2 % (c) The University of Glasgow 2006
3 % (c) The AQUA Project, Glasgow University, 1993-1998
5 \section[TcDefaults]{Typechecking \tr{default} declarations}
9 -- The above warning supression flag is a temporary kludge.
10 -- While working on this module you are encouraged to remove it and fix
11 -- any warnings in the module. See
12 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
15 module TcDefaults ( tcDefaults ) where
34 tcDefaults :: [LDefaultDecl Name]
35 -> TcM (Maybe [Type]) -- Defaulting types to heave
36 -- into Tc monad for later use
40 = getDeclaredDefaultTys -- No default declaration, so get the
41 -- default types from the envt;
42 -- i.e. use the curent ones
43 -- (the caller will put them back there)
44 -- It's important not to return defaultDefaultTys here (which
45 -- we used to do) because in a TH program, tcDefaults [] is called
46 -- repeatedly, once for each group of declarations between top-level
47 -- splices. We don't want to carefully set the default types in
48 -- one group, only for the next group to ignore them and install
51 tcDefaults [L locn (DefaultDecl [])]
52 = return (Just []) -- Default declaration specifying no types
54 tcDefaults [L locn (DefaultDecl mono_tys)]
56 addErrCtxt defaultDeclCtxt $
57 do { ovl_str <- doptM Opt_OverloadedStrings
58 ; num_class <- tcLookupClass numClassName
59 ; is_str_class <- tcLookupClass isStringClassName
60 ; let deflt_clss | ovl_str = [num_class, is_str_class]
61 | otherwise = [num_class]
63 ; tau_tys <- mapM (tc_default_ty deflt_clss) mono_tys
65 ; return (Just tau_tys) }
67 tcDefaults decls@(L locn (DefaultDecl _) : _)
69 failWithTc (dupDefaultDeclErr decls)
72 tc_default_ty deflt_clss hs_ty
73 = do { ty <- tcHsSigType DefaultDeclCtxt hs_ty
74 ; checkTc (isTauTy ty) (polyDefErr hs_ty)
76 -- Check that the type is an instance of at least one of the deflt_clss
77 ; oks <- mapM (check_instance ty) deflt_clss
78 ; checkTc (or oks) (badDefaultTy ty deflt_clss)
81 check_instance :: Type -> Class -> TcM Bool
82 -- Check that ty is an instance of cls
83 -- We only care about whether it worked or not; return a boolean
85 = do { (_, mb_res) <- tryTc (tcSimplifyDefault [mkClassPred cls [ty]])
86 ; return (isJust mb_res) }
88 defaultDeclCtxt = ptext (sLit "When checking the types in a default declaration")
90 dupDefaultDeclErr (L _ (DefaultDecl _) : dup_things)
91 = hang (ptext (sLit "Multiple default declarations"))
92 4 (vcat (map pp dup_things))
94 pp (L locn (DefaultDecl _)) = ptext (sLit "here was another default declaration") <+> ppr locn
97 = hang (ptext (sLit "Illegal polymorphic type in default declaration") <> colon) 4 (ppr ty)
99 badDefaultTy ty deflt_clss
100 = hang (ptext (sLit "The default type") <+> quotes (ppr ty) <+> ptext (sLit "is not an instance of"))
101 2 (foldr1 (\a b -> a <+> ptext (sLit "or") <+> b) (map (quotes. ppr) deflt_clss))