GHC can suggest "Probable fixes" that are impossible
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.)
Trac metadata
Trac field | Value |
---|---|
Version | 6.4.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | low |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | Unknown |
Architecture | Unknown |