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Examples

Crystal is a general-purpose, object-oriented programming language.With syntax inspired by Ruby, it’s a compiled language with static type-checking.Types are resolved by an advanced type inference algorithm.

Language specification

# A very basic HTTP serverrequire"http/server"server=HTTP::Server.newdo|context|context.response.content_type="text/plain"context.response.print"Hello world, got#{context.request.path}!"endaddress=server.bind_tcp(8080)puts"Listening on http://#{address}"# This call blocks until the process is terminatedserver.listen

Batteries included

Crystal’s standard library comes with a whole range of libraries that let you start working on your project right away.

Check the API docs

require"http/client"require"json"response=HTTP::Client.get("https://crystal-lang.org/api/versions.json")json=JSON.parse(response.body)version=json["versions"].as_a.find!{|entry|entry["released"]?!=false}["name"]puts"Latest Crystal version:#{version||"Unknown"}"

Type system

The compiler catches type errors early. Avoids null pointer exceptions at runtime.

The code is still clean and feels like a dynamic language.

More about the type system

defadd(a,b)a+bendadd1,2# => 3add"foo","bar"# => "foobar"

Flow typing

The compiler tracks the type of variables at each point,and restricts types according to conditions.

loopdocasemessage=gets# type is `String | Nil`whenNilbreakwhen""puts"Please enter a message"else# In this branch, `message` cannot be `Nil` so we can safely call `String#upcase`putsmessage.upcaseendend

Concurrency Model

Crystal uses green threads, called fibers, to achieve concurrency.Fibers communicate with each other via channels without having to turn to shared memory or locks (CSP).

C-bindings

Bindings for C libraries makes it easy to use existing tools.Crystal calls lib functions natively without any runtime overhead.

No need to implement the entire program in Crystal when there are alreadygood libraries for some jobs.

Learn how to bind to C libraries

# Define the lib bindings and link info:@[Link("m")]libLibMfunpow(x:LibC::Double,y:LibC::Double):LibC::Doubleend# Call a C function like a Crystal method:putsLibM.pow(2.0,4.0)# => 16.0

Macros

Crystal’s answer to metaprogramming is a powerful macro system, which ranges from basic templating and AST inspection, to types inspection and running arbitrary external programs.