Question

I’m new to haskell and I’m trying to make a function that will multiply an int by every element of a list.

Example:

mult 2 [2,4,6]

Returns:

[4,8,12]

Answer 1

Break the problem down into conceptual steps:

• You want to do something to every element of a list, which is a general operation provided by the map function. Using that, you can ignore the list, and consider only what you need to do with a single element.

• You want to multiply two numbers together. In any single use of the function, one of those numbers will be constant, so we can give it a name as an argument to the function: mult x = .... Now we can treat x as a constant and only worry about the other number.

• The other number is not constant, so you need a function, not just a simple expression. Haskell provides “operator sections” to do this with infix operators like (*), so using x we get (x *).

• Backing out the last few steps, you’re now giving x a name, and creating a function, which you pass to map

mult x = map (x *)

…and you’re actually done at this point, if you want to be. But it might be clearer for a beginner to make the list an explicit argument:

mult x ys = map (x *) ys

Both forms do the same thing, though.