Tengo by Example: Scope

Scope

Wikipedia: Scope
Tengo uses lexical (block) scoping. := always declares a new variable in the current scope; = assigns to an existing one in an enclosing scope.

fmt := import("fmt")

:= inside a block creates a new variable — the outer x is unaffected.

x := 10
if true {
    x := 20
    fmt.println(x)
}
fmt.println(x)

= inside a block modifies the nearest enclosing declaration.

if true {
    x = 30
}
fmt.println(x)

Loop variables are scoped to the loop body. Referencing i outside the loop would be a compile error.

for i := 0; i < 3; i++ {
    _ := i
}

Closures capture variables by reference, not by value.

counter := 0
inc := func() { counter++ }
inc()
inc()
fmt.println(counter)

Loop-closure trap: all closures share the same loop variable. When the loop ends i equals 3, so every closure returns 3.

fns := []
for i := 0; i < 3; i++ {
    fns = append(fns, func() { return i })
}
fmt.printf("%d %d %d\n", fns[0](), fns[1](), fns[2]())

Fix: use an immediately-called function to pass the current value as a parameter, creating a fresh binding for each iteration.

fns2 := []
for i := 0; i < 3; i++ {
    fns2 = append(fns2, (func(v) { return func() { return v } })(i))
}
fmt.printf("%d %d %d\n", fns2[0](), fns2[1](), fns2[2]())

try it

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