Conditionals

The if Expression

Code can be executed conditionally using if expressions.

if-else

An if expression with an else clause evaluates the given condition; if the condition is true it executes and evaluates to the expression in the if branch, and if the condition is false it executes and evaluates to the expression in the else branch. Both branches must evaluate to the same type.

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module Main

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

if is an expression, and therefore can be used like any other expression; in that way, if can be used like a ternary operator from other languages. For example, we can assign an if expression to a binding:

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module Main

let expr = true
let result = if (expr) "true branch" else "false branch"

print(result) // true branch

Block expressions are frequently used in combination with if expressions in order to execute several expressions together based on a condition, or to improve readability.

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module Main

let expr = false
if (expr) {
  print("true branch")
  print("with block expressions")
} else {
  print("false branch")
  print("with block expressions")
}

if-else expressions can be chained together to create more complex branching behavior:

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module Main

let expr = false
let otherExpr = true
if (expr) {
  print("First branch")
} else if (otherExpr) {
  print("Second branch") // This will be printed
} else {
  print("Third branch")
}

Single-sided if

The if expression can also 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")
}

An if with no else clause evaluates to void.

An aside on Void

void is a special value in Grain, and has the type Void. The Void type is used to represent the absence of a meaningful value; it is generally used in situations where an expression executes side effects and does not evaluate to a meaningful value itself.

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module Main

// The print function evaluates to void
let printValue: Void = print("Print")

let ifValue: Void = if (true) print("If")

assert printValue == void
assert ifValue == void