CODEWITHSUNDEEP
haskellrecursionmap-function
waterbottle334
waterbottle334

Reputation: 37

map function, recursively

So i have a function that takes in three variables. Variable y is called by another function that will turn it into a list.

I want to apply map to this list of variables. So that i can recurse through the function using all the elements of the list one at a time.

since this function takes in three variables, and one is a list that i want to recall the function and apply each element of the list until the list is empty. I am not sure how to do this and return the remaining two variables back each time.

search :: State -> Int -> Int -> Int 
search state n1 n2 = case n1 of 
  0 -> 1 * n2
  num -> average( map (search s) n1-1 n2)
     where s = turnStateToList s

i havent included the turn state to list but i dont think it is required to answer the question. basically it will produce a list of new states. think of it like a tree.

the errors i get are

* map is applied to too many functions 
* y has been given arguments however yy takes in 0 arguments

i understand how it is getting these errors however im not sure what else to do to fix it. Ive tried changing up the brackets and that just gives a different argument error.

Upvotes: 0

Views: 385

Answers (1)

Noughtmare
Noughtmare

Reputation: 10645

As was mentioned in the comments to your question, in Haskell you can define variables recursively, so s = turnStateToList s doesn't mean that you make a new s that gets the value of turnStateToList of the old s. Instead, it defines s recursively in terms of itself, which leads to the infinite expression s = turnStateToList (turnStateToList (turnStateToList ... which is probably not what you want.

To map over a list while keeping some arguments fixed you can introduce an anonymous function, in this case I think you want \s'' -> search s'' (n1 - 1) n2 (the parentheses around n1 - 1 are necessary!).

Oh, and I think you mixed up state and s by accident.

So, with minimal changes to your style, I think you want this code:

search :: State -> Int -> Int -> Int 
search s n1 n2 = case n1 of 
  0 -> 1 * n2
  num -> average( map (\s'' -> search s'' (n1 - 1) n2) s')
     where s' = turnStateToList s

I think it is more idiomatic to avoid the case statement and to avoid binding turnStateToList s to a variable, then you get this code:

search :: State -> Int -> Int -> Int
search _ 0  n2 = n2
search s n1 n2 = average $ map (\s' -> search s' (n1 - 1) n2) (turnStateToList s)

Upvotes: 1

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