[Haskell-cafe] Alex Lexer: Trying to get rid of "Alex"

[Bernie Pope]

On 23 February 2010 20:15, Amit Deshwar <amit.deshwar at gmail.com> wrote:
> Hi Haskell-cafe
> My problem:  I'm trying to obtain the current position of the lexer once it
> reaches the end of the file (line and row number).
> I'm trying to do this in a function:
> getEndPosition = do
>   (a,b,c) <- alexGetInput
>   return a
>
> Unfortunately, the type of a is 'Alex AlexPosn' instead of just 'AlexPosn'
> How do I strip the Alex so I'm left with just a AlexPosn object?

Hi Amit,

Are you sure about the type of a?

It looks like you are using the "monad" wrapper, described here:
   http://www.haskell.org/alex/doc/html/wrappers.html

If that is true, then:
   alexGetInput :: Alex AlexInput
and
   type AlexInput = (AlexPosn, Char, String)

>From that we can infer from your code:
   getEndPosition :: Alex AlexPosn
and thus:
   a :: AlexPosn

If you want to manipulate the value bound to a, you can simply apply a
function to it in the body of getEndPosition, and return the result of
that application (still inside the Alex type). Or you can use the
function:
    runAlex :: String -> Alex a -> Either String a

Cheers,
Bernie.