Replicate function using list comprehension in Haskell

Question

I came across this replicate function,

replicate' n x = [x|y <- [1..n]]

Why is y there? y is being assigned values from 1 to n despite not being featured in the output expression.

‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎‏‏‎ ‎

Because whoever wrote that code didn't know that you could put the wildcard pattern _ there. Having a y variable that's unused like that is just poor form. — Silvio Mayolo
– Silvio Mayolo, Commented Nov 5, 2022 at 1:40

K. A. Buhr · Accepted Answer · 2022-11-05 02:41:39Z

1

The idea is that the list comprehension:

[some_expr_involving_y | y <- [1..n]]

will produce a list of n elements that are the results of evaluating some_expr_involving_y with y set to each the numbers 1, 2, ... n in turn.

In the special case where some_expr_involving_y is just some constant x that doesn't depend on y, the result will be n copies of x, one for each (unused) value of y.

As pointed out in a comment, it may be better style to use a wildcard, as that's what it's for:

replicate' n x = [x | _ <- [1..n]]

answered Nov 5, 2022 at 2:41

K. A. Buhr

52.7k3 gold badges56 silver badges82 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Replicate function using list comprehension in Haskell

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related