guard may evaluate to false, but _ case present #271
Description
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)