Haskell Basics #1

Simple arithmetic

x = 18 - 7
main = print x

Gotcha with negative numbers - the follwing won't work,

x = 10 * -5
main = print x

But this will

x = 10 * (-5)
main = print x

Boolean logic

x = True && False
y = False || True
z = 7 == (10 - 3)
main = print (x, y)

Notice here that the print statement printed a single value, but that value was a a pair of parentheses, containing a comma separated list of values, in this case, x, y, and z.

This is called a pattern. More on this later.

Comparing different types

x = "derp" == 1337
main = print x

x = "derp" == "herp"
main = print x

As expected, the latter works, but the former fails to compile.

That is because, the Haskell compiler does not know how to compare (or add, or subtract) a number with a string.

Functions are values

All the values so far are actually Functions

x = 25

Is actually defining a function x, which takes no parameters, and always evaluates to 5.

(Note that I say "evaluates to", not "returns".)

Functions with parameters

x a b = a + b
main = print $ x 3 4

This defines a function x, which takes in two parameters, a and b, and evaluates it to the sum of a and b.

x (a, b) = a + b
main = print $ x 3 4

Notice that in the first definition, there were no parentheses surrounding x's only parameter - they are simply not required in Haskell, unlike most other programming languages.

This one does not compile, because this defines a function x which takes in just one parameter, (a, b), which happens to be a tuple containing two values.

x (a, b) = a + b
main = print $ x (3, 4)

Thus, this would be the correct way to invoke it.