CODEWITHSUNDEEP
haskell
PinkiePie-Z
PinkiePie-Z

Reputation: 553

how to assign values to a list in haskell

I have a data type:

data Numbers = Numbers {a::Int, b::Int}

How can I construct [Numbers] in order to get the same effect as 

[[a,b] | a <- [1,2], b <- (filter (/=a) [1,2])]

so the result will be similar to [[1,2],[2,1]]

Upvotes: 0

Views: 1077

Answers (2)

ertes
ertes

Reputation: 1

This seems to be nothing but a selection with removal. You can find efficient code to do that in an older question.

If it's certainly about exactly two elements, then this implementation will be efficient:

do x:ys <- tails [1..3]
   y <- ys
   [(x, y), (y, x)]

Upvotes: 0

Mischa Arefiev
Mischa Arefiev

Reputation: 5417

You have to use Numbers as the constructor (note: [] is also a constructor, only with a specific syntax sugar, so there is no fundamental difference).

data Numbers = Numbers {a::Int, b::Int}
               deriving Show

main = print [ Numbers a b | a <- [1, 2], b <- filter (/=a) [1, 2] ]

> main
[Numbers {a = 1, b = 2},Numbers {a = 2, b = 1}]

Upvotes: 2

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