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,,,,