On Jan 3, 2008 6:08 AM, Achim Schneider <barsoap@web.de> wrote:

Achim Schneider <barsoap@web.de> wrote:

> [...]

I'm trying to grok that

[] = id
++ = .

in the context of Hughes lists.

I guess it would stop to slip away if I knew what : corresponds to.

Well, (:) has type a -> [a] -> [a], so a function corresponding to (:) for Hughes lists should have type

foo :: a -> H a -> H a

that is,

foo :: a -> ([a] -> [a]) -> [a] -> [a]

so it can be written

foo x h = (x:) . h

which lambdabot informs me can also be written as (.) . (:).  But in the end I'm not sure how helpful that is for understanding Hughes lists! I think the key sentence from the paper is this: "by representing a list xs as the function (xs ++) that appends this list to another list that has still to be supplied."  If you understand that sentence, then you can understand why [] is id and (++) is (.).

-Brent