Example C-lib binding in Crystal

triangle.cr

module Radians
  class Triangle
    lib C
      # In C: double cos(double x)
      fun cos(value : Float64) : Float64
    end

    def cos(value : Float64)
      C.cos(value)
    end
  end
end

tri = Radians::Triangle.new
puts tri.cos(0.0)
puts tri.cos(3.1415926535)