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text/template: Templates should be able to access variables outside them #17454

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willfaught opened this issue Oct 15, 2016 · 7 comments
Closed

text/template: Templates should be able to access variables outside them #17454

willfaught opened this issue Oct 15, 2016 · 7 comments

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@willfaught
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@willfaught willfaught commented Oct 15, 2016

Templates cannot use variables defined outside them, like so:

{{$foo := .Foo}}
{{define "bar"}} {{$foo}} {{end}} {{/* error: unknown variable "foo" */}}

This means the template parameter pipeline must contain all the data needed by the template.

So if you've got a slice, and all the elements share some data, and in an iteration of the slice you want to pass everything about an element to a template, you can't factor that data out into a surrounding struct and then pull it into templates with variables:

type Names struct {
    First string
    Last []string
}

{{$first := .Names.First}}
{{range .Names.Last}} {{$first}} {{.}} {{end}} {{/* error: unknown variable "first" */}}

Note: See below for a correction.

You have to duplicate that shared data and push it down into each element of the slice:

type Name struct {
    First string
    Last string
}

{{range .Names}} {{.First}} {{.Last}} {{end}} {{/* .First is the same for all iterations */}}

I'm lazy and I don't want to do this. [Insert arguments here about DRY, compose-ability, simplicity, power, lexical scoping, etc.]

Note that this ability to reference outside variables isn't all that different from the ability to reference outside templates within the same "scope" (text/template doc term, not mine).

@quentinmit
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@quentinmit quentinmit commented Oct 17, 2016

Your second example works fine for me:
https://play.golang.org/p/ay4QlHnQFU

The first example doesn't work because you are defining a block. You can think of blocks as subroutines; the variables from one block do not leak into the scope of the second block. This is specifically documented:

A template invocation does not inherit variables from the point of its invocation.

@quentinmit quentinmit closed this Oct 17, 2016
@willfaught
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@willfaught willfaught commented Oct 17, 2016

Thanks for pointing that out. Yes, the second example should be inside a define block. I think they should be in scope at block definition, not when they're applied/run.

The scenario I had in mind:

type Names struct {
    First string
    Last []string
}

{{$first := .Names.First}}
{{define "names"}} {{range .}} {{$first}} {{.}} {{end}} {{end}}
[...]
{{template "names" .Names.Last}}

This won't work, but seems like it should, and it would be useful.

@quentinmit
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@quentinmit quentinmit commented Oct 17, 2016

Okay, so look at this example then:

https://play.golang.org/p/sAKdMc6Sbv

In that example, if the contents of the block had been {{ $first }}, what should the code have printed?

Remember that templates can be invoked without invoking their enclosing code.

@willfaught
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@willfaught willfaught commented Oct 18, 2016

I'd expect a not-defined error, like it works now.

@willfaught
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@willfaught willfaught commented Oct 18, 2016

I think I missed your last sentence. Are you saying that a template defined in another template can be used without running the outer template?

@quentinmit
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@quentinmit quentinmit commented Oct 18, 2016

Yes, that's what my example attempts to demonstrate.

@willfaught
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@willfaught willfaught commented Oct 18, 2016

Is the template namespace global? Is the whole thing parsed before scanning for templates to register?

@golang golang locked and limited conversation to collaborators Oct 18, 2017
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