Union inconsistency

[TechMagister]

Crystal 0.20.1 [18e7617] (2016-12-05)
OS : Linux

Union are working half as expected in this case :

alias MyType = Int32 | String | Array(MyType)

my_hash = Hash(String, MyType).new

# this is ok, giving [1, 2, 3] is recognized as a MyType
def do_something(t : MyType)
  t.is_a? Array(Int32)			# => true
end

arr = [1,2,3]
do_something arr             # Working
my_hash["a"] = arr           # Compilation error here ??

The Array(Int32) is well recognized as a MyType ( Array(MyType) ) when given as function argument but not when giving it to the hash.

instance variable '@value' of Hash::Entry(String, MyType) must be MyType, not Array(Int32)

[RX14]

I think we need a well written up answer in a section of the crystal docs about this. Unfortunately I can't find the great answer that I seem to remember @asterite giving about this a while ago.

[asterite]

Hi,

Maybe it's an inconsistency, but it's the way it works. Type restrictions are different than proper types. For example:

def foo(x : Range(Int, Int))
end

foo 1..10 # Works
p typeof(1..10) # => Range(Int32, Int32)

We are passing Range(Int32, Int32). The restriction says "I accept a Range where both components are ints", and since Int32 < Int, it works.

In the case of the alias, Array(Int32) can be matched against Array(MyType) because MyType can also be Int32... however, it's incorrect to put an Array(Int32) inside an Array(MyType).

Check #1297

I'm closing this. We'll eventually solve this. I think this is one of the points that bring people most confusion... but then again, you should really try to avoid using unions and recursive types if possible.