How to rewrite a fold with anonymous function in Haskell into a regular function?

Question

I am trying to teach myself functional programming using Haskell.

I have hard time understanding currying and lambdas.

This is a function that generates a list of prfixes of a list (outputs a list of lists).

foldr (\element accumulator -> [] : map (element:) accumulator) [[]]

I am trying to rewrite it as a regular function without lambda to help me understand how lambdas work. How would I do it? I am stuck. Would I need a helper function? Thank you.

Answer 1

Yes you will need a helper function. where clauses are a great place to put helpers like this. where clauses are attached to a definition, so I will need to name your function (I have named it inits). Start by just moving the expression out verbatim.

inits :: [a] -> [[a]]
inits = foldr helper [[]]
    where
    helper = \element accumulator -> [] : map (element:) accumulator

Then you can move the lambda arguments on the right to parameter bindings on the left, which means the same thing:

inits :: [a] -> [[a]]
inits = foldr helper [[]]
    where
    helper element accumulator = [] : map (element:) accumulator

(You can also just do one parameter:

    helper element = \accumulator -> [] : map (element:) accumulator

These are all equivalent.)