Question

there is a question really confused me when i am running this piece of code

data Tree a = Nil | Node a (Tree a) (Tree a)
type IntTree = Tree Int
type Counter = State Int

withCounter::Int -> Counter a -> a
withCounter init f = fst(runState f init)

nextCount::Counter Int
nextCount = do
    n <- get
    put (n+1)
    return n

incTree::IntTree -> IntTree
incTree tree = withCounter 1 (IncTree' tree)
incTree' Nil = return 0
incTree' (Node l e r) = do
    newl <- incTree' l
    n <- nextCount
    newr <- incTree' r
    return (Node newl (e+n) newr)

the error is as following:

parse error on input '<-'

and it appears to be raised at line 27, that is ‘n <- nextCount’

does anyone know why i am getting this error ? thanks !

Answer 1

As you appear not to have copied and pasted your code directly, the answer is probably that you are mixing tab characters and spaces.

Do not use tabs: indent using spaces only.