TIL Tuples vs Arrays in Crystal

Today I played around with Crystal... the programming language (🥁 ba-dum-tss).

Coming from Ruby, Crystal isn't a mystery in many cases, but in the end, I don't know much about it at all.

However, I learned something about Tuples and Arrays today.

---

It starts simple with a class that takes a String as an argument.

class TupleVsArray
  def initialize(@text : String)
    puts "Text: #{@text}"
  end
end

---

Now let's decomposit an array into two variables, and then pass the first variable, foo, into the class:

foo, bar = ["foo", "bar"]
TupleVsArray.new(foo)     # Text: foo

This works fine.
All elements in the array are of type String.
It compiles, it runs!

---

Let's change the second element in the array into a Symbol:

foo, bar = ["foo", :bar]
TupleVsArray.new(foo)

This fails during compiling with this error:

Error: no overload matches 'TupleVsArray.new' with type (String | Symbol)

To me, having little to 0 experience with type checking, this is fascinating... and confusing, because foo is allegedly never going to be a Symbol. Allegedly...

Then I realized that the array could be changed in the meantime, for example:

row = ["foo", :bar]
row[0] = :foo
foo, bar = row
TupleVsArray.new(foo)

# Error: no overload matches 'TupleVsArray.new' with type (String | Symbol)

This time, the error makes absolutely sense.

---

Then I also learned, that in Crystal we cannot change an array's element to an not-yet-existing type:

row = ["foo", :bar]
row[0] = 42
Error: no overload matches '=' with types Int32, Int32

If we want to change any element in row, we must only use String or Symbol.

---

So, in order to resolve the original problem, we need to use a Tuple, which is a

[...] fixed-size, immutable, stack-allocated sequence of values of possibly different types.

foo, bar = {"foo", :bar}
TupleVsArray.new(foo)     # Text: foo

Alright, got it. 😊

(Photo by Cristofer Jeschke on Unsplash)