Parse error while running a simple Haskell program

Question

I have just started learning Haskell. I'm beginning by writing a simple function that takes two values n and x, and then displays two integer values computed with it.

test n x = do
    cell1 = round(n*n*x)
    cell2 = n*n - cell1
    print cell1
    print cell2

But it doesn't run and keeps giving me a Parse error on input `=' error. What is happening?

Answer 1

You've hit your first trouble with Monads. What you probably want here is a let statement inside your do block

test n x = do
    let cell1 = round (n * n * x)
        cell2 = n * n - cell1
    print cell1
    print cell2

The difference here is that you can't assign directly inside of a do block, since all do blocks desugar to calls to >>= and >>. The let statement allows you to define a local value like you can inside a function definition like

f x =
    let y = 2 * x
        z = y * y * y
    in z + z + y

The way your function would desugar would be like

test n x =
    let cell1 = round (n * n * x)
        cell2 = n * n - cell1
    in (print cell1 >> print cell2)

Where >> just chains two monadic actions together. Note that this is not really how it desugars, I chose a representation that is equivalent in this case but it is not exactly what the compiler would actually generate.