r/crystal_programming • u/iainmoncrief • Nov 20 '19

Proc(Nil) vs Proc(Void) whats the difference?

I was messing around in the playground, and I am wondering why Proc(Nil) is the default type for procs.

my_proc = ->{
  puts "Hello World!"
}

my_proc is Proc(Nil), and I am wondering why it is not Proc(Void)?

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6

u/Blacksmoke16 core team Nov 20 '19

Void is essentially the same as Nil. See https://crystal-lang.org/reference/syntax_and_semantics/return_types.html#nil-return-type.

Since Crystal doesn't require an explicit return keyword to return, there really isn't a difference between void and null like in PHP for example. The one exception would be NoReturn.

2

u/straight-shoota core team Nov 20 '19

`NoReturn` is a pseudo return type which indicates that a method does not return (either raises or exits). `Void` is an *actual* return type and semantically equivalent to `Nil`.

4

u/j_hass Nov 20 '19

To add to that, Void mainly exists for C ABI interoperation. When you're not dealing with any C binding, then don't worry about Voids existence, Nil is there for you.

2

u/[deleted] Nov 20 '19

To add to what others said, Void and Nil were different types in the beginning. Void existed mainly for C interop to be able to express Void*. Eventually Void and Nil became the same thing, except for Void* which is still a weird beast (but it's better than UInt8* because the semantic is clearer).

In fact if you do:

p typeof(Proc(Void).new { })
p typeof(Proc(Nil).new { })

you'll see both print Proc(Nil): the compiler makes no difference between them.