[Haskell-cafe] 'data' syntax - a suggestion
isaacdupree at charter.net
Thu Sep 27 09:03:52 EDT 2007
Tomasz Zielonka wrote:
> On 9/27/07, jerzy.karczmarczuk at info.unicaen.fr
> <jerzy.karczmarczuk at info.unicaen.fr> wrote:
>> Thomas Conway writes:
>>> On 9/27/07, ok <ok at cs.otago.ac.nz> wrote:
>>>> I have often found myself wishing for a small extension to the syntax of
>>>> Haskell 'data' declarations. It goes like this:
>>> ['where' clause to allow locally defined names in type declarations]
>>> Quite a few times I've found myself declaring type synonyms for this
>>> reason, but you end up polluting the global namespace.
>>> +1 vote.
>> Data with where?
>> You haven't heard about GADTs?
> I think that you haven't read the question carefully, because "where"
> in GADTs is simply a syntactic sugar. However, this seems to be
> available already with GADTs and type equality constraints:
> data BST key val where
> Empty :: BST key val
> Fork :: (bst ~ BST key val) => key -> val -> bst -> bst -> BST key val
> It's a pity you can't use bst (or a type synonym) instead of the last
> "BST key val".
Indeed. GADT syntax looks like a type signature (except for strictness
annotations, which presently aren't part of function syntax!) but
apparently the (->)s and result-type aren't type-signature, because
type-synonyms can't be used for them. I tried. (because there were
several GADT constructors with slightly different signatures, so I made
a type-synonym with an argument to try to shorten them). It seems a
pity to me too.
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