"Inaccessible code" error reported in wrong place
Here is my file Bug.hs
:
{-# LANGUAGE GADTs, TypeFamilies #-}
module Bug where
data X a where
XInt :: X Int
XBool :: X Bool
XChar :: X Char
type family Frob a where
Frob Int = Int
Frob x = Char
frob :: X a -> X (Frob a)
frob XInt = XInt
frob _ = XChar
Compiling this file produces the error
Bug.hs:15:6:
Couldn't match type ‛Int’ with ‛Char’
Inaccessible code in
a pattern with constructor XInt :: X Int, in an equation for ‛frob’
In the pattern: XInt
In an equation for ‛frob’: frob XInt = XInt
The line/column number single out the pattern XInt
in the first clause of the function frob
. But, the real problem (as I see it), is the right-hand side of the second clause of frob
. Indeed, when I comment out the second line of the function, the error goes away, even though it was reported on the first line.
I do not believe that this error is caused by closed type families, per se, because I have run across it without them, just in code that was too hard to pare down enough to make a bug report out of.
This was tested on 7.7.20130702.
Trac metadata
Trac field | Value |
---|---|
Version | 7.7 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |