Why is the 'comma ok' pattern only applicable in assignment contexts when returning multiple values from a Go function?

Go Function Return Values: Deciphering the "Comma Ok" Pattern

In Go, a function can return multiple values, which can be useful in certain scenarios. However, not all multi-value returns are valid. The following code snippet demonstrates a valid case:

func FindUserInfo(id string) (Info, bool) {
    it, present := all[id]
    return it, present
}

Conversely, this code is invalid:

func FindUserInfo(id string) (Info, bool) {
    return all[id]
}

Why is the first example valid while the second is not? Understanding this requires delving into a technique known as the "comma ok" pattern.

The "Comma Ok" Idiom: Unlocking Missing Keys and Zero Values

The "comma ok" idiom is utilized to distinguish a missing entry in a map from a zero value. The pattern is particularly prevalent when accessing a map key. Consider the following:

var seconds int
var ok bool
seconds, ok = timeZone[tz]

If the tz key exists in the timeZone map, seconds will be assigned the corresponding value, and ok will be set to true. Otherwise, seconds will be set to 0, and ok will be set to false.

Implementation Details: Diving into Go's Source Code

Seeking clarification, we turn to Go's source code. The unpack method is responsible for handling multi-value assignments and returns. It determines whether a specific expression qualifies as a "comma ok" case based on the following criteria:

• Indexing a map
• Retrieving a value from a channel
• Performing a type assertion

In the case of multi-value assignments, the allowCommaOk parameter must be set to true to enable the "comma ok" pattern. However, in the Go compiler, this parameter is consistently set to false.

Consequences for Multi-Value Returns

This behavior implies that the "comma ok" pattern can only be used in assignment contexts. To retrieve multiple values from a function, variables on the left-hand side of the assignment are required.

In-Depth Example

To illustrate, we examine the Checker.initVars() method:

var x operand
if commaOk {
    var a [2]Type
    for i := range a {
        get(&x, i)
        a[i] = check.initVar(lhs[i], &x, returnPos.IsValid())
    }
    check.recordCommaOkTypes(rhs[0], a)
    return
}

In this code, the commaOk flag is only activated when the number of variables on the left-hand side of the assignment matches the number of values returned by the function. Consequently, it is necessary to declare variables for each value that needs to be retrieved.

Conclusion

The "comma ok" pattern in Go is a valuable tool for discriminating between missing map entries and zero values. However, its applicability is limited to assignment contexts, requiring the use of variables to store the returned values. Understanding these constraints ensures the correct and efficient implementation of multi-value returns in Go code.

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