Gleam Programming Language: How Deprecation Warnings, Generics, and Error Handling Work Together

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Gleam bakes deprecation markers, todo annotations, generics, pattern matching, and error handling directly into the language itself rather than leaving them to libraries or convention. That built-in consistency is what gives Gleam code its notable simplicity and readability. This piece breaks down what that actually means for developers evaluating statically typed functional languages.

What Gleam Does Differently

Gleam is a statically typed functional language that compiles to Erlang bytecode for the BEAM virtual machine and also targets JavaScript. What's drawing attention right now isn't a flashy new feature but how consistently the language handles a handful of everyday developer concerns: marking deprecated code, flagging unfinished work, generics, pattern matching, and error handling.

In most languages, these are bolted on through libraries, linters, or team conventions that drift out of sync with the actual code over time. Gleam instead treats them as first-class language constructs, which changes how reliable that information stays as a codebase evolves.

The @deprecated Annotation and todo

Two specific mechanisms stand out. The `@deprecated` annotation lets a developer mark a function or type as scheduled for removal, and the compiler surfaces that information directly, including a migration path toward the replacement API. This means the reason a function is deprecated and where to go instead lives inside the source code itself, not in a wiki page or a comment that nobody updates.

The `todo` keyword serves a related but distinct purpose. It lets incomplete work compile successfully while clearly signaling, at both compile time and runtime, that a piece of logic isn't finished yet. That's a small thing on paper, but it removes the common failure mode where a placeholder function silently ships to production because nobody remembered to finish it.

Generics, Pattern Matching, and Error Handling as One System

What makes Gleam's approach notable is that these pieces aren't isolated features — they're designed to work together. Generics, pattern matching, and error handling are integrated so that, for example, errors are represented as values and handled through pattern matching rather than thrown as exceptions. Developers coming from Rust or Erlang-family languages will recognize this style immediately.

This blog previously covered gomacro: An Interactive Go Interpreter with Generics and Macro Support, which also dealt with generics support, though in a very different context. gomacro brought generics into an interactive REPL environment built on top of Go's existing syntax, a practical retrofit for a language that added generics relatively late. Gleam's case is the opposite: generics were part of the language design from the start, integrated with pattern matching and error handling rather than added as a separate capability layered on top.

Why This Matters Beyond Gleam Itself

Few mainstream languages combine deprecation tracking, incomplete-work markers, generics, pattern matching, and error handling into a single coherent design. TypeScript and Kotlin have deprecation annotations, and Rust has strong pattern matching and error-as-value handling, but the specific combination Gleam offers — all of it consistent and compiler-enforced — is less common.

For teams maintaining large codebases, the practical value isn't abstract. Deprecation information that lives in the compiler rather than in external documentation is harder to lose track of. Incomplete functions that are explicitly marked rather than silently shipped are easier to catch before release. These are unglamorous problems, but they're the kind that accumulate technical debt quietly over years.

Takeaway

Gleam isn't gaining attention because of a single killer feature — it's the result of treating routine developer pain points as language-design problems rather than tooling afterthoughts. Developers interested in statically typed functional languages, or in how BEAM-based tooling is evolving, may find it worth examining the language's syntax and compiler behavior directly.

Reference: https://a.baez.link/3mtdbbp2dmc27

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