import HsSyn
import Name
+import Class
import TcRnMonad
import TcEnv
import TcHsType
import TcSimplify
import TcType
import PrelNames
+import DynFlags
import SrcLoc
+import Maybe
import Outputable
\end{code}
tcDefaults [L locn (DefaultDecl mono_tys)]
= setSrcSpan locn $
addErrCtxt defaultDeclCtxt $
- tcLookupClass numClassName `thenM` \ num_class ->
- tcLookupClass isStringClassName `thenM` \ num_class ->
- mappM tc_default_ty mono_tys `thenM` \ 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] `thenM_`
+ do { ovl_str <- doptM Opt_OverloadedStrings
+ ; num_class <- tcLookupClass numClassName
+ ; is_str_class <- tcLookupClass isStringClassName
+ ; let deflt_clss | ovl_str = [num_class, is_str_class]
+ | otherwise = [num_class]
+
+ ; tau_tys <- mappM (tc_default_ty deflt_clss) mono_tys
- returnM (Just tau_tys)
+ ; return (Just tau_tys) }
-tcDefaults decls@(L locn (DefaultDecl _) : _) =
- setSrcSpan locn $
+tcDefaults decls@(L locn (DefaultDecl _) : _)
+ = setSrcSpan locn $
failWithTc (dupDefaultDeclErr decls)
-tc_default_ty hs_ty
- = tcHsSigType DefaultDeclCtxt hs_ty `thenM` \ ty ->
- checkTc (isTauTy ty) (polyDefErr hs_ty) `thenM_`
- returnM ty
+tc_default_ty deflt_clss hs_ty
+ = do { ty <- tcHsSigType DefaultDeclCtxt hs_ty
+ ; checkTc (isTauTy ty) (polyDefErr hs_ty)
-defaultDeclCtxt = ptext SLIT("when checking that each type in a default declaration")
- $$ ptext SLIT("is an instance of class Num")
+ -- Check that the type is an instance of at least one of the deflt_clss
+ ; oks <- mapM (check_instance ty) deflt_clss
+ ; checkTc (or oks) (badDefaultTy ty deflt_clss)
+ ; return ty }
+check_instance :: Type -> Class -> TcM Bool
+ -- Check that ty is an instance of cls
+ -- We only care about whether it worked or not; return a boolean
+check_instance ty cls
+ = do { (_, mb_res) <- tryTc (tcSimplifyDefault [mkClassPred cls [ty]])
+ ; return (isJust mb_res) }
+
+defaultDeclCtxt = ptext SLIT("When checking the types in a default declaration")
dupDefaultDeclErr (L _ (DefaultDecl _) : dup_things)
= hang (ptext SLIT("Multiple default declarations"))
polyDefErr ty
= hang (ptext SLIT("Illegal polymorphic type in default declaration") <> colon) 4 (ppr ty)
+
+badDefaultTy ty deflt_clss
+ = hang (ptext SLIT("The default type") <+> quotes (ppr ty) <+> ptext SLIT("is not an instance of"))
+ 2 (foldr1 (\a b -> a <+> ptext SLIT("or") <+> b) (map (quotes. ppr) deflt_clss))
\end{code}
-- OK, so there are some errors
{ -- Use the defaulting rules to do extra unification
-- NB: irreds are already zonked
- ; extended_default <- if interactive then return True
- else doptM Opt_ExtendedDefaultRules
- ; disambiguate extended_default irreds1 -- Does unification
+ ; dflags <- getDOpts
+ ; disambiguate interactive dflags irreds1 -- Does unification
; (irreds2, binds2) <- topCheckLoop doc irreds1
-- Deal with implicit parameter
@void@.
\begin{code}
-disambiguate :: Bool -> [Inst] -> TcM ()
+disambiguate :: Bool -> DynFlags -> [Inst] -> TcM ()
-- Just does unification to fix the default types
-- The Insts are assumed to be pre-zonked
-disambiguate extended_defaulting insts
+disambiguate interactive dflags insts
| null defaultable_groups
= do { traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
; return () }
do { integer_ty <- tcMetaTy integerTyConName
; checkWiredInTyCon doubleTyCon
; string_ty <- tcMetaTy stringTyConName
- ; ovl_str <- doptM Opt_OverloadedStrings
- ; if ovl_str -- Add String if -foverloaded-strings
+ ; if ovl_strings -- Add String if -foverloaded-strings
then return [integer_ty,doubleTy,string_ty]
else return [integer_ty,doubleTy] }
; traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
; mapM_ (disambigGroup default_tys) defaultable_groups }
where
+ extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
+ ovl_strings = dopt Opt_OverloadedStrings dflags
+
unaries :: [(Inst,Class, TcTyVar)] -- (C tv) constraints
bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
(unaries, bad_tvs) = getDefaultableDicts insts
defaultable_classes clss
| extended_defaulting = any isInteractiveClass clss
- | otherwise = all isStandardClass clss && (any isNumericClass clss || any ((== isStringClassKey) . classKey) clss)
+ | otherwise = all is_std_class clss && (any is_num_class clss)
-- In interactive mode, or with -fextended-default-rules,
-- we default Show a to Show () to avoid graututious errors on "show []"
isInteractiveClass cls
- = isNumericClass cls
- || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey, isStringClassKey])
+ = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
+
+ is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
+ -- is_num_class adds IsString to the standard numeric classes,
+ -- when -foverloaded-strings is enabled
+ is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
+ -- Similarly is_std_class
disambigGroup :: [Type] -- The default types
-> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
class IsString a where
fromString :: String -> a
</programlisting>
-And the only predefined instance is the obvious one to make strings work as usual:
+The only predefined instance is the obvious one to make strings work as usual:
<programlisting>
instance IsString [Char] where
fromString cs = cs
</programlisting>
+The class <literal>IsString</literal> is not in scope by default. If you want to mention
+it explicitly (for exmaple, to give an instance declaration for it), you can import it
+from module <literal>GHC.Exts</literal>.
+</para>
+<para>
+Haskell's defaulting mechanism is extended to cover string literals, when <option>-foverloaded-strings</option> is specified.
+Specifically:
+<itemizedlist>
+<listitem><para>
+Each type in a default declaration must be an
+instance of <literal>Num</literal> <emphasis>or</emphasis> of <literal>IsString</literal>.
+</para></listitem>
+
+<listitem><para>
+The standard defaulting rule (<ulink url="http://haskell.org/onlinereport/decls.html#sect4.3.4">Haskell Report, Section 4.3.4</ulink>)
+is extended thus: defaulting applies when all the unresolved constraints involve standard classes
+<emphasis>or</emphasis> <literal>IsString</literal>; and at least one is a numeric class
+<emphasis>or</emphasis> <literal>IsString</literal>.
+</para></listitem>
+</itemizedlist>
</para>
<para>
A small example:
<programlisting>
+module Main where
+
+import GHC.Exts( IsString(..) )
+
newtype MyString = MyString String deriving (Eq, Show)
instance IsString MyString where
fromString = MyString
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