Fix defaulting for overloaded strings

This patch fixes the typechecking of the default declaration itself,
when overloaded strings are involved.  It also documents the behaviour
in the user manual.

nofib/spectral/power should work again now!

compiler/typecheck/TcDefaults.lhs

@@ -11,13 +11,16 @@ module TcDefaults ( tcDefaults ) where
 
 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}
 
@@ -45,29 +48,38 @@ tcDefaults [L locn (DefaultDecl [])]
 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 ->
+    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]
 
-    	-- 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_`
+	; 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)
+
+	-- 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 }
 
-defaultDeclCtxt =  ptext SLIT("when checking that each type in a default declaration")
-		    $$ ptext SLIT("is an instance of class Num")
+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"))
@@ -77,5 +89,9 @@ dupDefaultDeclErr (L _ (DefaultDecl _) : dup_things)
 
 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}
 

compiler/typecheck/TcSimplify.lhs

@@ -2194,9 +2194,8 @@ tc_simplify_top doc interactive wanteds
 	-- 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
@@ -2244,10 +2243,10 @@ Since we're not using the result of @foo@, the result if (presumably)
 @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 () }
@@ -2261,14 +2260,16 @@ disambiguate extended_defaulting insts
 				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
@@ -2287,14 +2288,19 @@ disambiguate extended_defaulting 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)

docs/users_guide/glasgow_exts.xml

@@ -4086,15 +4086,39 @@ The class <literal>IsString</literal> is defined as:
 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