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Doubling of last slice element when adding template.HTML #6188

MarkRosemaker opened this issue Aug 9, 2019 · 2 comments · Fixed by #6404


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@MarkRosemaker MarkRosemaker commented Aug 9, 2019

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.


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@vazrupe vazrupe commented Oct 9, 2019

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 (

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)

for _, f := range from {
fv := reflect.ValueOf(f)
if !fv.Type().AssignableTo(tot) {
// Fall back to a []interface{} slice.
return appendToInterfaceSlice(tov, from...)
tov = reflect.Append(tov, fv)

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.


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@bep bep commented Oct 9, 2019

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

@bep bep added the Bug label Oct 10, 2019
@bep bep added this to the v0.59 milestone Oct 10, 2019
@bep bep closed this in #6404 Oct 11, 2019
bep added a commit that referenced this issue Oct 11, 2019
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