2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[TcDefaults]{Typechecking \tr{default} declarations}
7 module TcDefaults ( tcDefaults ) where
9 #include "HsVersions.h"
11 import HsSyn ( HsDecl(..), DefaultDecl(..) )
12 import RnHsSyn ( RenamedHsDecl )
15 import TcEnv ( tcLookupClassByKey_maybe )
16 import TcMonoType ( tcHsType )
17 import TcSimplify ( tcSimplifyCheckThetas )
19 import TysWiredIn ( integerTy, doubleTy )
21 import Unique ( numClassKey )
22 import ErrUtils ( addShortErrLocLine )
28 default_default = [integerTy, doubleTy]
30 tcDefaults :: [RenamedHsDecl]
31 -> TcM s [Type] -- defaulting types to heave
32 -- into Tc monad for later use
34 tcDefaults decls = tc_defaults [default_decl | DefD default_decl <- decls]
36 tc_defaults [] = returnTc default_default
38 tc_defaults [DefaultDecl [] locn]
39 = returnTc [] -- no defaults
41 tc_defaults [DefaultDecl mono_tys locn]
42 = tcLookupClassByKey_maybe numClassKey `thenNF_Tc` \ maybe_num ->
44 Nothing -> -- Num has not been sucked in, so the defaults will
45 -- never be used; so simply discard the default decl.
46 -- This slightly benefits modules that don't use any
47 -- numeric stuff at all, by avoid the necessity of
48 -- always sucking in Num
51 Just num -> -- The common case
54 mapTc tcHsType mono_tys `thenTc` \ tau_tys ->
56 -- Check that all the types are instances of Num
57 -- We only care about whether it worked or not
58 tcAddErrCtxt defaultDeclCtxt $
61 [ (num, [ty]) | ty <- tau_tys ] `thenTc_`
66 tc_defaults decls@(DefaultDecl _ loc : _) =
68 failWithTc (dupDefaultDeclErr decls)
71 defaultDeclCtxt = ptext SLIT("when checking that each type in a default declaration")
72 $$ ptext SLIT("is an instance of class Num")
75 dupDefaultDeclErr (DefaultDecl _ locn1 : dup_things)
76 = hang (ptext SLIT("Multiple default declarations"))
77 4 (vcat (map pp dup_things))
79 pp (DefaultDecl _ locn) = ptext SLIT("here was another default declaration") <+> ppr locn