Haskell function reverse engineering

Question

I need to analyze the following Haskell function, which is part of a bigger program (extracted from here):

findMoves :: Position -> [Position]
findMoves (left,right) = elems $ Data.Set.filter validPos moves where
    moves | Farmer `member` left = Data.Set.map (move delItem addItem) left
          | otherwise = Data.Set.map (move addItem delItem) right
    move f1 f2 item = (f1 item left, f2 item right)
    delItem item = delete Farmer . delete item 
    addItem item = insert Farmer . insert item

I understand everything until the end of the where statement, but I haven't seen anything like the move f1 f2 item declaration before, I'm starting right now with Haskell. What is that? Something like an in-line function declaration? I just need to know which kind of statement is that, I'm not asking you to explain what the developer was trying to do (that's my task).

Thanks

@WillemVanOnsem which kind of functio is it? Shouldn't it be declared apart, with its own signature? — pitazzo, Jun 04 '18 at 15:31
no, not per se. Here it uses variables from the *outer* function, so it can not immediately be declared outside. Frequently a `where` clause contains functions that are tight to the outer function, but are not very generic, etc. to define separately. — Willem Van Onsem, Jun 04 '18 at 15:33
Its definition is exactly `move f1 f2 item = (f1 item left, f2 item right)`, as shown above. Some authors tend to add type signatures to local functions in where blocks (I do, since I believe it helps greatly), but many do not, relying on type inference. — chi, Jun 04 '18 at 15:34

JohEker · Accepted Answer · 2018-06-04T16:08:45.830

Maybe take a look at some easier example and see if we can figure out what's going on

foo :: Int -> (Int, Int)
foo x =  apply add sub x
    where
    apply f1 f2 someThing = (f1 x someThing, f2 x someThing)
    add k = (+) (1) --<---------------^
    sub s = (-) (10) -- <-----------------------------^

With the input 5, this would give output (6,5). It can often be useful to say something like "i want to apply x to some function", where this function itself takes other functions as input. So we can make it more general by saying: here is a function, that together with 2 other functions, gives me my desired output.

In the short example above we say, "here is a function, that together with two other functions, applies those functions with some values to make a pair". And we dont really care what those functions are, in this case we used the functions add and sub, but that doesnt have to be the case.

Haskell function reverse engineering

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