In the suggestions for Haskell-2 i wrote
>> class (Ring r,AddGroup (m r)) => RightModule m r where
>> cMul :: m r -> r -> m r
>> instance Ring r => RightModule r r where cMul = mul
>> ...
>> Haskell rejects this `=> RightModule r r'
>> How can we express the meaning
... RightModule m r where m is the identical constructor
(m a = a)
>> Scripting newtype Id a = Id a deriving(...)
>> instance Ring r => RightModule Id r where
>> cMul (Id r) r' = Id (mul r r')
>> is an awkward way-out.
Simon L Peyton Jones <[EMAIL PROTECTED]> replies
> So here, m :: *->*
> What you really want is to say
>
> instance Ring r => RightModule (\t->t) r where cMul = mul
>
> so that now you can use cMul at type cMul :: Foo -> Foo -> Foo
> (provided Foo is an instance of Ring).
>
> The '(\t->t)::*->*' is your 'transparent newtype' function.
Exactly. Thank you for explanation.
> ...
> What I don't know how to do is to perform type inference on such
> systems.
> ... "It's complicated".
OK.
Then, with the Haskell-2 instrument, we will, probably, script
newtype Id a = Id a deriving(...)
instance Ring r => RightModule Id r where
cMul (Id r) r' = Id (mul r r')
and tolerate the conversion between r and Id r.
And the application consideres Id as a parasitic constructor forced
by the language restriction.
So, to reduce this infelicity, we need here
newtype Id a = Id a deriving( all )
- as i proposed in the recent letters.
S.P.Jones:
> That would indeed be possible for newtype; but my guess is
> that if you want 'all' then you ought to be able to get away without
> a newtype at all. But I might well be wrong about this.
> Anyway, it's not an unreasonable suggestion.
`all' is necessary here.
Because the program deals with the values of type Ring r => r, and
after introducing of Id, it has also to deal with Ring r => Id r and
Ring (Id r) => Id r
(+,*,cMul applied to r as well as to Id r things),
and Ring is preceeded by certain superclass hierarhy ...
At least, it is needed here ...deriving( (upTo Ring) )
Am i missing something?
------------------
Sergey Mechveliani
[EMAIL PROTECTED]
