CODEWITHSUNDEEP
listhaskell
user5472204
user5472204

Reputation:

Confused about workings of a Haskell list comprehension

I'm a new programmer and using Haskell. I've found a line of code called find that matches up a String with its corresponding pair in a list. Like so

find a b = [x|(y,x) <- b, a==y]

I don't understand fully what this program is saying, for the list comprehension, as I've never seen the x|(y,x) used in such a way before, I've mainly seen it used as x|x or x|x^2. So, does this mean that find the string A, from your input list B = a list comprehension of x, where x is a pair of (String, Char) from your B, and for which the y equals your inputted a? A little confused here.

Upvotes: 2

Views: 171

Answers (3)

Kristiyan Tsvetanov
Kristiyan Tsvetanov

Reputation: 1047

Lets give you an example in order to visualize it:

find a b = [x|(y,x) <- b, a==y]

Let a=2, b=[(1,3),(2,4),(3,5),(2,7)]

(y,x) will get each of (1,3),(2,4),(3,5),(2,7) as a couple and check if the first element equals 2 (we said that a equals 2). If True, the function will return the second element of that couple - x, and put it in a list with the rest of the answers.

So the output for that function would be [4,7]

Upvotes: 2

MathematicalOrchid
MathematicalOrchid

Reputation: 62818

You seem to view "x|x" as a thing. Rather, list comprehensions work like this:

[ expression to return | stuff to iterate over ]

What your example basically says is "draw (x, y) from b, throw away anything which doesn't satisfy a == y, and then return x".

Upvotes: 3

dfeuer
dfeuer

Reputation: 48581

I would pronounce that comprehension

The list of all x such that (y, x) is drawn from b and a == y.

Another way to read it, more sequentially:

Draw pairs (y, x) from b. For each such pair, check whether a == y. If so, produce x; otherwise, move on to the next one.

I should also note that the variable names are a bit confusing. It's idiomatic in Haskell to give lists or other containers plural names. So something more like

find a abs = [x | (y, x) <- abs, a == y]

Upvotes: 7

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