CODEWITHSUNDEEP
haskell
Shury
Shury

Reputation: 578

What does x<-xs in Haskell mean?

In one of my courses I came across this:

numbers xs = [x | x<−xs , even x]

I understand that it takes a list of numbers and creates another list formed only with the even numbers from the original list. How exactly is it doing this? What does x<-xs mean?

Upvotes: 2

Views: 3229

Answers (2)

w.b
w.b

Reputation: 11238

As the other answers stated this a list comprehension notation, which means a way of defining a list in terms of another list. If you specify some predicates in the part after a |, only elements satisfying these predicates will be taken, for example [x | x <- xs, even x] is a list of all those elements x taken from the list xs that are even (meet the condition even x). There can be more than one predicate:

[x | x <- xs, even x, x > 10]

You can also apply transformations to the drawn elements, including changing the list type:

λ> [x*2 | x <- [1 .. 20], even x, x > 10]
[24,28,32,36,40]

or:

λ> [show x | x <- [1 .. 20], even x, x > 10]
["12","14","16","18","20"]

Upvotes: 0

John Coleman
John Coleman

Reputation: 52008

This is a list comprehension where the notation is designed to be similar to set builder notation with the token <- corresponding to the stylized lower case Greek epsilon which is the standard symbol for set membership. When I read list comprehensions I tend to read <- as "drawn from", so I would parse [x | x<−xs , even x] as "the list of all x such that x is drawn from xs and x is even" (, corresponds to and and is used to add conditions beyond mere membership in the list which is being drawn from).

Upvotes: 7

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