proposal: spec: for/else block

[VoxelPrismatic]

Go Programming Experience

Intermediate

Other Languages Experience

Go, Python, JS, Lua

Related Idea

• Has this idea, or one like it, been proposed before?
• Does this affect error handling?
• Is this about generics?
• Is this change backward compatible? Breaking the Go 1 compatibility guarantee is a large cost and requires a large benefit

Has this idea, or one like it, been proposed before?

I could not find one similar

Does this affect error handling?

No.

Is this about generics?

No.

Proposal

Python has a "for/else" block, which will only run the "else" block if the for-loop hasn't been broken out of. I am not a fan of the "else" keyword, but the pattern of only running some code if a loop doesn't break is incredibly handy, so you don't have to keep track of a "success" variable

Language Spec Changes

https://go.dev/ref/spec#For_statements

For statements¶

A "for" statement specifies repeated execution of a block. There are three forms: The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.

ForStmt   = "for" [ Condition | ForClause | RangeClause ] Block .
Condition = Expression .

changes to

ForStmt   = "for" [ Condition | ForClause | RangeClause ] Block [ "else" Block ] .
Condition = Expression .

(perhaps we could use the "default" keyword)

Informal Change

No response

Is this change backward compatible?

I believe so, but you can double check

Before:

broken := false
for e := range arr {
    // ...
    if cond {
        broken = true
        break
    }
    // ...
    if another_cond {
        broken = true
        break
    }
}

if broken {
    // only ran if never broken
    arr = append(arr, val)
}

// do more stuff with arr

After:

for e := range arr {
    // ...
    if cond {
        break
    }
    // ...
    if another_cond {
        break
    }
} else {
    // only ran if never broken
    arr = append(arr, val)
}

// do more stuff with arr

One such case is some sort of lookup in an array of structs, and creating a new such struct if one does not exist. Eg, editing an article. If an article does not exist, immediately create a new one and render the same edit page.

Orthogonality: How does this change interact or overlap with existing features?

It's mainly just syntactic sugar got managing a variable yourself

Would this change make Go easier or harder to learn, and why?

Coming from Python, it's easier, since it mirrors Py's existing syntax. Otherwise, no, because not many languages feature for/else blocks.

Cost Description

Intermediate, as now the compiler will have to keep track of an implicit variable before running the "else" block

Changes to Go ToolChain

minimal; gopls: same scope; gofmt: another indent block & follows if {} else {} chain; goimports: unchanged

Performance Costs

Minimal

Prototype

No response

[Mizommz]

#73857

[VoxelPrismatic]

@Mizommz is that supposed to be a reference to a different issue?

[seankhliao]

I think in Go where errors are values, the typical pattern is to use those directly rather than using an extra condition variable.

Given it's a python only syntax, it's not even clearer to the reader: on seeing it I thought it was only run of the for loop didn't execute at all.

[gabyhelp]

Related Issues

• proposal: Go 2: Do-While Extension to For-Loops #55868 (closed)
• proposal: Go 2: For Else branch #41348 (closed)
• proposal: add `do...for` loop #70064 (closed)
• language: add a for {} else {} construct #4884 (closed)
• proposal: spec: improve for-loop ergonomics #24282
• proposal: Go 2: permit two clause for statement #21855 (closed)
• proposal: Go 2: add syntax to repeat a Condition check for different StatementLists #27075 (closed)
• spec: add "with" conditional branch #68968 (closed)
• proposal: Go 2: spec: for range with defined types #47707 (closed)
• proposal: Go 2: add a new iterator syntax, package, interfaces #54047 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[VoxelPrismatic]

how do you find these tickets so quickly? searching "for else" gave me 20 pages

[VoxelPrismatic]

oh, it's a bot.

[VoxelPrismatic]

I think in Go where errors are values, the typical pattern is to use those directly rather than using an extra condition variable.

Given it's a python only syntax, it's not even clearer to the reader: on seeing it I thought it was only run of the for loop didn't execute at all.

Yeah, I wasn't a fan of the else keyword either. But the pattern is generally where a condition is never met, but not necessarily an error, since the case is handled immediately.

It is sparingly used, but is very handy when available. (But I suppose maybe there are better ways of writing code if no other language has implemented it)