frequently-asked-questions.md

layout	title	subtitle	description
page	Frequently asked questions	What? Why? Where? When? How?	The answers to some things you might be wondering about Gleam!

• Why is it called Gleam?
• What does Gleam compile to?
• Will Gleam have type classes?
• Will Gleam have metaprogramming?
• Does Gleam have mutable state?
• Does Gleam have side effects?
• How is message passing typed?
• Can Gleam use Erlang's hot code reloading?
• How does Gleam compare to Alpaca?
• How does Gleam compare to Caramel?
• How does Gleam compare to Elixir?
• How does Gleam compare to Purerl?
• Should I put Gleam in production?
• Why is the compiler written in Rust?
• Is it good?

Why is it called Gleam?

Gleam rhymes with and is a synonym of "beam", which is the name of the Erlang virtual machine.

It's also a short and cute word that's hopefully easy to spell and pronounce for most people.

What does Gleam compile to?

Gleam compiles to Erlang and also JavaScript.

Will Gleam have type classes?

Some form of ad-hoc polymorphism could be a good addition to the ergonomics of the language, though what shape that may take is unclear. Type classes are one option, OCaml style implicit modules are another, or perhaps it'll be something else entirely.

Will Gleam have metaprogramming?

We don't currently have any fixed ideas for what metaprogramming might look like in Gleam, but it is an area we are interested in. If you have any ideas please do share them!

Does Gleam have mutable state?

All data structures in Gleam are immutable and are implemented using structural sharing so they can be efficiently updated.

If your application needs to hold on to some mutable state then it can be held by an actor (which immutably wraps mutable state using recursion) or you can use ETS, the Erlang in-memory key-value database.

If you are compiling Gleam to JavaScript the gleam_javascript libraries offers mutable references.

Does Gleam have side effects?

Yes, Gleam is an impure functional language like OCaml or Erlang. Impure actions like reading to files and printing to the console is possible without special handling.

We may later introduce an effects system for identifying and tracking any impure code in a Gleam application, though this is still an area of research.

How is message passing typed?

Type safe message passing is implemented in Gleam as a set of libraries, rather than being part of the core language itself. This allows us to write safe concurrent programs that make use of Erlang's OTP framework while not locking us in to one specific approach to typing message passing. This lack of lock-in is important as typing message passing is an area of active research, we may discover an even better approach at a later date!

If you'd like to see more consider checking out Gleam's OTP library.

Can Gleam use Erlang's hot code reloading?

All the usual Erlang code reloading features work, but it is not possible to type check the upgrades themselves as we have no way knowing the types of the already running code. This means you would have the usual Erlang amount of safety rather than what you might have with Gleam otherwise.

Generally the OTP libraries for Gleam are optimised for type safety rather than upgrades, and use records rather than atom modules so the state upgrade callbacks may be slightly more complex to write.

How does Gleam compare to Alpaca?

Alpaca is similar to Gleam in that it is a statically typed language for the Erlang VM that is inspired by the ML family of languages. It's a wonderful project and it was an early inspiration for Gleam!

Here's a non-exhaustive list of differences:

• Alpaca functions are auto-curried, Gleam's are not.
• Alpaca's unions can be untagged, with Gleam all variants in a custom type need a name.
• Alpaca's compiler is written in Erlang, Gleam's is written in Rust.
• Alpaca's syntax is closer to ML family languages, Gleam's is closer to C family languages.
• Alpaca compiles to Core Erlang, Gleam compiles to regular Erlang and optionally JavaScript.
• Alpaca uses the Erlang build tool, Gleam has its own build tool.
• Gleam is more actively developed than Alpaca (at time of writing).

How does Gleam compare to Caramel?

Caramel is similar to Gleam in that it is a statically typed language for the Erlang VM. It is very cool, especially because of its OCaml heritage!

Here's a non-exhaustive list of differences:

• Caramel is based off of OCaml and forks the OCaml compiler, Gleam is an entirely new language, syntax, and compiler.
• Caramel's functions are auto-curried, Gleam's are not.
• Caramel's compiler is written in OCaml, Gleam's is written in Rust.
• Caramel uses OCaml syntax, Gleam has its own syntax that is closer to C family languages.
• Gleam is more actively developed than Caramel (at time of writing).

How does Gleam compare to Elixir?

Elixir is another language that runs on the Erlang virtual machine. It is very popular and a great language!

Here's a non-exhaustive list of differences:

• Elixir is dynamically typed, Gleam is statically typed.
• Elixir has a powerful macro system, Gleam has no metaprogramming features.
• Elixir's compiler is written in Erlang and Elixir, Gleam's is written in Rust.
• Elixir uses Ruby style syntax, Gleam has a C family style syntax.
• Elixir has a namespace for module functions and another for variables, Gleam has one unified namespace (so there's no special fun.() syntax).
• Gleam standard library is distributed as Hex packages, which makes interoperability with other BEAM languages easier.
• Both languages compile to Erlang but Elixir compiles to Erlang abstract format, while Gleam compiles to Erlang source. Gleam can also compile to JavaScript.
• Elixir is more mature than Gleam and has a much larger ecosystem.

How does Gleam compare to Purerl?

Purerl is a backend for the PureScript compiler that outputs Erlang. Both PureScript and Purerl are fantastic!

Here's a non-exhaustive list of differences:

• Purerl is a backend for the PureScript compiler, Gleam is its own language and compiler.
• PureScript has a more sophisticated type system than Gleam, featuring rows, HKTs, type classes, and more.
• Purerl's is written in Haskell, Gleam's is written in Rust.
• PureScript has an ML family style syntax, Gleam has a C family style syntax.
• Purerl code can be difficult to use from other BEAM languages, Gleam code is designed to be usable from all BEAM languages.
• PureScript has a more mature than Gleam and has a much larger ecosystem, though not all of it can be used with the Purerl compiler backend.

Should I put Gleam in production?

Gleam is a young language that has not reached version 1.0, so while it is robust it is likely to undergo breaking changes in the future, and there may be some annoying bugs in there somewhere. The Gleam ecosystem is also quite young, so many libraries that are found in other languages will need to be written, or Erlang/Elixir libraries will have to be used in place of pure Gleam versions.

The Erlang VM is extremely mature and well tested, so the runtime aspect of the language is ready for production.

If you decide to move away from Gleam the language you can compile your code to Erlang and maintain that in future.

Why is the compiler written in Rust?

Prototype versions of the Gleam compiler was written in Erlang, but a switch was made to Rust as the lack of static types was making refactoring a slow and error prone process. A full Rust rewrite of the prototype resulted in the removal of a lot of tech debt and bugs, and the performance boost is nice too!

One day Gleam may have a compiler written in Gleam, but for now we are focused on developing other areas of the language such as libraries, tooling, and documentation.

Is it good?

Yes, I think so. :)