TcSimplify.lhs 80.2 KB
Newer Older
1
%
2
% (c) The University of Glasgow 2006
3
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4
%
5

6
TcSimplify
7
8

\begin{code}
9
module TcSimplify (
10
	tcSimplifyInfer, tcSimplifyInferCheck,
11
	tcSimplifyCheck, tcSimplifyRestricted,
12
	tcSimplifyRuleLhs, tcSimplifyIPs, 
13
	tcSimplifySuperClasses,
14
	tcSimplifyTop, tcSimplifyInteractive,
15
	tcSimplifyBracket,
16

17
	tcSimplifyDeriv, tcSimplifyDefault,
18
	bindInstsOfLocalFuns
19
20
    ) where

21
#include "HsVersions.h"
22

23
import {-# SOURCE #-} TcUnify( unifyType )
24
import HsSyn
25

26
import TcRnMonad
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
import Inst
import TcEnv
import InstEnv
import TcMType
import TcType
import TcIface
import Id
import Var
import TyCon
import Name
import NameSet
import Class
import FunDeps
import PrelInfo
import PrelNames
import Type
import TysWiredIn
import ErrUtils
import BasicTypes
46
import VarSet
47
import VarEnv
48
import FiniteMap
49
import Bag
50
import Outputable
51
52
53
54
55
56
import ListSetOps
import Util
import SrcLoc
import DynFlags

import Data.List
57
58
59
60
61
\end{code}


%************************************************************************
%*									*
62
\subsection{NOTES}
63
64
65
%*									*
%************************************************************************

66
67
68
69
	--------------------------------------
	Notes on functional dependencies (a bug)
	--------------------------------------

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
Consider this:

	class C a b | a -> b
	class D a b | a -> b

	instance D a b => C a b	-- Undecidable 
				-- (Not sure if it's crucial to this eg)
	f :: C a b => a -> Bool
	f _ = True
	
	g :: C a b => a -> Bool
	g = f

Here f typechecks, but g does not!!  Reason: before doing improvement,
we reduce the (C a b1) constraint from the call of f to (D a b1).

Here is a more complicated example:

88
89
90
91
92
93
94
95
96
97
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
136
137
| > class Foo a b | a->b
| >
| > class Bar a b | a->b
| >
| > data Obj = Obj
| >
| > instance Bar Obj Obj
| >
| > instance (Bar a b) => Foo a b
| >
| > foo:: (Foo a b) => a -> String
| > foo _ = "works"
| >
| > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
| > runFoo f = f Obj
| 
| *Test> runFoo foo
| 
| <interactive>:1:
|     Could not deduce (Bar a b) from the context (Foo a b)
|       arising from use of `foo' at <interactive>:1
|     Probable fix:
|         Add (Bar a b) to the expected type of an expression
|     In the first argument of `runFoo', namely `foo'
|     In the definition of `it': it = runFoo foo
| 
| Why all of the sudden does GHC need the constraint Bar a b? The
| function foo didn't ask for that... 

The trouble is that to type (runFoo foo), GHC has to solve the problem:

	Given constraint	Foo a b
	Solve constraint	Foo a b'

Notice that b and b' aren't the same.  To solve this, just do
improvement and then they are the same.  But GHC currently does
	simplify constraints
	apply improvement
	and loop

That is usually fine, but it isn't here, because it sees that Foo a b is
not the same as Foo a b', and so instead applies the instance decl for
instance Bar a b => Foo a b.  And that's where the Bar constraint comes
from.

The Right Thing is to improve whenever the constraint set changes at
all.  Not hard in principle, but it'll take a bit of fiddling to do.  



138
	--------------------------------------
139
		Notes on quantification
140
	--------------------------------------
141
142
143
144
145
146

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

	G	the environment
	T	the type of the RHS
147
	C	the constraints from that RHS
148
149
150
151
152
153
154
155
156
157
158

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

159
and float the constraints Ct further outwards.
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197

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
198
		using all conceivable links from C.
199
200
201
202
203
204
205

		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.

206
Notice that
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
   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 ...
227

228
229
230
231
232
233
234
	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

235
  And then if the fn was called at several different c's, each of
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
  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.



simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
259
260
261
-------------------------------------
	Note [Ambiguity]
-------------------------------------
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300

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.

301
So here's the plan.  We WARN about probable ambiguity if
302
303
304
305
306

	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
307
in the environment, or by the variables in the type.
308
309
310

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

311
	class J a b c | a b -> c
312
313
314
	fv(G) = {a}

Is this ambiguous?
315
	forall b c. (J a b c) => b -> b
316
317

Only if we union {a} from G with {b} from T before using oclose,
318
do we see that c is fixed.
319

320
It's a bit vague exactly which C we should use for this oclose call.  If we
321
322
323
324
325
326
327
328
329
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

330
then c is a "bubble"; there's no way it can ever improve, and it's
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
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.
349
The definitely-ambiguous can then float out, and get smashed at top level
350
351
352
(which squashes out the constants, like Eq (T a) above)


353
	--------------------------------------
354
		Notes on principal types
355
	--------------------------------------
356
357
358

    class C a where
      op :: a -> a
359

360
361
362
363
364
365
366
    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


367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
	--------------------------------------
	The need for forall's in constraints
	--------------------------------------

[Exchange on Haskell Cafe 5/6 Dec 2000]

  class C t where op :: t -> Bool
  instance C [t] where op x = True

  p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
  q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)

The definitions of p and q differ only in the order of the components in
the pair on their right-hand sides.  And yet:

  ghc and "Typing Haskell in Haskell" reject p, but accept q;
  Hugs rejects q, but accepts p;
  hbc rejects both p and q;
  nhc98 ... (Malcolm, can you fill in the blank for us!).

The type signature for f forces context reduction to take place, and
the results of this depend on whether or not the type of y is known,
which in turn depends on which component of the pair the type checker
analyzes first.  

Solution: if y::m a, float out the constraints
	Monad m, forall c. C (m c)
When m is later unified with [], we can solve both constraints.


397
	--------------------------------------
398
		Notes on implicit parameters
399
	--------------------------------------
400

401
402
403
404
405
Question 1: can we "inherit" implicit parameters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this:

	f x = (x::Int) + ?y
406

407
408
409
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:
410

411
412
413
	f :: Int -> Int

(so we get ?y from the context of f's definition), or
414
415
416

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

417
418
419
420
421
422
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.

423
424
BOTTOM LINE: when *inferring types* you *must* quantify 
over implicit parameters. See the predicate isFreeWhenInferring.
425

426
427
428
429
430

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:
431
432
433
434
435
436
437
438
439
440
441

	(?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).
442

443
444
445
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:
446

447
	f :: Int -> Int
448
	f x = (x::Int) + ?y
449

450
At first sight this seems reasonable, but it has the nasty property
451
that adding a type signature changes the dynamic semantics.
452
Consider this:
453

454
	(let f x = (x::Int) + ?y
455
456
457
458
 	 in (f 3, f 3 with ?y=5))  with ?y = 6

		returns (3+6, 3+5)
vs
459
	(let f :: Int -> Int
460
	     f x = x + ?y
461
462
463
464
	 in (f 3, f 3 with ?y=5))  with ?y = 6

		returns (3+6, 3+6)

465
466
Indeed, simply inlining f (at the Haskell source level) would change the
dynamic semantics.
467

468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
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.
495

496

497
498
499
Question 3: monomorphism
~~~~~~~~~~~~~~~~~~~~~~~~
There's a nasty corner case when the monomorphism restriction bites:
500

501
502
	z = (x::Int) + ?y

503
504
The argument above suggests that we *must* generalise
over the ?y parameter, to get
505
506
	z :: (?y::Int) => Int,
but the monomorphism restriction says that we *must not*, giving
507
	z :: Int.
508
509
510
511
512
513
514
515
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.


516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
Question 4: top level
~~~~~~~~~~~~~~~~~~~~~
At the top level, monomorhism makes no sense at all.

    module Main where
	main = let ?x = 5 in print foo

	foo = woggle 3

	woggle :: (?x :: Int) => Int -> Int
	woggle y = ?x + y

We definitely don't want (foo :: Int) with a top-level implicit parameter
(?x::Int) becuase there is no way to bind it.  

531
532
533
534
535
536
537
538

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

    Consequences:
539
	* Inlining remains valid
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
	* 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.
577

578
It's really not clear what is the Right Thing To Do.  If you see
579

580
	z = (x::Int) + ?y
581

582
583
584
585
586
587
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.
588

589
Choice (C) really says "the monomorphism restriction doesn't apply
590
to implicit parameters".  Which is fine, but remember that every
591
592
593
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'
594
clauses that bind ?y, so a decent compiler should common up all
595
596
597
those function calls.  So I think I strongly favour (C).  Indeed,
one could make a similar argument for abolishing the monomorphism
restriction altogether.
598

599
BOTTOM LINE: we choose (B) at present.  See tcSimplifyRestricted
600

601

602

603
604
605
606
607
608
609
610
611
%************************************************************************
%*									*
\subsection{tcSimplifyInfer}
%*									*
%************************************************************************

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

    1. Compute Q = grow( fvs(T), C )
612
613

    2. Partition C based on Q into Ct and Cq.  Notice that ambiguous
614
       predicates will end up in Ct; we deal with them at the top level
615

616
    3. Try improvement, using functional dependencies
617

618
619
620
621
    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
622
Eq (a,b), we don't simplify to (Eq a, Eq b).  So Q won't be different
623
624
625
626
627
628
629
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 ...
630

631
632
633
634
635
636
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
637
again.
638

639
640

\begin{code}
641
tcSimplifyInfer
642
643
	:: SDoc
	-> TcTyVarSet		-- fv(T); type vars
644
	-> [Inst]		-- Wanted
645
646
647
	-> TcM ([TcTyVar],	-- Tyvars to quantify (zonked)
		TcDictBinds,	-- Bindings
		[TcId])		-- Dict Ids that must be bound here (zonked)
648
	-- Any free (escaping) Insts are tossed into the environment
649
\end{code}
650

651
652
653

\begin{code}
tcSimplifyInfer doc tau_tvs wanted_lie
654
  = inferLoop doc (varSetElems tau_tvs)
655
	      wanted_lie		`thenM` \ (qtvs, frees, binds, irreds) ->
656

657
658
    extendLIEs frees							`thenM_`
    returnM (qtvs, binds, map instToId irreds)
659
660
661

inferLoop doc tau_tvs wanteds
  =   	-- Step 1
662
663
664
    zonkTcTyVarsAndFV tau_tvs		`thenM` \ tau_tvs' ->
    mappM zonkInst wanteds		`thenM` \ wanteds' ->
    tcGetGlobalTyVars			`thenM` \ gbl_tvs ->
665
    let
666
 	preds = fdPredsOfInsts wanteds'
667
	qtvs  = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
668
669

	try_me inst
670
671
	  | isFreeWhenInferring qtvs inst = Free
	  | isClassDict inst 		  = DontReduceUnlessConstant	-- Dicts
672
	  | otherwise	    		  = ReduceMe NoSCs		-- Lits and Methods
673
    in
674
675
    traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds, 
				      ppr (grow preds tau_tvs'), ppr qtvs])	`thenM_`
676
		-- Step 2
677
    reduceContext doc try_me [] wanteds'    `thenM` \ (no_improvement, frees, binds, irreds) ->
678

679
680
		-- Step 3
    if no_improvement then
681
	returnM (varSetElems qtvs, frees, binds, irreds)
682
    else
683
684
685
686
687
688
689
690
691
692
693
694
	-- 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)

695
696
697
	-- However, NOTICE that when we are done, we might have some bindings, but
	-- the final qtvs might be empty.  See [NO TYVARS] below.
				
698
	inferLoop doc tau_tvs (irreds ++ frees)	`thenM` \ (qtvs1, frees1, binds1, irreds1) ->
699
	returnM (qtvs1, frees1, binds `unionBags` binds1, irreds1)
700
\end{code}
701

702
703
704
705
706
707
708
709
710
711
712
713
714
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)
715
and improve by binding l->T, after which we can do some reduction
716
717
718
on both the Lte and If constraints.  What we *can't* do is start again
with (Max Z (S x) y)!

719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
[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
738
739
740
polymorphic in.  

The net effect of [NO TYVARS] 
741

742
\begin{code}
743
744
isFreeWhenInferring :: TyVarSet -> Inst	-> Bool
isFreeWhenInferring qtvs inst
745
746
747
  =  isFreeWrtTyVars qtvs inst		-- Constrains no quantified vars
  && isInheritableInst inst		-- And no implicit parameter involved
					-- (see "Notes on implicit parameters")
748
749
750
751
752
753
754
755

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

756
isFreeWrtTyVars qtvs inst = tyVarsOfInst inst `disjointVarSet` qtvs
757
isFreeWrtIPs     ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
758
\end{code}
759

760

761
762
763
764
765
%************************************************************************
%*									*
\subsection{tcSimplifyCheck}
%*									*
%************************************************************************
766

767
@tcSimplifyCheck@ is used when we know exactly the set of variables
768
we are going to quantify over.  For example, a class or instance declaration.
769
770

\begin{code}
771
tcSimplifyCheck
772
	 :: SDoc
773
774
	 -> [TcTyVar]		-- Quantify over these
	 -> [Inst]		-- Given
775
776
	 -> [Inst]		-- Wanted
	 -> TcM TcDictBinds	-- Bindings
777

778
-- tcSimplifyCheck is used when checking expression type signatures,
779
-- class decls, instance decls etc.
780
781
782
783
--
-- NB: tcSimplifyCheck does not consult the
--	global type variables in the environment; so you don't
--	need to worry about setting them before calling tcSimplifyCheck
784
tcSimplifyCheck doc qtvs givens wanted_lie
785
786
787
788
  = ASSERT( all isSkolemTyVar qtvs )
    do	{ (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie
	; extendLIEs frees
	; return binds }
789
  where
790
791
--  get_qtvs = zonkTcTyVarsAndFV qtvs
    get_qtvs = return (mkVarSet qtvs)	-- All skolems
792
793
794
795
796
797


-- 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
798
	 :: SDoc
799
800
	 -> TcTyVarSet		-- fv(T)
	 -> [Inst]		-- Given
801
	 -> [Inst]		-- Wanted
802
803
804
805
	 -> TcM ([TcTyVar],	-- Variables over which to quantify
		 TcDictBinds)	-- Bindings

tcSimplifyInferCheck doc tau_tvs givens wanted_lie
806
807
808
  = do	{ (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie
	; extendLIEs frees
	; return (qtvs', binds) }
809
810
811
812
813
814
815
816
817
818
819
820
  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)

821
822
    get_qtvs = zonkTcTyVarsAndFV all_tvs	`thenM` \ all_tvs' ->
	       tcGetGlobalTyVars		`thenM` \ gbl_tvs ->
823
824
825
	       let
	          qtvs = all_tvs' `minusVarSet` gbl_tvs
			-- We could close gbl_tvs, but its not necessary for
826
			-- soundness, and it'll only affect which tyvars, not which
827
828
			-- dictionaries, we quantify over
	       in
829
	       returnM qtvs
830
831
832
833
\end{code}

Here is the workhorse function for all three wrappers.

834
\begin{code}
835
836
tcSimplCheck doc get_qtvs want_scs givens wanted_lie
  = do	{ (qtvs, frees, binds, irreds) <- check_loop givens wanted_lie
837

838
839
840
841
842
		-- Complain about any irreducible ones
	; if not (null irreds)
	  then do { givens' <- mappM zonkInst given_dicts_and_ips
		  ; groupErrs (addNoInstanceErrs (Just doc) givens') irreds }
	  else return ()
843

844
	; returnM (qtvs, frees, binds) }
845
  where
846
847
848
849
    given_dicts_and_ips = filter (not . isMethod) givens
	-- For error reporting, filter out methods, which are 
	-- only added to the given set as an optimisation

850
851
    ip_set = mkNameSet (ipNamesOfInsts givens)

852
853
    check_loop givens wanteds
      =		-- Step 1
854
855
    	mappM zonkInst givens	`thenM` \ givens' ->
    	mappM zonkInst wanteds	`thenM` \ wanteds' ->
856
    	get_qtvs 		`thenM` \ qtvs' ->
857

858
859
860
861
 		    -- Step 2
    	let
 	    -- When checking against a given signature we always reduce
 	    -- until we find a match against something given, or can't reduce
862
 	    try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
863
 			| otherwise  			       = ReduceMe want_scs
864
    	in
865
    	reduceContext doc try_me givens' wanteds'	`thenM` \ (no_improvement, frees, binds, irreds) ->
866

867
868
 		    -- Step 3
    	if no_improvement then
869
 	    returnM (varSetElems qtvs', frees, binds, irreds)
870
    	else
871
 	    check_loop givens' (irreds ++ frees) 	`thenM` \ (qtvs', frees1, binds1, irreds1) ->
872
 	    returnM (qtvs', frees1, binds `unionBags` binds1, irreds1)
873
874
875
\end{code}


876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
%************************************************************************
%*									*
		tcSimplifySuperClasses
%*									*
%************************************************************************

Note [SUPERCLASS-LOOP 1]
~~~~~~~~~~~~~~~~~~~~~~~~
We have to be very, very careful when generating superclasses, lest we
accidentally build a loop. Here's an example:

  class S a

  class S a => C a where { opc :: a -> a }
  class S b => D b where { opd :: b -> b }
  
  instance C Int where
     opc = opd
  
  instance D Int where
     opd = opc

From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
Simplifying, we may well get:
	$dfCInt = :C ds1 (opd dd)
	dd  = $dfDInt
	ds1 = $p1 dd
Notice that we spot that we can extract ds1 from dd.  

Alas!  Alack! We can do the same for (instance D Int):

	$dfDInt = :D ds2 (opc dc)
	dc  = $dfCInt
	ds2 = $p1 dc

And now we've defined the superclass in terms of itself.

Solution: never generate a superclass selectors at all when
satisfying the superclass context of an instance declaration.

Two more nasty cases are in
	tcrun021
	tcrun033

\begin{code}
tcSimplifySuperClasses qtvs givens sc_wanteds
  = ASSERT( all isSkolemTyVar qtvs )
    do	{ (_, frees, binds1) <- tcSimplCheck doc get_qtvs NoSCs givens sc_wanteds
924
925
	; ext_default        <- doptM Opt_ExtendedDefaultRules
	; binds2	     <- tc_simplify_top doc ext_default NoSCs frees
926
927
928
929
930
931
932
	; return (binds1 `unionBags` binds2) }
  where
    get_qtvs = return (mkVarSet qtvs)
    doc = ptext SLIT("instance declaration superclass context")
\end{code}


933
934
935
936
937
938
%************************************************************************
%*									*
\subsection{tcSimplifyRestricted}
%*									*
%************************************************************************

939
940
941
942
943
944
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
977
978
979
980
981
tcSimplifyRestricted infers which type variables to quantify for a 
group of restricted bindings.  This isn't trivial.

Eg1:	id = \x -> x
	We want to quantify over a to get id :: forall a. a->a
	
Eg2:	eq = (==)
	We do not want to quantify over a, because there's an Eq a 
	constraint, so we get eq :: a->a->Bool	(notice no forall)

So, assume:
	RHS has type 'tau', whose free tyvars are tau_tvs
	RHS has constraints 'wanteds'

Plan A (simple)
  Quantify over (tau_tvs \ ftvs(wanteds))
  This is bad. The constraints may contain (Monad (ST s))
  where we have 	instance Monad (ST s) where...
  so there's no need to be monomorphic in s!

  Also the constraint might be a method constraint,
  whose type mentions a perfectly innocent tyvar:
	  op :: Num a => a -> b -> a
  Here, b is unconstrained.  A good example would be
	foo = op (3::Int)
  We want to infer the polymorphic type
	foo :: forall b. b -> b


Plan B (cunning, used for a long time up to and including GHC 6.2)
  Step 1: Simplify the constraints as much as possible (to deal 
  with Plan A's problem).  Then set
	qtvs = tau_tvs \ ftvs( simplify( wanteds ) )

  Step 2: Now simplify again, treating the constraint as 'free' if 
  it does not mention qtvs, and trying to reduce it otherwise.
  The reasons for this is to maximise sharing.

  This fails for a very subtle reason.  Suppose that in the Step 2
  a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
  In the Step 1 this constraint might have been simplified, perhaps to
  (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
  This won't happen in Step 2... but that in turn might prevent some other
982
983
  constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..}) 
  and that in turn breaks the invariant that no constraints are quantified over.
984
985
986
987
988
989
990
991
992
993
994
995

  Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
  the problem.


Plan C (brutal)
  Step 1: Simplify the constraints as much as possible (to deal 
  with Plan A's problem).  Then set
	qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
  Return the bindings from Step 1.
  

996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
A note about Plan C (arising from "bug" reported by George Russel March 2004)
Consider this:

      instance (HasBinary ty IO) => HasCodedValue ty

      foo :: HasCodedValue a => String -> IO a

      doDecodeIO :: HasCodedValue a => () -> () -> IO a
      doDecodeIO codedValue view  
        = let { act = foo "foo" } in  act

You might think this should work becuase the call to foo gives rise to a constraint
(HasCodedValue t), which can be satisfied by the type sig for doDecodeIO.  But the
restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
constraint using the (rather bogus) instance declaration, and now we are stuffed.
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031

I claim this is not really a bug -- but it bit Sergey as well as George.  So here's
plan D


Plan D (a variant of plan B)
  Step 1: Simplify the constraints as much as possible (to deal 
  with Plan A's problem), BUT DO NO IMPROVEMENT.  Then set
	qtvs = tau_tvs \ ftvs( simplify( wanteds ) )

  Step 2: Now simplify again, treating the constraint as 'free' if 
  it does not mention qtvs, and trying to reduce it otherwise.

  The point here is that it's generally OK to have too few qtvs; that is,
  to make the thing more monomorphic than it could be.  We don't want to
  do that in the common cases, but in wierd cases it's ok: the programmer
  can always add a signature.  

  Too few qtvs => too many wanteds, which is what happens if you do less
  improvement.

1032

1033
1034
\begin{code}
tcSimplifyRestricted 	-- Used for restricted binding groups
1035
			-- i.e. ones subject to the monomorphism restriction
1036
	:: SDoc
1037
1038
	-> TopLevelFlag
	-> [Name]		-- Things bound in this group
1039
	-> TcTyVarSet		-- Free in the type of the RHSs
1040
	-> [Inst]		-- Free in the RHSs
1041
1042
	-> TcM ([TcTyVar],	-- Tyvars to quantify (zonked)
		TcDictBinds)	-- Bindings
1043
1044
1045
	-- tcSimpifyRestricted returns no constraints to
	-- quantify over; by definition there are none.
	-- They are all thrown back in the LIE
1046

1047
tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1048
1049
1050
1051
	-- Zonk everything in sight
  = mappM zonkInst wanteds			`thenM` \ wanteds' ->

   	-- 'reduceMe': Reduce as far as we can.  Don't stop at
1052
1053
1054
1055
	-- 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.
1056
1057
	--
	-- BUT do no improvement!  See Plan D above
1058
1059
	-- HOWEVER, some unification may take place, if we instantiate
	-- 	    a method Inst with an equality constraint
1060
1061
    reduceContextWithoutImprovement 
	doc reduceMe wanteds' 		`thenM` \ (_frees, _binds, constrained_dicts) ->
1062
1063

	-- Next, figure out the tyvars we will quantify over
1064
1065
1066
    zonkTcTyVarsAndFV (varSetElems tau_tvs)	`thenM` \ tau_tvs' ->
    tcGetGlobalTyVars				`thenM` \ gbl_tvs' ->
    mappM zonkInst constrained_dicts		`thenM` \ constrained_dicts' ->
1067
    let
1068
1069
1070
	constrained_tvs' = tyVarsOfInsts constrained_dicts'
	qtvs' = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
			 `minusVarSet` constrained_tvs'
1071
    in
1072
    traceTc (text "tcSimplifyRestricted" <+> vcat [
1073
		pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts',
1074
		ppr _binds,
1075
		ppr constrained_tvs', ppr tau_tvs', ppr qtvs' ])	`thenM_`
1076

1077
1078
1079
1080
	-- The first step may have squashed more methods than
	-- necessary, so try again, this time more gently, knowing the exact
	-- set of type variables to quantify over.
	--
1081
	-- We quantify only over constraints that are captured by qtvs';
1082
1083
1084
1085
1086
1087
1088
1089
	-- these will just be a subset of non-dicts.  This in contrast
	-- to normal inference (using isFreeWhenInferring) in which we quantify over
	-- all *non-inheritable* constraints too.  This implements choice
	-- (B) under "implicit parameter and monomorphism" above.
	--
	-- 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
1090
1091
1092
1093
1094
	--
	-- At top level, we *do* squash methods becuase we want to 
	-- expose implicit parameters to the test that follows
    let
	is_nested_group = isNotTopLevel top_lvl
1095
        try_me inst | isFreeWrtTyVars qtvs' inst,
1096
		      (is_nested_group || isDict inst) = Free
1097
	            | otherwise  		       = ReduceMe AddSCs
1098
    in
1099
1100
1101
    reduceContextWithoutImprovement 
	doc try_me wanteds' 		`thenM` \ (frees, binds, irreds) ->
    ASSERT( null irreds )
1102
1103
1104
1105

	-- See "Notes on implicit parameters, Question 4: top level"
    if is_nested_group then
	extendLIEs frees	`thenM_`
1106
        returnM (varSetElems qtvs', binds)
1107
1108
1109
1110
1111
1112
    else
	let
    	    (non_ips, bad_ips) = partition isClassDict frees
	in    
	addTopIPErrs bndrs bad_ips	`thenM_`
	extendLIEs non_ips		`thenM_`
1113
        returnM (varSetElems qtvs', binds)
1114
1115
\end{code}

1116
1117
1118

%************************************************************************
%*									*
1119
		tcSimplifyRuleLhs
1120
1121
1122
%*									*
%************************************************************************

1123
1124
1125
1126
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.

1127
1128
1129
Example.  Consider the following left-hand side of a rule
	
	f (x == y) (y > z) = ...
1130

1131
If we typecheck this expression we get constraints
1132

1133
	d1 :: Ord a, d2 :: Eq a
1134

1135
We do NOT want to "simplify" to the LHS
1136

1137
1138
	forall x::a, y::a, z::a, d1::Ord a.
	  f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1139

1140
Instead we want	
1141

1142
1143
	forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
	  f ((==) d2 x y) ((>) d1 y z) = ...
1144

1145
Here is another example:
1146
1147
1148
1149

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

1150
1151
In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
we *dont* want to get
1152
1153

	forall dIntegralInt.
1154
	   fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1155

1156
because the scsel will mess up RULE matching.  Instead we want
1157
1158

	forall dIntegralInt, dNumInt.
1159
	  fromIntegral Int Int dIntegralInt dNumInt = id Int
1160

1161
Even if we have 
1162

1163
	g (x == y) (y == z) = ..
1164

1165
where the two dictionaries are *identical*, we do NOT WANT
1166

1167
1168
1169
1170
1171
	forall x::a, y::a, z::a, d1::Eq a
	  f ((==) d1 x y) ((>) d1 y z) = ...

because that will only match if the dict args are (visibly) equal.
Instead we want to quantify over the dictionaries separately.
1172

1173
1174
1175
In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
all dicts unchanged, with absolutely no sharing.  It's simpler to do this
from scratch, rather than further parameterise simpleReduceLoop etc
1176

1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
\begin{code}
tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
tcSimplifyRuleLhs wanteds
  = go [] emptyBag wanteds
  where
    go dicts binds []
	= return (dicts, binds)
    go dicts binds (w:ws)
	| isDict w
	= go (w:dicts) binds ws
	| otherwise
	= do { w' <- zonkInst w  -- So that (3::Int) does not generate a call
				 -- to fromInteger; this looks fragile to me
	     ; lookup_result <- lookupInst w'
	     ; case lookup_result of
		 GenInst ws' rhs -> go dicts (addBind binds w rhs) (ws' ++ ws)
		 SimpleInst rhs  -> go dicts (addBind binds w rhs) ws
		 NoInstance	 -> pprPanic "tcSimplifyRuleLhs" (ppr w)
	  }
\end{code}
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207

tcSimplifyBracket is used when simplifying the constraints arising from
a Template Haskell bracket [| ... |].  We want to check that there aren't
any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
Show instance), but we aren't otherwise interested in the results.
Nor do we care about ambiguous dictionaries etc.  We will type check
this bracket again at its usage site.

\begin{code}
tcSimplifyBracket :: [Inst] -> TcM ()
tcSimplifyBracket wanteds
1208
  = simpleReduceLoop doc reduceMe wanteds	`thenM_`
1209
1210
    returnM ()
  where
1211
    doc = text "tcSimplifyBracket"
1212
1213
1214
\end{code}


1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
%************************************************************************
%*									*
\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
1225
step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1226
1227
1228
1229
1230
1231

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

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

1232
then the constraint (?x::Int) arising from the call to f will
1233
force the binding for ?x to be of type Int.
1234
1235

\begin{code}
1236
tcSimplifyIPs :: [Inst]		-- The implicit parameters bound here
1237
1238
1239
1240
1241
1242
	      -> [Inst]		-- Wanted
	      -> TcM TcDictBinds
tcSimplifyIPs given_ips wanteds
  = simpl_loop given_ips wanteds	`thenM` \ (frees, binds) ->
    extendLIEs frees			`thenM_`
    returnM binds
1243
  where
1244
1245
    doc	     = text "tcSimplifyIPs" <+> ppr given_ips
    ip_set   = mkNameSet (ipNamesOfInsts given_ips)
1246

1247
	-- Simplify any methods that mention the implicit parameter
1248
    try_me inst | isFreeWrtIPs ip_set inst = Free
1249
		| otherwise		   = ReduceMe NoSCs
1250
1251

    simpl_loop givens wanteds
1252
1253
      = mappM zonkInst givens		`thenM` \ givens' ->
        mappM zonkInst wanteds		`thenM` \ wanteds' ->
1254

1255
        reduceContext doc try_me givens' wanteds'    `thenM` \ (no_improvement, frees, binds, irreds) ->
1256
1257
1258

        if no_improvement then
	    ASSERT( null irreds )
1259
	    returnM (frees, binds)
1260
	else
1261
	    simpl_loop givens' (irreds ++ frees)	`thenM` \ (frees1, binds1) ->
1262
	    returnM (frees1, binds `unionBags` binds1)
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
\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}
1292
1293
1294
1295
1296
1297
1298
bindInstsOfLocalFuns ::	[Inst] -> [TcId] -> TcM TcDictBinds
-- Simlifies only MethodInsts, and generate only bindings of form 
--	fm = f tys dicts
-- We're careful not to even generate bindings of the form
--	d1 = d2
-- You'd think that'd be fine, but it interacts with what is
-- arguably a bug in Match.tidyEqnInfo (see notes there)
1299

1300
bindInstsOfLocalFuns wanteds local_ids
1301
  | null overloaded_ids
1302
	-- Common case
1303
  = extendLIEs wanteds		`thenM_`
1304
    returnM emptyLHsBinds
1305
1306

  | otherwise
1307
  = simpleReduceLoop doc try_me for_me	`thenM` \ (frees, binds, irreds) ->
1308
    ASSERT( null irreds )
1309
    extendLIEs not_for_me	`thenM_`
1310
1311
    extendLIEs frees		`thenM_`
    returnM binds
1312
1313
1314
  where
    doc		     = text "bindInsts" <+> ppr local_ids
    overloaded_ids   = filter is_overloaded local_ids
1315
    is_overloaded id = isOverloadedTy (idType id)
1316
    (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1317
1318

    overloaded_set = mkVarSet overloaded_ids	-- There can occasionally be a lot of them
1319
						-- so it's worth building a set, so that
1320
						-- lookup (in isMethodFor) is faster
1321
    try_me inst | isMethod inst = ReduceMe NoSCs
1322
		| otherwise	= Free
1323
\end{code}
1324

1325

1326
1327
%************************************************************************
%*									*
1328
\subsection{Data types for the reduction mechanism}
1329
1330
1331
%*									*
%************************************************************************

1332
1333
The main control over context reduction is here

1334
\begin{code}
1335
data WhatToDo
1336
 = ReduceMe WantSCs	-- Try to reduce this
1337
			-- If there's no instance, behave exactly like
1338
1339
			-- DontReduce: add the inst to the irreductible ones, 
			-- but don't produce an error message of any kind.
1340
			-- It might be quite legitimate such as (Eq a)!
1341

1342
1343
 | DontReduceUnlessConstant	-- Return as irreducible unless it can
				-- be reduced to a constant in one step
1344

1345
 | Free			  -- Return as free
1346

1347
reduceMe :: Inst -> WhatToDo
1348
reduceMe inst = ReduceMe AddSCs
1349

1350
1351
data WantSCs = NoSCs | AddSCs	-- Tells whether we should add the superclasses
				-- of a predicate when adding it to the avails
1352
1353
	-- The reason for this flag is entirely the super-class loop problem
	-- Note [SUPER-CLASS LOOP 1]
1354
\end{code}
1355
1356
1357
1358



\begin{code}
1359
type Avails = FiniteMap Inst Avail
1360
emptyAvails = emptyFM
1361

1362
data Avail
1363
1364
  = IsFree		-- Used for free Insts
  | Irred		-- Used for irreducible dictionaries,
1365
1366
			-- which are going to be lambda bound

1367
  | Given TcId 		-- Used for dictionaries for which we have a binding
1368
1369
			-- e.g. those "given" in a signature

1370
  | Rhs 		-- Used when there is a RHS
1371
	(LHsExpr TcId) 	-- The RHS
1372
	[Inst]		-- Insts free in the RHS; we need these too
1373

1374
pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
1375
			| (inst,avail) <- fmToList avails ]
1376
1377
1378
1379

instance Outputable Avail where
    ppr = pprAvail

1380
1381
pprAvail IsFree	       	= text "Free"
pprAvail Irred	       	= text "Irred"
1382
pprAvail (Given x)   	= text "Given" <+> ppr x
1383
pprAvail (Rhs rhs bs)   = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1384
1385
1386
1387
1388
1389
1390
1391
1392
\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}
1393
extractResults :: Avails
1394
	       -> [Inst]		-- Wanted
1395
	       -> TcM (TcDictBinds, 	-- Bindings
1396
1397
			[Inst],		-- Irreducible ones
			[Inst])		-- Free ones
1398

1399
extractResults avails wanteds
1400
  = go avails emptyBag [] [] wanteds
1401
  where
1402
    go avails binds irreds frees [] 
1403
      = returnM (binds, irreds, frees)
1404

simonpj's avatar