\section[TcDefaults]{Typechecking \tr{default} declarations}
\begin{code}
-module TcDefaults ( tcDefaults ) where
+module TcDefaults ( tcDefaults, defaultDefaultTys ) where
#include "HsVersions.h"
import RnHsSyn ( RenamedHsDecl )
import TcMonad
-import TcEnv ( tcLookupClassByKey )
+import TcEnv ( tcLookupGlobal_maybe )
import TcMonoType ( tcHsType )
-import TcSimplify ( tcSimplifyCheckThetas )
+import TcSimplify ( tcSimplifyDefault )
import TysWiredIn ( integerTy, doubleTy )
-import Type ( Type )
-import Unique ( numClassKey )
-import ErrUtils ( addShortErrLocLine )
+import TcType ( Type, mkClassPred, isTauTy )
+import PrelNames ( numClassName )
import Outputable
-import Util
+import HscTypes ( TyThing(..) )
\end{code}
\begin{code}
-default_default = [integerTy, doubleTy ]
+defaultDefaultTys = [integerTy, doubleTy]
tcDefaults :: [RenamedHsDecl]
- -> TcM s [Type] -- defaulting types to heave
+ -> TcM [Type] -- defaulting types to heave
-- into Tc monad for later use
-- in Disambig.
tcDefaults decls = tc_defaults [default_decl | DefD default_decl <- decls]
-tc_defaults [] = returnTc default_default
+tc_defaults [] = returnTc defaultDefaultTys
-tc_defaults [DefaultDecl mono_tys locn]
- = tcAddSrcLoc locn $
- mapTc tcHsType mono_tys `thenTc` \ tau_tys ->
-
- case tau_tys of
- [] -> returnTc [] -- no defaults
-
- _ ->
- -- Check that all the types are instances of Num
- -- We only care about whether it worked or not
-
- tcAddErrCtxt defaultDeclCtxt $
- tcLookupClassByKey numClassKey `thenNF_Tc` \ num ->
- tcSimplifyCheckThetas
- [{- Nothing given -}]
- [ (num, [ty]) | ty <- tau_tys ] `thenTc_`
-
- returnTc tau_tys
-
-tc_defaults decls
- = failWithTc (dupDefaultDeclErr decls)
+tc_defaults [DefaultDecl [] locn]
+ = returnTc [] -- no defaults
+tc_defaults [DefaultDecl mono_tys locn]
+ = tcLookupGlobal_maybe numClassName `thenNF_Tc` \ maybe_num ->
+ case maybe_num of
+ Just (AClass num_class) -> common_case num_class
+ other -> returnTc []
+ -- In the Nothing case, Num has not been sucked in, so the
+ -- defaults will never be used; so simply discard the default decl.
+ -- This slightly benefits modules that don't use any
+ -- numeric stuff at all, by avoid the necessity of
+ -- always sucking in Num
+ where
+ common_case num_class
+ = tcAddSrcLoc locn $
+ tcAddErrCtxt defaultDeclCtxt $
+ mapTc tc_default_ty mono_tys `thenTc` \ tau_tys ->
+
+ -- Check that all the types are instances of Num
+ -- We only care about whether it worked or not
+ tcSimplifyDefault [mkClassPred num_class [ty] | ty <- tau_tys] `thenTc_`
+
+ returnTc tau_tys
+
+tc_defaults decls@(DefaultDecl _ loc : _) =
+ tcAddSrcLoc loc $
+ failWithTc (dupDefaultDeclErr decls)
+
+
+tc_default_ty hs_ty
+ = tcHsType hs_ty `thenTc` \ ty ->
+ checkTc (isTauTy ty) (polyDefErr hs_ty) `thenTc_`
+ returnTc ty
defaultDeclCtxt = ptext SLIT("when checking that each type in a default declaration")
$$ ptext SLIT("is an instance of class Num")
dupDefaultDeclErr (DefaultDecl _ locn1 : dup_things)
- = vcat (item1 : map dup_item dup_things)
+ = hang (ptext SLIT("Multiple default declarations"))
+ 4 (vcat (map pp dup_things))
where
- item1
- = addShortErrLocLine locn1 (ptext SLIT("multiple default declarations"))
+ pp (DefaultDecl _ locn) = ptext SLIT("here was another default declaration") <+> ppr locn
- dup_item (DefaultDecl _ locn)
- = addShortErrLocLine locn (ptext SLIT("here was another default declaration"))
+polyDefErr ty
+ = hang (ptext SLIT("Illegal polymorphic type in default declaration") <> colon) 4 (ppr ty)
\end{code}
+