On 8/17/07, rodrigo.bonifacio
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.
Regards,
Rodrigo.
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, Bryan