Crystal - A language for humans and computers

https://fosdem.org/2026/schedule/event/N3AFSF-crystal/

• Manas worked in Ruby, but there were things about Ruby that weren't ideal:
  • Lack of static type system
  • Poor concurrency situation
  • What if we could evolve Ruby to compile statically?
• And thus Crystal was born
• Compiler was initially written in Ruby, and then the language grew into its own ecosystem
• "General-purpose programming language that's object-oriented, garbage-collected, with [...] lots of goodies from Ruby [...] static typing and inference [...] compiles to native code [...] strong native concurrency model"
• Best from Ruby, add more
  • Very dynamic, so you can do lots of stuff
  • You don't get that much flexibility from a static language
    • Crystal moves into the static direction, without losing the dynamic feel of Ruby
• Function without typed arguments will use generic types, so add(a, b) can be instantiated with both integers and strings
  • Code is instantiated at compile-time, so you know add "foo", 2 won't work before you run it

crystalmessage = gets
puts message.upcase

will fail because message can be String | Nil; we have to prove a message is a String:

crystalmessage = gets
if message.is_a?(String)
  puts message.upcase
end

Also supports metaprogramming:

crystalmacro getter(var)
  @{{ var }}

  def {{ var.var }}: {{ var.type }}
    @{{ var.var.id }}
  end
end

getter foo : Int32 = 0

expands to

crystal@def : Int32 = 0

def foo : Int32
  @foo
end

This example is simplified, so much more is possible.

• Batteries included
  • HTTP server with request-response behaviou, fully-concurrent (tens-of-thousands connections)
• Concurrency
  • Crystal has native support for fibers, so you can spawn fibres and run code in them and send messages to them etc

crystaldone = Channel(Int32).new

spawn do
  result = do_some_work
  done.send result
end

done.receive

Everything here will be scheduled appropriately.

• In the future:
  • Concurrency support is great, but multithreading is not really there yet
    • Multithreading support exists, but was implemented as single-thread schedulers running on independent threads with no movement
    • This is being addressed by execution contexts, which enable more multithreading
    • Multithreading at scale is still largely unsafe, so more work to be done
  • Crystal was designed for thread-safety by default, but the existing code out there is largely targeting single-threaded code, so much work must be done to get it fully multithreading-safe
  • Need to add support for io_uring; support is in near-final phase, it's been being worked on for a while, preview might come in the next few weeks
  • Windows support! Has been there for a while, but there are a few tiny things that need to be resolved; almost there. Won't say it's supported until it's fully supported.
• Can learn about Crystal from the language reference/Exercism track
• Question: Compatible with Ruby?
  • Very simple programs could work, but Crystal needs some type annotations here and there.
  • Things have diverged a fair bit
  • There's a library that allows for interacting between Crystal and Ruby; can compile Crystal and make it available to Ruby