Haskell中的闭包和列表推导

Question

I'm playing around with Haskell at the moment and thus stumbled upon the list comprehension feature. Naturally, I would have used a closure to do this kind of thing:

Prelude> [x|x<-[1..7],x>4] -- list comprehension
[5,6,7]
Prelude> filter (\x->x>4) [1..7] -- closure
[5,6,7]

I still don't feel this language, so which way would a Haskell programmer go? What are the differences between these two solutions?

Answer 1

Idiomatic Haskell would be filter (> 4) [1..7]

Note that you are not capturing any of the lexical scope in your closure, and are instead making use of a sectioned operator. That is to say, you want a partial application of > , which operator sections give you immediately. List comprehensions are sometimes attractive, but the usual perception is that they do not scale as nicely as the usual suite of higher order functions ("scale" with respect to more complex compositions). That kind of stylistic decision is, of course, largely subjective, so YMMV.

Answer 2

List comprehensions come in handy if the elements are somewhat complex and one needs to filter them by pattern matching, or the mapping part feels too complex for a lambda abstraction, which should be short (or so I feel), or if one has to deal with nested lists. In the latter case, a list comprehension is often more readable than the alternatives (to me, anyway).

For example something like:

[ (f b, (g . fst) a) | (Just a, Right bs) <- somelist, a `notElem` bs, (_, b) <- bs ]

But for your example, the section (>4) is a really nice way to write (\\a -> a > 4) and because you use it only for filtering, most people would prefer ANthonys solution.