id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
671	"GHC can suggest ""Probable fixes"" that are impossible"	glasser@…		"Let's say I type in [""foo"", 4] to GHCI:

Prelude> [""foo"", 4]

<interactive>:1:8:
    No instance for (Num [Char])
      arising from the literal `4' at <interactive>:1:8
    Probable fix: add an instance declaration for (Num [Char])
    In the list element: 4
    In the definition of `it': it = [""foo"", 4]

It suggests a ""Probable fix"": add an instance declaration for (Num [Char]).  OK, great.  However, as soon as I try to evaluate ""instance Num [Char] where"" I get

    Illegal instance declaration for `Num [Char]'
        (The instance type must be of form (T a b c)
         where T is not a synonym, and a,b,c are distinct type variables)
    In the instance declaration for `Num [Char]'

Now, I'm not enough of an expert on the type class system to know if this means that it's absolutely impossible to declare instances on [Char], but it certainly seems like that.  Thus, the ""Probable fix"" is in error, since [Char] can't get a Num instance.  (Though I guess (Num a) => [a] can, so maybe this whole issue is a little moot.)"	bug	closed	low		Compiler	6.4.1	wontfix			Unknown/Multiple	Unknown/Multiple		Unknown