elm-bcp47-language-tag

This package aims to help you build BCP47 language tags with some type-safety. It's important to note that just because a language tag is valid and correct doesn't mean that it will be supported by browsers and tools. So building up a correct tag with this package should give you added confidence but be sure to test the results.

Project goals

• Provide a way to create valid BCP47 tags with confidence that the result is well-formed and valid.
• The resulting Elm bundle will be able to include only the data that is explicitly referenced (for example, LanguageTag.build Language.en { emptySubtags | region = Just Country.gb } only includes the data for the English language and the Great Britain region, but doesn't cause your bundle to include data for French, Spanish, etc.)
• For unusual cases, there's an escape hatch (LanguageTag.custom) where you're on your own making sure you have a valid and meaningful value (much like the elm/html API). If you encounter a use case that isn't supported directly and requires this escape hatch, please open a GitHub issue to describe your use case to help me get context!
• This package is generated from a script with some data sources, so it can be relatively kept up-to-date through automation.

Why it matters

• BCP47 tags can be used in the lang attribute in HTML elements.
• lang attributes are recommended by Lighthouse for the top-level <html> tag
• lang tags can also be used for individual sections of a page, like <p lang="en-GB">...</p><p lang="fr-CA">...</p>
• Some CSS, like text-transform, relies on lang tags to determine how to apply capitalize, uppercase, etc. to the content. See https://developer.mozilla.org/en-US/docs/Web/CSS/text-transform.
• Screen readers rely on lang attributes to determine how to pronounce the text. See https://dequeuniversity.com/rules/axe/3.3/html-has-lang.
• The Intl API in browsers uses BCP47 language tags as locales for some functions
• navigator.language gives the current user's browser language in the BCP47 format

Usage

import LanguageTag exposing (emptySubtags)
import LanguageTag.Language as Language
import LanguageTag.Country as Country
import LanguageTag.Script as Script
import LanguageTag.Variant as Variant

Language.no
    |> LanguageTag.build emptySubtags
    |> LanguageTag.toString
    --> "no"

Language.en
    |> LanguageTag.build  { emptySubtags | region = Just Country.gb }
    |> LanguageTag.toString
    --> "en-gb"

Language.zh
    |> LanguageTag.build  { emptySubtags | region = Just Country.tw }
    |> LanguageTag.toString
    --> "zh-tw"

Language.hy
    |> LanguageTag.build  
        { emptySubtags | 
          region = Just Country.it
        , script = Just Script.latn
        , variants = [ Variant.arevela ] 
        }
    |> LanguageTag.toString
    --> "hy-latn-it-arevela"

-- Chinese, Simplified script, as used in China
Language.zh
|> LanguageTag.build { emptySubtags | region = Just Country.cn, script = Just Script.hans }
    |> LanguageTag.toString
    --> "zh-hans-cn"

Custom Tags

The following features and tag types are supported indirectly through the custom constructor that lets you supply your own custom string:

• Schema extensions
• Private use
• Irregular tags

Reference

• Learn more about the BCP47 spec at https://www.w3.org/International/questions/qa-choosing-language-tags