hugo/docs/content/en/functions/apply.md

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apply	Given an array or slice, `apply` returns a new slice with a function applied over it.	functions	docs parent functions	advanced	apply COLLECTION FUNCTION [PARAM...]

apply expects at least three arguments, depending on the function being applied.

1. The first argument is the sequence to operate on.
2. The second argument is the name of the function as a string, which must be the name of a valid Hugo function.
3. After that, the arguments to the applied function are provided, with the string "." standing in for each element of the sequence the function is to be applied against.

Here is an example of a content file with names: as a front matter field:

{{< code-toggle file="content/example.md" fm=true copy=false >}} title: Example names: [ "Derek Perkins", "Joe Bergevin", "Tanner Linsley" ] {{< /code-toggle >}}

You can then use apply as follows:

{{ apply .Params.names "urlize" "." }}

Which will result in the following:

"derek-perkins", "joe-bergevin", "tanner-linsley"

This is roughly equivalent to using the following with range:

{{ range .Params.names }}{{ . | urlize }}{{ end }}

However, it is not possible to provide the output of a range to the delimit function, so you need to apply it.

If you have post-tag-list.html and post-tag-link.html as partials, you could use the following snippets, respectively:

{{< code file="layouts/partials/post-tag-list.html" copy=false >}} {{ with .Params.tags }}

Tags: {{ $len := len . }} {{ if eq $len 1 }} {{ partial "post-tag-link.html" (index . 0) }} {{ else }} {{ $last := sub $len 1 }} {{ range first $last . }} {{ partial "post-tag-link.html" . }}, {{ end }} {{ partial "post-tag-link.html" (index . $last) }} {{ end }}

{{ end }} {{< /code >}}

{{< code file="layouts/partials/post-tag-link.html" copy=false >}} {{ . }} {{< /code >}}

This works, but the complexity of post-tag-list.html is fairly high. The Hugo template needs to perform special behavior for the case where there’s only one tag, and it has to treat the last tag as special. Additionally, the tag list will be rendered something like Tags: tag1 , tag2 , tag3 because of the way that the HTML is generated and then interpreted by a browser.

This first version of layouts/partials/post-tag-list.html separates all of the operations for ease of reading. The combined and DRYer version is shown next:

{{ with .Params.tags }}
  <div class="tags-list">
    Tags:
    {{ $sort := sort . }}
    {{ $links := apply $sort "partial" "post-tag-link.html" "." }}
    {{ $clean := apply $links "chomp" "." }}
    {{ delimit $clean ", " }}
  </div>
{{ end }}

Now in the completed version, you can sort the tags, convert the tags to links with layouts/partials/post-tag-link.html, chomp off stray newlines, and join the tags together in a delimited list for presentation. Here is an even DRYer version of the preceding example:

{{< code file="layouts/partials/post-tag-list.html" >}} {{ with .Params.tags }}

Tags: {{ delimit (apply (apply (sort .) "partial" "post-tag-link.html" ".") "chomp" ".") ", " }}

{{ end }} {{< /code >}}

{{% note %}} apply does not work when receiving the sequence as an argument through a pipeline. {{% /note %}}