On Tue, Mar 25, 2014 at 6:00 PM, Patrick Walton <pcwal...@mozilla.com>wrote:
> On 3/24/14 11:46 PM, Nick Cameron wrote:
>
>> Currently we forbid bounds on type parameters in structs, enums, and
>> types. So the following is illegal:
>>
>> struct S<X: B> {
>> f: ~T<X>,
>> }
>>
>
> IIRC Haskell allows bounds on type parameters (and we did once too), but I
> heard that considered deprecated and not preferred. I don't recall the
> exact reasons, but that's why we removed the feature (and also just for
> language simplicity).
>
> Patrick
>
>
If I remember the reason cited in Haskell design it was that some functions
require more bounds than others.
For example a HashMap generally requires that the key be hashable somehow,
but the "isEmpty" or "size" functions on a HashMap have no such requirement.
Therefore, you would end up with a "minimal" bounds precised at Type level,
and then each function could add some more bounds depending on their needs:
that's 2 places to specify bounds. In the name of simplicity (and maximum
reusability of types) Haskell therefore advise to only use bounds on
functions.
However, I seem to remember than in Haskell the bounds are only Traits;
whereas in Rust some bounds may actually be required to be able to
instantiate the type (Sized ?).
-- Matthieu
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