[Haskell-cafe] zip comprehensions and local decls
Christian Sievers
sievers at math2.nat.tu-bs.de
Thu Mar 25 23:26:55 EST 2004
Hello!
This is about a misconception I had about zip comprehensions and a possible
extension to get the desired result. As zip compr. are implemented in hugs
and in ghc (though I only tried hugs), I thought that haskell-cafe might be a
good place for it.
In hugs mode, I expected something like the following to work:
[ (a,a*x) | x <- [1..5] | y <- [10..], let a=x+y ]
but hugs says: ERROR - Undefined variable "x"
I think I can see why:
The expression is rewritten to
[ (a,a*x) | (x,y) <- zip [ x | x<-[1..5] ]
[ y | y <- [10..], let a=x+y ] ]
This scheme is useful if one has ...|y<-list, let a=..., sometest a
which drops some elements from the list before zipping.
But it would be nice to have a way to drop already zipped tuples and to allow
such usages of local declarations. Maybe a final '|' branch without
generator, so to get what was intended with the expression above one might
write
[ (a,a*x) | x <- [1..5] | y <- [10..] | let a=x+y ]
or, as another example, one could have
[ (x,y) | x <- [1..5] | y <- [10..] | even x ]
giving [(2,11),(4,13)].
So the idea is to rewrite
[ e | oldstyle parallel list comprehension | declsandpreds ]
to
[ e | zipN <as before...> , declsandpreds ]
Is this clear enough?
Does it sound useful? Can you think of a nicer syntax?
All the best
Christian Sievers
More information about the Haskell-Cafe
mailing list