I'm really new to Haskell and programming in general. I'm trying to add up the contents of a list without using the sum function from Data.List. Here's what I've got so far:
module Summ
where
summ :: [Int] -> Int
summ xs =
if null xs == False
then let y = x + (head xs)
let xs = tail xs
else print y
I'm pretty sure there's a lot wrong with this code, but for now the latest error is "parse error on input" for the else statement. What's wrong?
The syntax for a let expression is let BINDINGS in EXPRESSION, e.g.
let x = 21 in x + x
It's probably complaining about else because it was expecting to see in.
There's a form of let without in, but that only works in do blocks (or list comprehensions).
let. What is an alternative that would still do those operations?
let in both cases where I used it, and I don't really know a lot of functions. What else could I use there, or is in applicable here?y to be the sum of x and the first element of xs, but x doesn't exist. Then you're defining xs to be its own tail. I can't tell you what to use instead because I don't understand what you're trying to do or how any of this would help you with summing up a list.x. I'm trying to take the first part of the list, add it to a variable, then get rid of the first element of xs and repeat until it's empty. Once it's empty I planned on having it print the variable which had been added to.That doesn't look quite right to me. print returns IO (), which isn't Int, and the let section doesn't seem to lead up to an expression at all, and x wasn't defined. Were you attempting to write a recursive function?
summ [] = 0
summ (x:xs) = x + summ xs
The recursion isn't syntax; this simply uses two partial function definitions with pattern matching. One of them calls summ again, which is recursion, and eventually (given a finite list) that call will lead to the simpler, non-recursive function. The pattern also deconstructs the list into head and tail, so the function is equivalent to:
summ xs = if null xs
then 0
else head xs + summ (tail xs)
Note that both then and else branches are expressions of the same type.
Each of these definitions have type summ :: Num t => [t] -> t
IO () instead of Int?
IO anywhere in your summ function. The compiler may have suggested that, because you had a stray print call in there, but you don't want to print inside your summ function; you want to simply return the value and let main print it out or do whatever it wants to it.
yis defined in the first branch of your expression, but used in the second half. What are you expecting to happen?yas sort of storage for the expressions I'm using to add up the elements of the list. I couldn't think of another way to do it...