Improve the cond func

[bep]

There are two items that I want to address:

1. Make it use the same "truth" function as if/with/not/and -- but we should think a little if this would break stuff
2. Lazily invocation of passed funcs/methods

Basically I want to be able to do something ala:

n := cond .File .File.Filename .Name

Maybe not the best example, but imagine a nilpointer scenario etc.

[jamesgeorge007]

@bep I would love to give it a try 👏 I'm new here 🔰
It would be great if you could point me to the right files to work with 👍

[bep]

@jamesgeorge007 this is a particulary hard problem. I don't know how to do it myself.

[regisphilibert]

One line is always better than 4, especially in Go Template.

{{ $n := .Name }}
{{ with .File }}
  {{ $n = .Filename }}
{{ end }}

[jamesgeorge007]

@bep @regisphilibert if that's the case, can you point me to something that is good to start with 👏

[moorereason]

Given:

n := cond .File .File.Filename .Name

@bep, as I under the existing implementation, it would blow up if .File is nil because the text/template engine would resolve .File.Filename before sending it to cond. Correct?

[bep]

@moorereason It would 2 days ago ... I changed it from a pointer to a struct (which is how was in 0.54) for a very similar reason.

But you are right, there are (at least) 2 reasons why I want to fix this:

• To avoid nilpointers (a comon use case for cond
• To avoid costly initialization for values I don't use

The problem, I have no idea how to do this without making modifications to Go's stdlib, so I might bring it up with them (would not be too hopeful). Because now:

{{ if and .Foo .Bar }}{{ end }}

(same with or)

Is very different from:

if .Foo && .Bar {
}

[bep]

#5792

[bep]

I think any help from the Go stdlib in this area is long down the road. But for cond I think we could solve it by handling it as (another) special case in the AST transformer:

{{ $n := cond .File .File.Filename .Name }}

Becomes

{{ $_tmp := "" }}
{{ if .File }}
{{ $_tmp = .File.Filename }}
{{ else }}
{{ $_tmp = .Name }}
{{ end }}
{{ $n := $_tmp }}

Or similar.