How to use continue and break keywords in golang templates?

Question

For example:

{{range .Users}}
    {{if .IsAdmin}}
        {{/* How to use "break" or "continue"? */}}
    {{end}}
{{end}}

The documentation for "break" or "continue" in templates is not available in golang.org

Mcgannon

Confirmed for Go 1.18 (Q4 2021). See my answer below – Montage

Answer 1

They are not documented because they do not exist.

To make sure - check the tests for the text/template lexer: https://github.com/golang/go/blob/master/src/text/template/parse/lex_test.go

Tippet

But how can I accomplish this is go? Is there other way? – Mcgannon

You cannot simulate break, and you don't need continue as long as you have {{if. Golang templates are not suited to have too complicated logic. – Tippet

Can you share an example on how can I skip or stop an iteration? – Mcgannon

You cannot stop an iteration. But you can skip with a simple {{if .IsAdmin}}. What is inside the {{if }} would only be rendered if the condition is true. – Tippet

Ok I get it. Thanks. – Mcgannon

Answer 2

Go101 does mention (May 2021, 4+ years later):

Since Go 1.18, "break" and "continue" might be supported in range loops of Go templates.

Note: Go 1.18 should be released in Q1 2022.

That would solve issue 20531 text/template: add break and continue support.
And is currently implemented in CL 321491: html/template, text/template: implement break and continue for range loops.

This is still a work in progress for now (Q1 2021)

Update Sept. 2021: confirmed, with commit d0dd26a

html/template, text/template: implement break and continue for range loops

break and continue for range loops was accepted as a proposal in June 2017.
It was implemented in CL 66410 (Oct 2017)
but then rolled back in CL 92155 (Feb 2018) because html/template changes had not been implemented.

This CL reimplements break and continue in text/template and then adds support for them in html/template as well.

Montage

Answer 3

break and continue statements are part of text/template and html/template in Go 1.10 (in Beta at time of writing). From the release notes:

The new actions {{break}} and {{continue}} break out of the innermost {{range ...}} loop, like the corresponding Go statements.

Prior releases of Go (Before 1.10) do not support break or continue statements.

Looking at the beta documentation you can see the new itemContinue and itemBreak items in the lexer, the new nodes like ContinueNode in Parser to follow the code.

Afflict

The new break and continue actions do not work in html/template, and fixing them requires thinking about security issues that seem too tricky at this stage of the release. We will try again for 1.11. github.com/golang/go/commit/… – Pharyngoscope

Answer 4

You can use a variable to add a in processing check and skip any existing checks for the remainder of the loop.

// method 1
{{ $dothing := false }}
{{ range $i, $A := .List }}
  {{ if $A.Check }}
    {{ $dothing = true }}
  {{end}}
{{ end }}
{{ if $dothing }}
  // do thing
{{ end }}

// method 2
{{ $already := false }}
{{ range $i, $A := .List }}
  {{ if $already }}
  {{ else }}
    {{ if $A.Check }}
      {{ $already = true }}
      // do thing
    {{ end }}
  {{ end }}
{{ end }}

Eisen