Just curious about a code in Base

General Usage
1

Hi, I was hanging around Base and I found this method.

@edit Base.hash_32_32(UInt32(1)) # juila 1.6.1

function hash_32_32(n::UInt32)
    a::UInt32 = n # what is it the purpose of this line?
    a = a + 0x7ed55d16 + a << 12
    a = a ⊻ 0xc761c23c ⊻ a >> 19
    a = a + 0x165667b1 + a << 5
    a = a + 0xd3a2646c ⊻ a << 9
    a = a + 0xfd7046c5 + a << 3
    a = a ⊻ 0xb55a4f09 ⊻ a >> 16
    return a
end

I have no idea of the purpose of the first line of the method!
Why is it necessary the type assertion?

2

The first line probably exists to avoid mutating n. If you got rid of that line, it would be customary to call the function hash_32_32! to make it clear that it returns the answer in place (EDIT: This is nonsense, sorry!). If you are only curious about the type assertion, I agree it seems redundant.

3

That is not right I think. The value of the variable n is immutable. It is not necessary to work with a copy.

4

Thanks, I’m actually more curious now! What do you mean with mutating a UInt32?

5

Operations on numbers (like UInt32) are never “in place” (since they’re immutable) so the function wouldn’t need the !. The reason for the line a::UInt32 is that, in that context, the type annotation is a “guarantee” that a will always be a UInt32, no matter what happens to it. In practice, this means that every a = a... operation is followed by a convert(UInt32, a).

See:

julia> function foo(x::Int)
           y::Int = x
           y += 1.0
           y
       end
foo (generic function with 1 method)

julia> function bar(x::Int)
           y = x
           y += 1.0
           y
       end
bar (generic function with 1 method)

julia> foo(1)
2

julia> bar(1)
2.0

edit with another example, with a number that cannot be converted back to Int

julia> function baz(x)
           y::Int = x
           y += 1.1
           y
       end
baz (generic function with 1 method)

julia> baz(1)
ERROR: InexactError: Int64(2.1)
Stacktrace:
 [1] Int64 at ./float.jl:710 [inlined]
 [2] convert at ./number.jl:7 [inlined]
 [3] baz(::Int64) at ./REPL[39]:3
 [4] top-level scope at REPL[40]:1
6

Thanks, I didn’t know it!

7

Those this feature of the type assertions have a name? It it for changing the title to a more informative one!

8

It doesn’t seem to have its own name. See the second paragraph of this section for more info Types · The Julia Language