Do we really want to treat every newtype wrappers as a form of 'id'?
For example:
newtype Nat = Nat Integer -- must always be positive
A possible rule (doesn't actually typecheck, but you get the idea):
forall (x :: Nat). sqrt (x * x) = x
If we ignore newtyping we get an incorrect rewrite rule. It depends
on the exact implementation of which 'id's would be recognised.
On 20 October 2010 21:08, James Andrew Cook <[email protected]> wrote:
> On Oct 20, 2010, at 11:58 AM, Gregory Crosswhite <[email protected]>
> wrote:
>
>> On 10/20/10 4:09 AM, Simon Peyton-Jones wrote:
>>> No, this isn't optimised. The trouble is that you write (map Foo xs), but
>>> GHC doesn't know about 'map'. We could add a special case for map, but
>>> then you'd soon want (mapTree Foo my_tree).
>>
>> How about a special case for fmap? That seems like it should handle a lot
>> of cases.
>>
>
> Or even better, a special handling of 'id' in rules pragmas that would cause
> any rule matching id to also match any newtype constructor or projection. Or
> have the compiler automatically add rules that map all newtype wrappers and
> unwrappers to unsafeCoerce and make sure that unsafeCoerce has rules for map,
> fmap, (.), etc.
>
> --James_______________________________________________
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--
Push the envelope. Watch it bend.
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