| instBindingRequired w = addToFM avails w (Given (instToId w) True)
	| otherwise		= addToFM avails w NoRhs
	-- NB: make sure that CCallable/CReturnable use NoRhs rather
	--	than Given, else we end up with bogus bindings.

    add_free avails w | isMethod w = avails
		      | otherwise  = add_given avails w
	-- NB: Hack alert!  
	-- Do *not* replace Free by Given if it's a method.
	-- The following situation shows why this is bad:
	--	truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
	-- From an application (truncate f i) we get
	--	t1 = truncate at f
	--	t2 = t1 at i
	-- If we have also have a second occurrence of truncate, we get
	--	t3 = truncate at f
	--	t4 = t3 at i
	-- When simplifying with i,f free, we might still notice that
	--   t1=t3; but alas, the binding for t2 (which mentions t1)
	--   will continue to float out!
	-- (split n i a) returns: n rhss
	--			  auxiliary bindings
	--			  1 or 0 insts to add to irreds


split :: Int -> TcId -> Avail -> Inst 
      -> NF_TcM (TcDictBinds, [TcExpr], [Inst])
-- (split n split_id avail wanted) returns
--	* a list of 'n' expressions, all of which witness 'avail'
--	* a bunch of auxiliary bindings to support these expressions
--	* one or zero insts needed to witness the whole lot
--	  (maybe be zero if the initial Inst is a Given)
split n split_id avail wanted
  = go n
  where
    ty  = linearInstType wanted
    pair_ty = mkTyConApp pairTyCon [ty,ty]
    id  = instToId wanted
    occ = getOccName id
    loc = getSrcLoc id

    go 1 = case avail of
	     Given id _ -> returnNF_Tc (EmptyMonoBinds, [HsVar id], [])
	     Irred      -> cloneDict wanted		`thenNF_Tc` \ w' ->
			   returnNF_Tc (EmptyMonoBinds, [HsVar (instToId w')], [w'])

    go n = go ((n+1) `div` 2)		`thenNF_Tc` \ (binds1, rhss, irred) ->
	   expand n rhss		`thenNF_Tc` \ (binds2, rhss') ->
	   returnNF_Tc (binds1 `AndMonoBinds` binds2, rhss', irred)

	-- (expand n rhss) 
	-- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
	--  e.g.  expand 3 [rhs1, rhs2]
	--	  = ( { x = split rhs1 },
	--	      [fst x, snd x, rhs2] )
    expand n rhss
	| n `rem` 2 == 0 = go rhss 	-- n is even
	| otherwise  	 = go (tail rhss)	`thenNF_Tc` \ (binds', rhss') ->
			   returnNF_Tc (binds', head rhss : rhss')
	where
	  go rhss = mapAndUnzipNF_Tc do_one rhss	`thenNF_Tc` \ (binds', rhss') ->
		    returnNF_Tc (andMonoBindList binds', concat rhss')

	  do_one rhs = tcGetUnique 			`thenNF_Tc` \ uniq -> 
		       tcLookupGlobalId fstIdName	`thenNF_Tc` \ fst_id -> 
		       tcLookupGlobalId sndIdName	`thenNF_Tc` \ snd_id -> 
		       let 
			  x = mkUserLocal occ uniq pair_ty loc
		       in
		       returnNF_Tc (VarMonoBind x (mk_app split_id rhs),
				    [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x])

mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var

mk_app id rhs = HsApp (HsVar id) rhs

addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs
1209
1210
1211
1212
1213
\end{code}


%************************************************************************
%*									*
1214
\subsection[reduce]{@reduce@}
1215
%*									*
1216
1217
%************************************************************************

1218
1219
1220
1221
1222
When the "what to do" predicate doesn't depend on the quantified type variables,
matters are easier.  We don't need to do any zonking, unless the improvement step
does something, in which case we zonk before iterating.

The "given" set is always empty.
1223

1224
\begin{code}
1225
1226
1227
simpleReduceLoop :: SDoc
	 	 -> (Inst -> WhatToDo)		-- What to do, *not* based on the quantified type variables
		 -> [Inst]			-- Wanted
1228
		 -> TcM ([Inst],		-- Free
1229
1230
1231
1232
1233
1234
1235
1236
1237
			 TcDictBinds,
			 [Inst])		-- Irreducible

simpleReduceLoop doc try_me wanteds
  = mapNF_Tc zonkInst wanteds			`thenNF_Tc` \ wanteds' ->
    reduceContext doc try_me [] wanteds'	`thenTc` \ (no_improvement, frees, binds, irreds) ->
    if no_improvement then
	returnTc (frees, binds, irreds)
    else
1238
1239
	simpleReduceLoop doc try_me (irreds ++ frees)	`thenTc` \ (frees1, binds1, irreds1) ->
	returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
1240
\end{code}
1241

1242
1243
1244
1245
1246
1247
1248
1249


\begin{code}
reduceContext :: SDoc
	      -> (Inst -> WhatToDo)
	      -> [Inst]			-- Given
	      -> [Inst]			-- Wanted
	      -> NF_TcM (Bool, 		-- True <=> improve step did no unification
1250
			 [Inst],	-- Free
1251
1252
1253
1254
1255
			 TcDictBinds,	-- Dictionary bindings
			 [Inst])	-- Irreducible

reduceContext doc try_me givens wanteds
  =
1256
    traceTc (text "reduceContext" <+> (vcat [
1257
	     text "----------------------",
1258
	     doc,
1259
1260
1261
	     text "given" <+> ppr givens,
	     text "wanted" <+> ppr wanteds,
	     text "----------------------"
1262
1263
	     ]))					`thenNF_Tc_`

1264
        -- Build the Avail mapping from "givens"
1265
    foldlNF_Tc addGiven emptyFM givens			`thenNF_Tc` \ init_state ->
1266
1267

        -- Do the real work
1268
    reduceList (0,[]) try_me wanteds init_state		`thenNF_Tc` \ avails ->
1269
1270
1271
1272

	-- Do improvement, using everything in avails
	-- In particular, avails includes all superclasses of everything
    tcImprove avails					`thenTc` \ no_improvement ->
1273

1274
1275
    extractResults avails wanteds			`thenNF_Tc` \ (binds, irreds, frees) ->

1276
    traceTc (text "reduceContext end" <+> (vcat [
1277
	     text "----------------------",
1278
	     doc,
1279
1280
	     text "given" <+> ppr givens,
	     text "wanted" <+> ppr wanteds,
1281
	     text "----",
1282
	     text "avails" <+> pprAvails avails,
1283
	     text "frees" <+> ppr frees,
1284
	     text "no_improvement =" <+> ppr no_improvement,
1285
	     text "----------------------"
1286
	     ])) 					`thenNF_Tc_`
1287
1288

    returnTc (no_improvement, frees, binds, irreds)
1289
1290
1291
1292

tcImprove avails
 =  tcGetInstEnv 				`thenTc` \ inst_env ->
    let
1293
1294
1295
1296
1297
1298
	preds = [ (pred, pp_loc)
		| inst <- keysFM avails,
		  let pp_loc = pprInstLoc (instLoc inst),
		  pred <- predsOfInst inst,
		  predHasFDs pred
		]
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
		-- Avails has all the superclasses etc (good)
		-- It also has all the intermediates of the deduction (good)
		-- It does not have duplicates (good)
		-- NB that (?x::t1) and (?x::t2) will be held separately in avails
		--    so that improve will see them separate
	eqns  = improve (classInstEnv inst_env) preds
     in
     if null eqns then
	returnTc True
     else
1309
	traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns))	`thenNF_Tc_`
1310
        mapTc_ unify eqns	`thenTc_`
1311
	returnTc False
1312
  where
1313
1314
1315
1316
    unify ((qtvs, t1, t2), doc)
	 = tcAddErrCtxt doc			$
	   tcInstTyVars (varSetElems qtvs)	`thenNF_Tc` \ (_, _, tenv) ->
	   unifyTauTy (substTy tenv t1) (substTy tenv t2)
1317
\end{code}
1318

1319
1320
1321
The main context-reduction function is @reduce@.  Here's its game plan.

\begin{code}
1322
1323
1324
1325
reduceList :: (Int,[Inst])		-- Stack (for err msgs)
					-- along with its depth
       	   -> (Inst -> WhatToDo)
       	   -> [Inst]
1326
1327
       	   -> Avails
       	   -> TcM Avails
1328
1329
1330
1331
1332
1333
1334
1335
1336
\end{code}

@reduce@ is passed
     try_me:	given an inst, this function returns
		  Reduce       reduce this
		  DontReduce   return this in "irreds"
		  Free	       return this in "frees"

     wanteds:	The list of insts to reduce
1337
     state:	An accumulating parameter of type Avails
1338
		that contains the state of the algorithm
1339

1340
  It returns a Avails.
1341

1342
The (n,stack) pair is just used for error reporting.
1343
1344
1345
n is always the depth of the stack.
The stack is the stack of Insts being reduced: to produce X
I had to produce Y, to produce Y I had to produce Z, and so on.
1346
1347

\begin{code}
1348
1349
1350
reduceList (n,stack) try_me wanteds state
  | n > opt_MaxContextReductionDepth
  = failWithTc (reduceDepthErr n stack)
1351

1352
1353
1354
  | otherwise
  =
#ifdef DEBUG
1355
   (if n > 8 then
1356
1357