#include "HsVersions.h"
import {-# SOURCE #-} TcUnify( unifyType )
-import HsSyn ( HsBind(..), HsExpr(..), LHsExpr, emptyLHsBinds )
-import TcHsSyn ( mkHsApp, mkHsTyApp, mkHsDictApp )
+import HsSyn ( HsBind(..), HsExpr(..), LHsExpr,
+ ExprCoFn(..), (<.>), nlHsTyApp, emptyLHsBinds )
+import TcHsSyn ( mkHsApp )
import TcRnMonad
import Inst ( lookupInst, LookupInstResult(..),
isMethodFor, isMethod,
instToId, tyVarsOfInsts, cloneDict,
ipNamesOfInsts, ipNamesOfInst, dictPred,
- fdPredsOfInst,
+ fdPredsOfInst, mkInstCoFn,
newDictsAtLoc, tcInstClassOp,
getDictClassTys, isTyVarDict, instLoc,
zonkInst, tidyInsts, tidyMoreInsts,
Notes on functional dependencies (a bug)
--------------------------------------
+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:
+
| > class Foo a b | a->b
| >
| > class Bar a b | a->b
- --------------------------------------
- Notes on ambiguity
- --------------------------------------
+-------------------------------------
+ Note [Ambiguity]
+-------------------------------------
It's very hard to be certain when a type is ambiguous. Consider
tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
-- Zonk everything in sight
= mappM zonkInst wanteds `thenM` \ wanteds' ->
- zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
- tcGetGlobalTyVars `thenM` \ gbl_tvs' ->
-- 'reduceMe': Reduce as far as we can. Don't stop at
-- dicts; the idea is to get rid of as many type
-- immediately, with no constraint on s.
--
-- BUT do no improvement! See Plan D above
+ -- HOWEVER, some unification may take place, if we instantiate
+ -- a method Inst with an equality constraint
reduceContextWithoutImprovement
doc reduceMe wanteds' `thenM` \ (_frees, _binds, constrained_dicts) ->
-- Next, figure out the tyvars we will quantify over
+ zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
+ tcGetGlobalTyVars `thenM` \ gbl_tvs' ->
+ mappM zonkInst constrained_dicts `thenM` \ constrained_dicts' ->
let
- constrained_tvs = tyVarsOfInsts constrained_dicts
- qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
- `minusVarSet` constrained_tvs
+ constrained_tvs' = tyVarsOfInsts constrained_dicts'
+ qtvs' = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
+ `minusVarSet` constrained_tvs'
in
traceTc (text "tcSimplifyRestricted" <+> vcat [
- pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts,
+ pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts',
ppr _binds,
- ppr constrained_tvs, ppr tau_tvs', ppr qtvs ]) `thenM_`
+ ppr constrained_tvs', ppr tau_tvs', ppr qtvs' ]) `thenM_`
-- 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.
--
- -- We quantify only over constraints that are captured by qtvs;
+ -- We quantify only over constraints that are captured by qtvs';
-- 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
-- expose implicit parameters to the test that follows
let
is_nested_group = isNotTopLevel top_lvl
- try_me inst | isFreeWrtTyVars qtvs inst,
+ try_me inst | isFreeWrtTyVars qtvs' inst,
(is_nested_group || isDict inst) = Free
| otherwise = ReduceMe AddSCs
in
-- See "Notes on implicit parameters, Question 4: top level"
if is_nested_group then
extendLIEs frees `thenM_`
- returnM (varSetElems qtvs, binds)
+ returnM (varSetElems qtvs', binds)
else
let
(non_ips, bad_ips) = partition isClassDict frees
in
addTopIPErrs bndrs bad_ips `thenM_`
extendLIEs non_ips `thenM_`
- returnM (varSetElems qtvs, binds)
+ returnM (varSetElems qtvs', binds)
\end{code}
new_binds = addBind binds w rhs
new_avails = addToFM avails w (LinRhss rhss)
+ -- get_root is just used for Linear
get_root irreds frees (Given id _) w = returnM (irreds, frees, id)
get_root irreds frees Irred w = cloneDict w `thenM` \ w' ->
returnM (w':irreds, frees, instToId w')
returnM (L span (VarBind x (mk_app span split_id rhs)),
[mk_fs_app span fst_id ty x, mk_fs_app span snd_id ty x])
-mk_fs_app span id ty var = L span (HsVar id) `mkHsTyApp` [ty,ty] `mkHsApp` (L span (HsVar var))
+mk_fs_app span id ty var = nlHsTyApp id [ty,ty] `mkHsApp` (L span (HsVar var))
mk_app span id rhs = L span (HsApp (L span (HsVar id)) rhs)
| is_given sc_dict = return avails
| otherwise = addSCs is_loop avails' sc_dict
where
- sc_sel_rhs = mkHsDictApp (mkHsTyApp (L (instSpan dict) (HsVar sc_sel)) tys) [instToId dict]
+ sc_sel_rhs = L (instSpan dict) (HsCoerce co_fn (HsVar sc_sel))
+ co_fn = mkInstCoFn tys [dict]
avails' = addToFM avails sc_dict (Rhs sc_sel_rhs [dict])
is_given :: Inst -> Bool
ptext SLIT("to the") <+> what] ]
fix2 | null instance_dicts = []
- | otherwise = [ ptext SLIT("add an instance declaration for")
- <+> pprDictsTheta instance_dicts ]
+ | otherwise = [ sep [ptext SLIT("add an instance declaration for"),
+ pprDictsTheta instance_dicts] ]
instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)]
-- Insts for which it is worth suggesting an adding an instance declaration
-- Exclude implicit parameters, and tyvar dicts
projects / ghc-hetmet.git / blobdiff