Tuple-like constructors
Robert Dockins
robdockins at fastmail.fm
Sat Feb 4 23:03:52 EST 2006
On Feb 4, 2006, at 7:56 PM, Pablo Barenbaum wrote:
> An awkwardness in Haskell I would like to see solved in
> Haskell', is the fact that the behavior of tuple-like
> constructors must be either built-in or "limited".
One thing I recall seeing on haskell-cafe some time back was the
notion that an n-tuple is semantically equivalent to n nested right-
strict 2-tuples (essentially right-strict heterogeneous lists).
Perhaps we should consider the idea of the tuple notation simply
being syntactic sugar for nested right-strict 2-tuples. Consider:
data Tuple a b = Tuple a !b
-- (a,b) === Tuple a (Tuple b ())
-- (a,b,c) === Tuple a (Tuple b (Tuple c ()))
-- etc...
fst (Tuple x _) = x
snd (Tuple x (Tuple y _)) = y
fst ('a',b') = 'a'
snd (a','b) = 'b'
fst ('a','b','c') = 'a'
snd ('a','b','c') = 'b'
fst ('a','b','c','d','e','f') = 'a'
-- etc...
It seems like compiler cleverness could recover the identical
strictness and unboxing information available now when the "shape" of
the tuple is known at compile time.
> As far as I can see, these are two issues:
>
> 1. There is not a way, for the programmer, to define
> infinite constructors for infinite associated types (such as
> (,) (,,) (,,,) ... / (a, b) (a, b, c) (a, b, c, d) ...)
> these must be built-in.
>
> 2. There is not a way to define functions operating on all
> of these types. Instead, different functions (like zip,
> zip3) must be defined. Something similar happens with
> liftM, liftM2, ... these are "limited".
>
> It seems the language is lacking abstraction, or being
> misused, when the standard prelude issues this code:
>
> zip :: [a] -> [b] -> [(a,b)]
> zip = zipWith (\a b -> (a,b))
>
> zip3 :: [a] -> [b] -> [c] -> [(a,b,c)]
> zip3 = zipWith3 (\a b c -> (a,b,c))
>
> Clearly, at least for a human being, it's redundant.
> I don't know if there already are proposals to solve this.
>
> Sorry if I sound aggresive, I'm just trying to help.
> Excuse my English...
>
> Best regards.
>
> --
> Pablo Barenbaum
> _______________________________________________
> Haskell-prime mailing list
> Haskell-prime at haskell.org
> http://haskell.org/mailman/listinfo/haskell-prime
Rob Dockins
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
-- TMBG
More information about the Haskell-prime
mailing list