[project @ 2001-08-24 07:58:29 by simonpj]

Improve error message for nullary class

diff --git a/ghc/compiler/typecheck/TcClassDcl.lhs b/ghc/compiler/typecheck/TcClassDcl.lhs
index ff99a46..97e5d5b 100644 (file)
--- a/ghc/compiler/typecheck/TcClassDcl.lhs
+++ b/ghc/compiler/typecheck/TcClassDcl.lhs
@@ -261,8 +261,8 @@ checkValidClass cls
     doptsTc Opt_GlasgowExts                            `thenTc` \ gla_exts ->
 
        -- Check that the class is unary, unless GlaExs
-    checkTc (gla_exts || unary)
-           (classArityErr cls)                         `thenTc_`
+    checkTc (arity > 0)                (nullaryClassErr cls)   `thenTc_`
+    checkTc (gla_exts || unary) (classArityErr cls)    `thenTc_`
 
        -- Check the super-classes
     checkValidTheta (ClassSCCtxt (className cls)) theta        `thenTc_`
@@ -277,7 +277,8 @@ checkValidClass cls
 
   where
     (tyvars, theta, sel_ids, op_stuff) = classBigSig cls
-    unary      = length tyvars == 1
+    arity      = length tyvars
+    unary      = arity == 1
     no_generics = null [() | (_, GenDefMeth) <- op_stuff]
 
     check_op (sel_id, dm) 
@@ -614,8 +615,12 @@ find_prags sel_name meth_name (prag:prags) = find_prags sel_name meth_name prags
 Contexts and errors
 ~~~~~~~~~~~~~~~~~~~
 \begin{code}
+nullaryClassErr cls
+  = ptext SLIT("No parameters for class")  <+> quotes (ppr cls)
+
 classArityErr cls
-  = ptext SLIT("Too many parameters for class") <+> quotes (ppr cls)
+  = vcat [ptext SLIT("Too many parameters for class") <+> quotes (ppr cls),
+         parens (ptext SLIT("Use -fglasgow-exts to allow multi-parameter classes"))]
 
 defltMethCtxt clas
   = ptext SLIT("When checking the default methods for class") <+> quotes (ppr clas)