Functions in Haskell

Question

I'm new to functional programming. I have a basic question.

I'm using the Hugs interpreter,

I would like to write a function in Haskell; I went though several tutorials, but I'm not getting it.

fact :: Int -> Int
fact n = if n == 0 then
1
else
n * fact (n-1)

This gives me a syntax error :-S

ERROR - Syntax error in input (unexpected `=')

Answer 1

I assume you type this right into the interactive prompt. Sadly, these are relatively primitive in Haskell - complex definitions, such as fact, can't be entered at the prompt, at least not in the same way you'd normally write them.

You need to put function definitions etc. into modules, then load those via (e.g.) :load fact.hs. There are resources for Hugs specifically that provide more information on this and other topic (I used http://cvs.haskell.org/Hugs/pages/hugsman/index.html to check my assumptions).

Also note that indentation matters, so the code won't work the way you posted it here even when in a module. Those tutorials will have correct versions. If not, they're useless and you should forget them.

Answer 2

The syntax is incorrect. In Haskell, whitespace matters, much like it does in Python. More specifically, if you have text that starts on the first column of a line, the interpreter will think it's a top-level declaration. The correct syntax would be (for example):

fact :: Int -> Int
fact n = if n == 0
  then 1
  else n * fact (n-1)

You could also put the if in one line if you'd like to. So if you're using an interactive prompt you could do:

λ> let fact n = if n == 0 then 1 else n * fact (n-1)

Notice that you'll need to use let in order to define functions on the prompt (at least this is how it's done in GHCi, I'm not sure about Hugs). You'll be better off putting them in a separate file and then loading that in the interpreter. But anyway, a much nicer solution would use pattern-matching in my opinion anyway:

fact :: Int -> Int
fact 0 = 1
fact n = n * fact (n-1)

Here, the interpreter would pattern-match the first argument of the function against the possible cases listed. So if the first argument is null, the result if 1, otherwise apply the function recursively.