[Haskell-cafe] Why are these record accesses ambiguous

[Joe Fredette]

The error is because of the way records work in Haskell. Recall that a 
record is just sugar for the normal datatype syntax. Namely:

    data FooA a b c = FooA {getA :: a, getB:: b, getC :: c}

can be accessed as either

    f (FooA a b c) = ...

or

   f fooA = ... (getA fooA) ... etc

That is, Record syntax just creates functions for each label that take a 
record and return the content of that label. eg

    getA :: FooA a b c -> a
    getA (FooA a _ _ ) = a
    ...

So when you have two records with the same label in it:

    data Bar = Bar { badlabel :: Int }
    data Baz = Baz { badlabel :: String }

even though they are not the same type, you end up with the following 
definitions:

badlabel :: Bar -> Int
badlabel :: Baz -> String

this is a type error, one that is not trivially resolved. Thats where 
your problem is coming from, two fields both named `x` which result in 
this error.


HTH,

/Joe



John Ky wrote:
> Hi Luke,
>
> You're right.  My code had a typo.  Unfortunately, I still get the 
> same error 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 <lrpalmer at gmail.com 
> <mailto:lrpalmer at gmail.com>> wrote:
>
>     On Sat, Jun 6, 2009 at 1:48 AM, John Ky <newhoggy at gmail.com
>     <mailto:newhoggy at 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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>
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