Scope

Updated: 2020/9/18 by

1 min read

Every function and block ({ ... }) introduces a new scope for declarations. Each function and block can refer to declarations in its scope or any of the outer scopes.

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let x = 10
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fun f(): Int {
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    let y = 10
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    return x + y
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}
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f()  // is `20`
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// Invalid: the identifier `y` is not in scope.
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//
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y

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fun doubleAndAddOne(_ n: Int): Int {
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    fun double(_ x: Int) {
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        return x * 2
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    }
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    return double(n) + 1
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}
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// Invalid: the identifier `double` is not in scope.
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//
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double(1)

Each scope can introduce new declarations, i.e., the outer declaration is shadowed.

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let x = 2
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fun test(): Int {
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    let x = 3
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    return x
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}
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test()  // is `3`

Scope is lexical, not dynamic.

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let x = 10
2

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fun f(): Int {
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   return x
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}
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fun g(): Int {
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   let x = 20
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   return f()
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}
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g()  // is `10`, not `20`

Declarations are not moved to the top of the enclosing function (hoisted).

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let x = 2
2

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fun f(): Int {
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    if x == 0 {
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        let x = 3
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        return x
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    }
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    return x
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}
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f()  // is `2`