Conditionals

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The if Expression

Code can be executed conditionally using if expressions.

Single-sided if

The if expression can be used with no else in order to execute side effects:

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module Main

let expr = true
if (expr) print("Single-sided conditional")
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module Main

let expr = true
if (expr) {
  print("Single-sided conditional")
  print("with multiple statements")
}

An if with no else clause always evaluates to Void.

Double-sided if

An if expression with an else clause executes the code in the if branch and evaluates to the last expression in the branch if the condition evaluates to true, and vice versa if the condition evaluates to false. Both branches must typecheck to the same type.

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module Main

let expr = true
if (expr) print("true branch") else print("false branch")
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module Main

let expr = false
if (expr) {
  print("true branch")
  print("with multiple statements")
} else {
  print("false branch")
  print("with multiple statements")
}

As an expression

Since the if expression evaluates to a value, it can be assigned to a binding like any other value:

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module Main

let expr = true
let result = if (expr) {
  print("got into if branch")
  "true branch"
} else {
  print("got into else branch")
  "false branch"
}

print(result) // true branch

It can also be used inline, like a ternary operator:

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module Main

let expr = false
print(if (expr) "true branch" else "false branch") // false branch
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