A Simple Example of Calling an Elixir Library from Gleam

Michael Lynch wanted to see Gleam’s flagship interop feature in action, calling an Elixir library, but couldn’t find a worked example anywhere. So he built one himself: a Gleam wrapper around the Elixir CSV package’s encode function, documented step by step as a beginner learning both languages at once.

The core difficulty is the type mismatch between a statically typed language and a dynamically typed one running on the same virtual machine. Elixir and Gleam both compile to BEAM bytecode, so Gleam can call Elixir functions directly, but Gleam has no native concept of Elixir’s Enumerable protocol, and the wrapper has to bridge that gap by hand.

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• Gleam calls Elixir code through the @external(erlang, "Elixir.CSV", "encode") attribute, using the Elixir. prefix because that’s the namespace Elixir functions get compiled to on BEAM.
• CSV.encode takes a list of lists of strings and returns something typed as Elixir’s Enumerable, which has no Gleam equivalent, so Lynch defines an opaque ElixirEnumerable placeholder type just to receive it.
• Converting that placeholder into something Gleam can actually use requires a second external call, to Elixir’s Enum.to_list, which turns the Enumerable into a plain Elixir list that matches Gleam’s List type.
• The final public wrapper is three lines: pipe the input through csv_encode, then through enum_to_list, exposing only that combined function and keeping the two low-level externals private.
• He hit a version bug immediately: gleam new scaffolded a gleeunit dependency requiring Gleam 1.11.0 while he had 1.10.0 installed, fixed by pinning gleeunit back to 1.3.1.
• Before writing any Gleam code, he explored CSV.encode’s behavior directly in iex, Elixir’s interactive shell, since he didn’t yet know Elixir’s sigil syntax or the exact shape of the function’s output.