Hugo: Doubling of last slice element when adding template.HTML

[MarkRosemaker]

I came across a weird error that I could boil down to this minimal example:

{{ $c := delimit (slice "c") " " }}<!-- should be 'c' -->
{{ $abc_slice := slice "a" | append "b" $c  }}<!-- should be ['a','b','c'] -->
{{ $abc := delimit $abc_slice "+" }}<!-- should be 'a+b+c' -->
{{ errorf "'%s' (%T) - '%s' (%T)" $c $c $abc $abc }}

Put that in a shortcode and strangely enough, the result is:

'c' (template.HTML) - 'a+b+b+c' (template.HTML)

As you can see, b is doubled.

The problem goes away if you add | print to $c, making it a string. I wonder why it's not a string in the first place.

Thanks in advance for any comments and fixing.

[vazrupe]

There are two causes of this problem.

The return of the delimit function is template.HTML and a bug in the collections.Append function.

template.HTML is actually the same as string, but due to the nature of golang you cannot append to [] string. This can be verified with a simple code.

package main

import (
    "fmt"
)

type inherit string

func main() {
    var s []string

    var txt inherit
    txt = "test"

    s = append(s, txt) // ./prog.go:15:12: cannot use txt (type inherit) as type string in append

    fmt.Printf("%v %s\n", s, txt)
}

https://github.com/gohugoio/hugo/blob/339ee37143ca5a6bb22bbc1b0468d785f450cfb7/common/collections/append.go#L64-L71

For collections.Append, the problem arises in the code above. If the type cannot be added to an existing slice, add the whole element once again.

[bep]

template.HTML isn't "the same as string"; if it was, it would have no purpose.