英文:
How to have short circuit with and/or in text/template
问题
我有这个Go模板：
{{ if and $b.Trigger $b.Trigger.Name }}
Name is {{ $b.Trigger.Name }}.
{{ else }}
...其他内容...
{{ end }}
我试图让这个模板实现以下功能：
if b.Trigger != nil && $b.Trigger.Name != "" { ...
然而它不起作用，因为根据text/template godoc所说，and/or函数的两个参数都会被求值。
当求值$b.Trigger.Name时，它会出错，因为$b.Trigger可能为nil。所以它返回错误：
template: builds.html:24:46: 执行“content”时出错，位于<$b.Trigger.Name>：无法在类型*myType中评估字段Name
我尝试重构为：
{{ if and (ne $b.Trigger nil) (ne $b.Trigger.Name "") }}
但奇怪的是，这也失败了，它说我不能将$b.Trigger与nil进行比较，这是没有意义的，因为该字段是一个指针类型：
template: builds.html:24:31: 执行“content”时出错，位于<ne $b.Trigger nil>：调用ne时出错：比较的类型无效
有什么想法吗？
英文:
I have this Go template:
{{ if and $b.Trigger $b.Trigger.Name }}
Name is {{ $b.Trigger.Name }}.
{{ else }}
...other stuff...
{{ end }}
I'm trying to get this template to do:
if b.Trigger != nil && $b.Trigger.Name != "" { ...
however it doesn't work, because as text/template godoc says, both arguments to and/or functions are evaluated.
When the $b.Trigger.Name is evaluated, it errors out because $b.Trigger can be nil. So it returns error:
> template: builds.html:24:46: executing "content" at <$b.Trigger.Name>: can't evaluate field Name in type *myType
I tried refactoring this to:
{{ if and (ne $b.Trigger nil) (ne $b.Trigger.Name "") }}
and weird enough, this fails as well, it says I can't compare $b.Trigger with nil, which doesn't make sense because that field is a pointer type:
> template: builds.html:24:31: executing "content" at <ne $b.Trigger nil>: error calling ne: invalid type for comparison
Any ideas?
答案1
得分: 5
如Volker在上面提到的，嵌套if语句：
{{if $b.Trigger}}{{if $b.Trigger.Name}}
名称是{{ $b.Trigger.Name }}。
{{end}}{{end}}
或者更简洁地：
{{with $b.Trigger}}{{with .Name}}
名称是{{.}}。
{{end}}{{end}}
不幸的是，上述方法无法处理else子句。下面是一个（相当丑陋的）可能性：
{{$bTriggerName := ""}}
{{with $b.Trigger}}{{$bTriggerName = .Name}}{{end}}
{{with $bTriggerName}}
名称是{{.}}。
{{else}}
...其他内容...
{{end}}
我查看了gobuffalo/plush是否可以更优雅地实现这一点，但截至2019-04-30，它无法做到。
英文:
As Volker noted above, nest the ifs:
{{if $b.Trigger}}{{if $b.Trigger.Name}}
Name is {{ $b.Trigger.Name }}.
{{end}}{{end}}
Or, more succinctly:
{{with $b.Trigger}}{{with .Name}}
Name is {{.}}.
{{end}}{{end}}
Unfortunately, the above won't handle else clauses. Here's one (rather ugly) possibility:
{{$bTriggerName := ""}}
{{with $b.Trigger}}{{$bTriggerName = .Name}}{{end}}
{{with $bTriggerName}}
Name is {{.}}.
{{else}}
...other stuff...
{{end}}
I looked to see if gobuffalo/plush could do this more elegantly, but as of 2019-04-30 it cannot.
答案2
得分: 0
这里解释了为什么在管道中会评估所有的参数。它是通过Go模板函数进行评估的。
https://golang.org/pkg/text/template/#hdr-Functions
英文:
Here explains why all arguments are evaluated in a pipeline.
It is evaluated through go template functions
https://golang.org/pkg/text/template/#hdr-Functions
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