Just curious about a code in Base

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

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


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.

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

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

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).


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

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

julia> foo(1)

julia> bar(1)

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

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

julia> baz(1)
ERROR: InexactError: Int64(2.1)
 [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

Thanks, I didn’t know it!

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

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