guard may evaluate to false, but _ case present

[mabre]

The following code runs fine with ghc (printing Nothing):

module WhereGuardBug where

readDriveShare :: [Char] -> Maybe ([Char], [Char])
readDriveShare (x:xs) | x == '/' = Just (x:a,b)
    where (a,b) = (['f'],['g'])
readDriveShare _ = Nothing

main = print $ readDriveShare ('f':'i':'l':'e':[])

When compiling with frege, I get

W whereGuardBug.hs:4: guard (== x '/') may evaluate to false.

and at runtime

Exception in thread "main" frege.runtime.NoMatch: readDriveShare at line 4 no match for value frege.prelude.PreludeBase$TTuple2@70177ecd
        at WhereGuardBug.readDriveShare(WhereGuardBug.java:90)
        at WhereGuardBug.lambda$null$2(WhereGuardBug.java:135)
        at frege.run8.Thunk.call(Thunk.java:231)
        at WhereGuardBug.lambda$null$0(WhereGuardBug.java:127)
        at frege.run8.Thunk.call(Thunk.java:231)
        at WhereGuardBug.lambda$static$3(WhereGuardBug.java:153)
        at frege.run8.Thunk.call(Thunk.java:231)
        at WhereGuardBug.main(WhereGuardBug.java:162)

[Ingo60]

This is yet another silly bug arising from the pattern binding in the where clause (a construct I never use, which may explain why I didn't recognize this before).

This gets translated into a something like

readDriveShare (x:xs) = case (['f'], ['g']) of (a,b) | x == '/' = Just (x:a,b)

but this wrong here, because the guard must not slip inside.

The workaround is to write

 where
      a = ['f']
      b = ['g']

[mabre]

The workaround is to write

 where
     a = ['f']
     b = ['g']

That doesn’t work in my case, because (a,b) is actually the result of a function, so I used let (a,b) = (['f'],['g']) in … instead.

[Ingo60]

In that case you can write

foo pattern | guard = case something of (a,b) -> .....

But since it is fixed anyway, this is no issue anymore.

[Ingo60]

Fixed.