[Haskell-cafe] Basic question....
bryan.burgers at gmail.com
Fri Aug 17 13:24:01 EDT 2007
On 8/17/07, rodrigo.bonifacio <rodrigo.bonifacio at uol.com.br> wrote:
> Hi all.
> I want to create the following polymorphic type (EnvItem) that we can apply two functions (envKey and envValue). I tried the following:
> > type Key = String
> > data EnvItem a = EnvItem (Key, a)
> > envKey :: EnvItem (Key, a) -> String
> > envKey EnvItem (key, value) = key
> > envValue :: EnvValue(Key, a) -> a
> > envValue EnvItem (key, value) = value
> But this is resulting in the error: [Constructor "EnvItem" must have exactly 1 argument in pattern]
> I think this is a very basic problem, but I don't know what is wrong.
In addition to what others have already said, I'd like to point out
that you do not really need a tuple in your data item.
> data EnvItem a = EI Key a
> envKey :: EnvItem a -> Key
> envKey (EI key _) = key
> envValue :: EnvValue a -> a
> envValue (EI _ value) = value
Also, you made a distinction between 'Key' and 'String', which is
good. But then in 'envKey', you used 'String' in the signature instead
of 'Key'.That's a little confusing, and also should you ever want to
change the representation of 'Key', you would then have to change the
signature of envKey.
Just my two cents,
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