Haskell beginner here.

I have a list of tuples type (a, b) like so: [(1, "One"), (2, "2")], and a single pair (key, value).

I want to create a recursive function that accepts a pair and a list of tuples all type (a, b) and returns the list after replacing all values associated to the passed key with the passed value:

changeTup :: (Eq a) => (a,b) -> [(a,b)] -> [(a,b)]

Example output:

> changeTup (1, "Hello") [(1, "One"), (2, "Two")] 
=> [(1, "Hello"), (2, "Two")]

I did some digging and it seems like the best way to do this is with map. I tried this but haven't gotten any good results.

This is my code:

changeTup :: (Eq a) => (a,b) -> [(a,b)] -> [(a,b)]
changeTup (y,z) [] = [(y,z)]
changeTup (y,z) (x:xs) = map replace
    where replace (key, value)
            | y == key    = (key, z)
            | otherwise   = (key, value)

I'm not sure how to approach increments the list (x:xs) within map. I'm getting an expected type error.

CodePudding user response：

The thing about the map function is that iterating over a list (or another vaguely list-shaped thing) is its entire job. In the code you have, you have a perfectly fine replace function, and you want to apply to the elements of the list (x : xs) - and doing that is as simple as map replace (x : xs)