On type annotations

New to Julia
82

To be absolutely clear, erroring at declaring another local variable of the same name does not involve the annotations at all:

julia> function foo(x)
        local x
       end
ERROR: syntax: local variable name "x" conflicts with an argument

It’s very clearly documented that argument annotations serve multiple dispatch. Nowhere does it say that it restricts the type of instances assigned to a variable. I would argue that would be needlessly restrictive. Say you have a function foo that takes in either a x::Int or x::Float64, and x could switch types in the body x = bar()::Union{Int, Float64}. However, you need to dispatch to separate methods depending on what type the input x is. If the argument annotation restricted x’s type, it’s impossible to pull this off.

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83

Regardless whether it’s considered a bug it would be quite breaking to change and not really an option before 2.0. It could be a candidate for the mechanism proposed in Add `strict` mechanism for opting into stricter subsets of the language · Issue #54903 · JuliaLang/julia · GitHub though.

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84

Actually, what is the reason of not allowing introducing const within a local scope like a function definition?

1 Like
85

See implement local const · Issue #5148 · JuliaLang/julia · GitHub

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86

I’m prototyping a macro to get rid of the conversions here:

julia> @noconvert function foo(x::Number)
           let x::Int = x
               x
           end
       end
foo (generic function with 2 methods)

julia> foo(3)
3

julia> foo(3.0)
ERROR: TypeError: in typeassert, expected Int64, got a value of type Float64

julia> foo(3.5)
ERROR: TypeError: in typeassert, expected Int64, got a value of type Float64

The code generation looks pretty clean:

julia> @code_llvm foo(3)
;  @ none within `foo`
define i64 @julia_foo_1572(i64 signext %0) #0 {
top:
  ret i64 %0
}

julia> @code_llvm foo(3.0)
;  @ none within `foo`
; Function Attrs: noreturn
define void @julia_foo_1574(double %0) #0 {
top:
  call void @j_overdub_1576(double %0) #6
  unreachable
}
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