Question

I am trying to compile a regexp and get an error message that can be presented to the user. I tried this with Text.Regex.TDFA and Text.Regex.Posix and it seems to behave the same:

Prelude Text.Regex.TDFA Data.Maybe Data.Either.Utils> fromLeft $ (makeRegexM ".[" :: Either String Regex)
"*** Exception: parseRegex for Text.Regex.TDFA.String failed:".[" (line 1, column 3):
unexpected end of input
expecting "^", "]", "-" or Failed to parse bracketed string
Prelude Text.Regex.TDFA Data.Maybe Data.Either.Utils> isJust $ (makeRegexM ".[" :: Maybe Regex)
False
Prelude Text.Regex.TDFA Data.Maybe Data.Either.Utils> isJust $ (makeRegexM "." :: Maybe Regex)
True

The Maybe monad seems to work; the Either does not. However the documentation says, it should use ‘fail’ – which, as far as I know, is defined in Either monad. Am I doing something wrong?

Answer 1

The reason probably is that the monad instance for Either e recently changed. In mtl-1.*, there used to be an

instance Error e => Monad (Either e) where
    ...
    fail msg = Left (strMsg msg)  -- I may misremember the exact names

so calling fail there didn’t cause an exception. Now, there is a monad instance in base (Control.Monad.Instances)

instance Monad (Either e) where
    ...
    fail s = error s  -- implicitly from the default method for fail

so you get the above.