[image] How can we maintain easy generics in the face of demand for a “safe” language? Ah, right, I forgot that return type annotations are convert statements, not type checks, making them much less useful. That utterly baffled me when I found out. A return annotation has the same appearance a…

The :: operator generally means “this will be of this type”. When does it convert? The simple answer is that we try to autoconvert whenever it’s sensible to do so. This rule gives Julia a lot of it’s “dynamic language” flavor. For example, the fact that you can make a Vector{String} array and then a…

[image] StefanKarpinski: My point is this. reinterpret is asking what the literal bits of the representation of Char happen to be. Note also that the mismatch between reinterpret(T, x) and convert(T, x) or T(x) also holds for many other types. For example: julia> x = 1.0f0 1.0f0 julia> con…

[image] sadish-d: Could we have a ::: then for people who want a non-converting declaration? Making methods is easier than changing the parser to allow a whole new syntax that people have to learn and distinguish from the verrrrry similar ::. julia> struct A a::Int end julia> A('a') # we do…

[image] StefanKarpinski: As I said before, this is really more of an argument for why the convert(Integer, ::Char) method shouldn’t exist. This much we vigorously agree on. [image] StefanKarpinski: Returning the internal representation of the Char would not be fine. This on the other …

While I agree that convert(Integer, ::Char) is a problem, there are other aspects of :: declarations which I find inconsistent (though maybe others might not). :: declarations privilage primative types over user-defined types. If I have struct A a end I can’t just go ahead and do x::A = 1 I …

[image] mnemnion: I might be the only person on the planet who thought it would do the same thing for Char, for all I know. This isn’t unreasonable — UInt32(char) == codepoint is probably the most obvious representation of a Unicode Char, and I expect that it is the one used in most other lan…

[image] sadish-d: I can’t just go ahead and do x::A = 1 It makes sense that you can’t. Passing something into A’s constructor is not semantically equivalent to converting that thing to type A. Ref(1) makes sense, but convert(Ref, 1) doesn’t. [image] sadish-d: I need to first define a Ba…

Right, that makes sense. My point was just that a Base.convert method needs to be defined for user-defined types, which disadvantages them over primitive types. This was a counterargument that conversion of Char to Int is not the (only) thing that’s weird to me about ::, but that a convert is called…

[image] sadish-d: My point was just that a Base.convert method needs to be defined for user-defined types, which disadvantages them over primitive types. “Primitive types” like Char and Int are implemented entirely in Julia (with a couple exceptions like Array), so in that sense they don’t ha…

[image] sadish-d: function foo(x::Integer)::Float64 answer = pi * x return answer end But since I learned that it applies conversion, I do: function foo(x::Integer) answer = pi * x return answer::Float64 end This is a trick that I never thought about. Sometimes I use the first option…

So question. I have the functions f and g: function f(x::Int64)::Float64 return x end function g(x::Int64) return Base.convert(Float64, x)::Float64 end g is doing a conversion before returning. Is f doing that too? Because that would be wild. f lowers to: CodeInfo( 1 ─ %1 = Base.convert │ %2 =…

Semantics of :: in return type vs. argument type annotations

General Usage

stevengj Split this topic February 8, 2024, 9:58pm 42

22 posts were split to a new topic: Why assignment operators return the right-hand-side

Topic		Replies	Views
Is there a reason *not* to explicitly define return types? New to Julia question	35	1311	November 3, 2024
On type annotations New to Julia	88	3197	March 8, 2025
Should Julia raise errors for illegal return type annotation? Internals & Design	6	2669	September 4, 2021
How is it that new Julia programmers tend to abuse type annotations? Teaching & Outreach	96	5527	January 15, 2024
Type stable Base.return_types - is it possible? General Usage	21	712	September 6, 2025

Semantics of :: in return type vs. argument type annotations

Related topics