On Thu, 11 Oct 2007, Tom Davies wrote:
Andrew Wagner <wagner.andrew <at> gmail.com> writes:
If you change your type declarations to 'newtype' declarations, I
believe you would get the effect that you want, depending on what you
mean by 'equivalent'. In that case, Foo and Bar would essentially be
strings, but you could not use either of them in a place where the
other is expected, nor where a String is expected. See
http://haskell.org/haskellwiki/Newtype for more information. Hope this
helps!
I wanted to avoid wrapping the string with a constructor.
I suppose what I'm really asking for is for each type to implicitly define a
'type class with no methods', and to be able to create new instances of
that type class which simply behave as the underlying
type.
For custom types you can add phantom type parameters. This way you can
make types distinct while being able to apply the same functions to their
values.
Say
data SpecialString usage = SpecialString String
Then you can define a generic function like
take :: Int -> SpecialString usage -> SpecialString usage
or a specialised function like
foo :: SpecialString Foo -> SpecialString Foo
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