Conditionals in PureScript

There are a couple ways to execute code conditionally in PureScript: If-then-else, case expressions, pattern matching (there is a whole section below on this topic) and guards. Let’s start with the classic one, the if-else-then syntax in PureScript

if-then-else

biggerThan10 :: Int -> String
biggerThan10 num =
  if num > 10
  then "Number is > 10"
  else "Number is NOT > 10"


main = log (biggerThan10(2))

The output: “Number is NOT > 10”.

You can also pass a condition itself as a parameter, using the type Boolean:

test :: Boolean -> String
test condition =
  if condition
  then "true"
  else "false"

main = log (test(1 > 2))

The output: “false”

Case expressions

Using the case-of keyword, we can have something similar like a switch-case syntax:

printNumber :: Int -> String
printNumber n = case n of
  0 -> "zero"
  1 -> "one"
  2 -> "two"
  _ -> "another number"

printNumber 2
--- "two"

The underscore catches all other cases for n, which is required.

Pattern matching

Depending on your knowledge of other languages, you might view this concept as PureScript’s style of function-overloading. Through repeating our function definition without mentioning our parameter-variable, we do pattern matching. Instead of writing the variable, we instead provide the exact function definition for the exact given parameter. Here is our printNumber function again:

printNumber :: Int -> String
printNumber 0 = "zero"
printNumber 1 = "one"
printNumber 2 = "two"
printNumber n = "another number"

Make sure to cover every other case again.

Guards

Last but not least, Guards can help us to realize the same function as with the ways I showed you before. Regarding the syntax, there are two important things:

1. There is no equal-sign in the beginning.
2. After the | (OR-sign) a condition must follow. That’s why we always write 1 == n, etc. in the following code example.

printNumber :: Int -> String
printNumber n
  | 0 == n = "zero"
  | 1 == n = "one"
  | 2 == n = "two"
  | otherwise = "another number"

Guards can be combined with case-expressions.