# functions not in type classes

**Lauri Alanko
**
[email protected]

*Fri, 18 Jan 2002 20:33:45 +0200*

On Fri, Jan 18, 2002 at 12:27:09AM -0800, Ashley Yakeley wrote:
>* Well, what classes should such functions as const, id and (.) be members
*>* of?
*>*
*>* const :: a -> b -> a;
*>* const x y = x;
*>*
*>* id :: a -> a;
*>* id x = x;
*>*
*>* (.) :: (b -> c) -> (a -> b) -> (a -> c);
*>* (.) f g x = f (g x);
*
Arrows, of course. :) In fact, this is directly from my personal,
somewhat tweaked arrow library:
class Arrow z where
arr :: (a -> b) -> z a b
-- at least two of >>>, >>>= and first must must be defined
-- (preferably >>> and first, or things get slow...)
(>>>) :: z a b -> z b c -> z a c
a >>> b = a >>>= first b >>>= arr (\((a,_),_) -> a)
(>>>=) :: z a b -> z (b,a) c -> z a c
a >>>= b = save a >>> b
first :: z a b -> z (a, c) (b, c)
first a = (fst >>> a) >>>= arr (\(b,(a,c)) -> (b,c))
id :: z a a
id = arr (P.id)
const :: a -> z q a
const a = arr (P.const a)
The idea is that arrows that id-arrows and const-arrows can be given
specialized implementations. For instance, we can have a direct term
implementation for the arrows:
data Term z a b
= Arr (a -> b)
| Lift (z a b)
| forall q . Term z a q :>>> Term z q b
| forall q r s . First (Term z a (q,s)) (Term z q r) (Term z (r,s) b)
| Id (Term z a b)
| Label String (Term z a b)
| Const b
| Null -- unsafe
instance Arrow z => Arrow (Term z) where
arr f = Arr f
a >>> b = a :>>> b
first a = First Null a Null
const = Const
id = Id Null
And then one can optimize them right inside the Haskell program:
reduce :: Arrow z => Term z a b -> Term z a b
reduce (Const a :>>> Arr b) = reduce (Const (b a))
reduce (Arr a :>>> Const b) = reduce (Const b)
reduce (Arr a :>>> Arr b) = reduce (Arr (b . a))
-- ...
And after optimization run it again as a real arrow:
runTerm :: Arrow z => Term z a b -> z a b
runTerm (Arr f) = arr f
runTerm (Lift z) = z
runTerm (a :>>> b) = runTerm a >>> runTerm b
-- ...
I never actually pursued this idea to the end, though, so I don't know
if this would be useful in practice. But still, it's a neat idea, and
gives a reason why const should be in a class. :)
Lauri Alanko
[email protected]