Value restrtictions in function parameters

Help & Support
1

When I write a function I can declare the type of the parameters fibonacci(n : Int32), but can I further restrict it to, for example, only positive integers, or only even numbers? What is the recommended way for input validation for functions?

2

There is no such functionality. You just raise an exception in such a case.

https://crystal-lang.org/api/1.0.0/ArgumentError.html

3

Or… you can use macros!

Without the error…

macro fib(n)
  {% raise "fib expected n > 0, got #{n}" if n < 0 %}
  {% if n < 2 %}
    {{ n }}
  {% else %}
    {% prev, curr = 0, 1 %}
    {% (0...n-1).each { prev, curr = curr, curr + prev } %}
    {{ curr }}
  {% end %}
end

puts fib(1)
puts fib(10)
4

I can’t begin to imagine how dependent types would work in crystal, but they sure would be cool.

2 Likes
5

erlang guard?

fibonacci(n : Int32) where(i>0)
or
fibonacci(n : Int32) where(i.odd?)

1 Like
6

Nice, but I’d only do the check in macro code and compute normally at runtime.
fib(rand(10)) (or any non-constant value for that matter) doesn’t work if it’s all done in macro.
And I bet if the argument is constant the compiler will optimize the whole thing at compile time anyways.

8

Maybe something like GitHub - Groogy/crystal-clear: Design by Contract for Crystal could be useful for this? I really think this should use annotations, though.

2 Likes