text/template: allow callers to override IsTrue

[bep]

The IsTrue, which is used in templates to determine if if and when etc. conditionals should be invoked makes sense in most situations. Some examples:

package main

import (
	"fmt"
	"text/template"
	"time"
)

func main() {

	type s struct{}

	printIsTrue(1)         // true
	printIsTrue(0)         // false
	printIsTrue(-1)        // true
	printIsTrue(s{})       // true
	printIsTrue(&s{})      // true
	printIsTrue((*s)(nil)) // false
	printIsTrue(nil)       // false
	printIsTrue("")        // false
	printIsTrue("foo")     // true

	printIsTrue(time.Now())  // true
	printIsTrue(time.Time{}) // true

}

func printIsTrue(in interface{}) {
	b, _ := template.IsTrue(in)
	fmt.Println(b)
}

My main challenge with the above is that Struct values are always true-- even the zero time.Time value above.

It would be useful if the above method could check some additional truther interface:

type truther interface {
	IsTrue() bool
}

If the above is considered a breaking change, an alternative suggestion would be to make the template.IsTrue function settable.

[bcmills]

Please use https://play.golang.org for examples in issue reports when possible. That makes the details much easier for others to investigate.

https://play.golang.org/p/mvC2Cr0_guo

[bcmills]

My main challenge with the above is that Struct values are always true-- even the zero time.Time value above.

The fact that the zero time.Time reports as true is either a bug in the documentation or the implementation. The documentation for template.IsTrue says:

IsTrue reports whether the value is 'true', in the sense of not the zero of its type, and whether the value has a meaningful truth value.

The zero time.Time is the zero of its type, so according to the documentation it should be considered to be true, but the implementation clearly intends otherwise.

[bcmills]

CC @robpike and @mvdan to clarify whether the fix belongs in the documentation or the implementation. (At this point I suspect that it will have to be the documentation.)

[bcmills]

Actually, I think the documentation problem is a separate issue. I'll split it out.