How to code the output value in Haskell that equals a function?

Question

I have a function that finds empty cells in a chess game I am making, and I want to replace one of my pieces into that empty spot that I found. I have a function for findCells, so I have a code something like this

movePawnBack :: GameState -> Player -> [[Cell]]
movePawnBack g p = if p == Black then BP = findCells g E else WP = findCells g E


findCells :: GameState -> Cell -> [(Int, Int)]
findCells b c = [(x, y) | x <- [0..4], y <-[0..4], getFromBoard (theBoard b) (x, y) == c]

However, it gives me an error that I don't have an else cause and if I do something like this BP ==... then I just return a boolean type which is not what I want. Is there an easier way to do this?

First, you should put a complete working example: What's a cell? What's GameState? With this code, is impossible to understand what's happening beside a type error on your function movePawnBack. — Rodrigo Ribeiro
– Rodrigo Ribeiro, Commented Feb 12, 2016 at 1:48
First of all you have a syntax error BP = findCells g E this is invalid, there is no assignment or mutation or whatever you are trying to do there. Secondly what is E, WP, BP and where did they come from? — lehins
– lehins, Commented Feb 12, 2016 at 1:58
@AlexeyKuleshevich, I don't think it's a syntax error, per se, but it's clearly a mistake. It means "calculate findCells g E, check if the result is BP, and if it's not, throw an error." — dfeuer
– dfeuer, Commented Feb 12, 2016 at 2:22
@dfeuer I don't think it means (or is intended to mean) that. After all, BP doesn't look at all like a [(Int, Int)], which is the kind of thing findCells returns. But it's very unclear what it is intended to mean! — Daniel Wagner
– Daniel Wagner, Commented Feb 12, 2016 at 2:55
Ok I will put more code up. WK and BP are coming from a different module and E means Empty, So if I have an empty cell I want to fill it, so BP is now in the empty spot — AVC
– AVC, Commented Feb 12, 2016 at 3:20

lehins · Accepted Answer · 2016-02-12 02:12:27Z

1

Is that what you are trying to do?

movePawnBack :: GameState -> Player -> (Cell -> [(Int, Int)])
movePawnBack g p = if p == Black then (findCells g) else (findCells g)

In order to return a function, besides currying, as it is done above, you can also use lambda

movePawnBack :: GameState -> Player -> (Cell -> [(Int, Int)])
movePawnBack g p = if p == Black then (\bp -> findCells g bp) else (\wp -> findCells g wp)

Also note the return type (Cell -> [(Int, Int)]), whenever it is in parenthesis, it usually means a function, although it is not required in above case, I put it there because of your question title: output value in Haskell that equals a function?. For instance map takes a function:

>>> :t map
map :: (a -> b) -> [a] -> [b]

So is that what you were looking for?

edited Feb 12, 2016 at 2:12

answered Feb 12, 2016 at 2:04

lehins

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AVC Over a year ago

Hello sorry was away. I am trying to replace the empty spot with a certain pawn, I will see if that works!

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