[Haskell-cafe] Re: Why are these record accesses ambiguous

Benedikt Huber Sat, 06 Jun 2009 04:32:55 -0700

Hi John,

The record field disambiguation only works if you
use the form


>  C{ field-name = variable }

where C is a datatype constructor.
In your example you have to write

> let TypeA{ x = v } = getA
> print v

You're right, after type inference it is clear (for us) that x should
mean A.x, but this kind of reasoning (disambiguate names based on the
results of type inference) is not supported by ghc - and that's a good
thing, in my opinion, as otherwise it would be incredibly hard to find
the definition in scope.
There was a long thread on cafe on this subject.


cheers,
benedikt


John Ky schrieb:

Hi Luke,

You're right. My code had a typo. Unfortunately, I still get the sameerror whichever way I do it.


For example:

 > {-# LANGUAGE DisambiguateRecordFields #-}
 > import A
 > import B
 >
 > main = do
 >    let xx = getA
 >    print (x xx)

and:

#!/usr/bin/env runhaskell

 > {-# LANGUAGE DisambiguateRecordFields #-}
 > import A
 > import B
 >
 > main = do
 >    let xx = getA
 >    putStrLn $ show (x xx)

both give me:

test.lhs:8:22:
    Ambiguous occurrence `x'
    It could refer to either `A.x', imported from A at test.lhs:3:2-9
                                      (defined at A.hs:5:5)
                          or `B.x', imported from B at test.lhs:4:2-9
                                      (defined at B.hs:5:5)

Any ideas?

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.10.3

Thanks,

-John

On Sat, Jun 6, 2009 at 6:41 PM, Luke Palmer <lrpal...@gmail.com<mailto:lrpal...@gmail.com>> wrote:


    On Sat, Jun 6, 2009 at 1:48 AM, John Ky <newho...@gmail.com
    <mailto:newho...@gmail.com>> wrote:

        Hi Haskell Cafe,

        In the following code, I get an error saying Ambiguous
        occurrence `x'.  Why can't Haskell work out which x to call
        based on the type of getA?

        Thanks

        -John

        #!/usr/bin/env runhaskell

         > {-# LANGUAGE DisambiguateRecordFields #-}
         > import A
         > import B
         >
         > main = do
         >    let xx = getA
         >    putStrLn $ show x xx


    This is parsed as two arguments passed to the show function (which
    only takes one argument).

    putStrLn $ show (x xx)

    Or because putStrLn . show = print;

    print $ x xx



        ----------------------

        module A where

        data TypeA = TypeA
           { a :: Int
           , x :: Int
           }

        getA = TypeA { a = 1, x = 2 }

        -------------------------

        module B where

        data TypeB = TypeB
           { b :: Int
           , x :: Int
           }

        getB = TypeB { b = 1, x = 3 }

        --------------------------

        ./test.lhs:8:21:
            Ambiguous occurrence `x'
            It could refer to either `A.x', imported from A at
        ./test.lhs:3:2-9
                                              (defined at A.hs:5:5)
                                  or `B.x', imported from B at
        ./test.lhs:4:2-9
                                              (defined at B.hs:5:5)


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[Haskell-cafe] Re: Why are these record accesses ambiguous

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