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.)

[simonpj]

I've changed "Probable" to "Possible" in the HEAD. But I can't see another easy way to make this msg more helpful.

It's be enough to give an instance for Num [a], for example, but I'm leery about trying to second-guess the programmer. And saying "Make Num [Char] satisfiable" which is more correct, probably won't help.

Simon