Can I Explicitly Type a Variable in a Go For Loop\'s Initialization Statement?

Variable Declaration in the Initialization Statement of a For Loop

A common question for Go beginners is regarding variable declaration in the initialization statement of a for loop. While you can write a loop like for i := 0; i < 10; i , specifying a type explicitly within the initialization statement seems impossible.

Attempting to specify a type directly within the initialization, such as for var i int64 = 0; i < 10; i , will result in an error. Contrary to expectations, you must declare the variable outside of the loop initialization and assign it within:

var i int64
for i = 0; i < 10; i++ {
    // i here is of type int64
}

The language specification for a for loop states that the initialization statement may be a short variable declaration, which assigns a value (e.g., i := 0) but not a full variable declaration (var i = 0).

The reason behind this is likely to maintain language simplicity. However, it's worth noting that you can achieve a similar result by using a type conversion:

for i := int64(0); i < 10; i++ {
    // i here is of type int64
}

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