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Introduce 'f' sigil for string interpolation #9512

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Introduce 'f' sigil for string interpolation #9512

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@williamthome williamthome commented Feb 28, 2025 •
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The Triple-Quoted Strings introduced in EEP 64 are awesome!
However, they can be tedious to use when manipulating dynamic content. For example:

render(Bindings) ->
    <<
    "<html>\n"
    "  <head>\n"
    "    <title>", (maps:get(title, Bindings))/binary, "</title>\n"
    "  </head>\n"
    "  <body>\n"
    "    Hello, ", (maps:get(name, Bindings, ~"World"))/binary, "!\n"
    "  </body>\n"
    "</html>"
    >>.

AFAIK, it's not possible to do the above using Triple-Quoted Strings.

This PR aims to simplify such cases by introducing string interpolation via the f sigil.
EEP 62 also proposes string interpolation but uses a different approach.

Changes

  • The parser now supports the ~f"""...""" sigil for interpolated strings.
  • The result is a binary, and interpolated values must also be binaries.
  • Erlang expressions are enclosed in {} within the string.
  • Nesting interpolated strings is allowed.
  • Escape { with a backslash \ to treat it as a literal.

With this PR, the earlier example becomes:

render(Bindings) ->
    ~f"""
    <html>
      <head>
        <title>{maps:get(title, Bindings)}</title>
      </head>
      <body>
        Hello, {maps:get(name, Bindings, ~"World")}!
      </body>
    </html>
    """.

Example Usage

$ bin/erl
Erlang/OTP 28 [RELEASE CANDIDATE 1] [erts-15.2.2] [source] [64-bit] [smp:24:24] [ds:24:24:10] [async-threads:1] [jit:ns]

Eshell V15.2.2 (press Ctrl+G to abort, type help(). for help)
1> render(Bindings) ->
       ~f"""
       <html>
         <head>
           <title>{maps:get(title, Bindings)}</title>
         </head>
         <body>
           Hello, {maps:get(name, Bindings, ~"World")}!
         </body>
       </html>
       """.
ok
2> render(#{title => ~"Example", name => ~"Erlang"}).
<<"<html>\n  <head>\n    <title>Example</title>\n  </head>\n  <body>\n    Hello, Erlang!\n  </body>\n</html>">>

Discussion

No major drawbacks come to mind with this approach, but feedback is welcome.
It would be fantastic to include this feature in OTP-28!

Erlang Forums thread

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github-actions bot commented Feb 28, 2025 •
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CT Test Results

    2 files     97 suites   1h 8m 4s ⏱️
2 196 tests 2 149 ✅ 47 💤 0 ❌
2 563 runs  2 514 ✅ 49 💤 0 ❌

Results for commit 9513781.

♻️ This comment has been updated with latest results.

To speed up review, make sure that you have read Contributing to Erlang/OTP and that all checks pass.

See the TESTING and DEVELOPMENT HowTo guides for details about how to run test locally.

Artifacts

// Erlang/OTP Github Action Bot

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@williamthome