On Wed, Jun 8, 2011 at 3:15 PM, Malcolm Wallace <[email protected]> wrote:
>
>> data Bar f a = Foo f => Bar {bar :: f a}
>
> The class context on the data constructor buys you nothing extra in terms of
> expressivity in the language. All it does is force you to repeat the context
> on every function that uses the datatype. For this reason, the language
> committee has decided that the feature will be removed in the next revision
> of Haskell.
You're thinking of a context on the type constructor, i.e.,
data Foo f => Bar f a = Bar { bar :: f a }
The reason the original code does not work is that the constructor
only adds Foo f to the class context during pattern matching. So, for
example, this works:
baz :: Bar f a -> a -> f a -- n.b., no Foo context
baz (Bar _) = foo
But the code in the original post is trying to create a value of type
Bar f a, so the context is needed.
--
Dave Menendez <[email protected]>
<http://www.eyrie.org/~zednenem/>
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