proposal: Go 2: Do-While Extension to For-Loops

[assyrianic]

Author background

• Would you consider yourself a novice, intermediate, or experienced Go programmer? --Intermediate.
• What other languages do you have experience with? --C, C++, and Python.

Related proposals

• Has this idea, or one like it, been proposed before? --Not that I've seen.
• Does this affect error handling? --Possibly but shouldn't have too much of an effect.
• Is this about generics? --No.

Proposal

• What is the proposed change? --Extending the for-loop in Go to also be able to function as a do-while loop.

// traditional for-loop.
for i := 0; i < n; i++ {
    
}

// do-while-style for-loop.
for {
    
} if i < n /// <-- newline/semicolon inserted to end it.

• Who does this proposal help, and why? --Provide a simpler mechanism for writing a do-while loop.
• Please describe as precisely as possible the change to the language. --This would change the syntax grammar for the for-statement construct.
• What would change in the language spec? --The ForStmt syntax.
• Please also describe the change informally, as in a class teaching Go.

At the end of a for-loop statement, an optional if and a conditional expression following the if would allow the for-loop to act similar to a do-while loop. The conditional would work as an "after-check" when the code block of the for-loop finished an iteration.

• Is this change backward compatible? --Yes.
• Orthogonality: how does this change interact or overlap with existing features? --It does not.
• Is the goal of this change a performance improvement? --No.

Costs

• Would this change make Go easier or harder to learn, and why? --Debatebly Easier as the Go For-loop would be the all encompassing loop.
• What is the cost of this proposal? (Every language change has a cost). --Needing to work on any parsing such as the go/ast library.
• How many tools (such as vet, gopls, gofmt, goimports, etc.) would be affected? --Any tool that has to analyze/manipulate the source code.
• What is the compile time cost? --Insignificant.
• What is the run time cost? --None.

[rittneje]

You can already essentially do this today:

for {
    // ...
    if i >= n {
        break
    }
}