Question

I have a problem with implementing FP (Backus) interpreter in Haskell.

FP functions look like this:

[+,*]:<2,3>

should result with

<+:<2,3>, *:<2,3>> ->
<5,6>

meaning that every function from list on left side should be performed on every every element of a list on a right side.

What I understand is that I need something similar to “map” function, but map applies one function on a list, and I need list of functions on a list of values.

Thank you in advance! 🙂

EDIT:

Since I wasn’t precise, here is my code that doesn’t work:

apply :: [String] -> [Integer] -> [Integer]
apply fs v = [((apply_f f x) | f <- fs | x <- v)]

apply_f :: String -> [Integer] -> [Integer]
apply_f "+" v = [(sum v)]
apply_f "*" v = [(product v)]

I can’t figure out why… 🙁

EDIT 2:

Sorry, I was too tired from working all day on this.
Problem is that I don’t need second pipe, just first one:

apply fs v = [ apply_f f v | f <- fs ]

Now, everything works just fine, thank you very much! 🙂

Answer 1

Do you want something like this?

[f x | f <- [(+3),(*2)], x <- [1..2]]

Output:

[4,5,2,4]

EDIT:

So something like this?

[[f x | f <- [(+3),(*2)]] | x <- [1..2]]
[[4,2],[5,4]]