# Performance of type annotations
**URL:** https://discourse.julialang.org/t/performance-of-type-annotations/8152
**Category:** Internals & Design
**Created:** [January 4, 2018, 10:59am UTC](https://discourse.julialang.org/t/performance-of-type-annotations/8152 "2018-01-04T10:59:45Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@julbinb](https://discourse.julialang.org/u/julbinb)
#### Post date: [January 4, 2018, 10:59am UTC](https://discourse.julialang.org/t/performance-of-type-annotations/8152/1 "2018-01-04T10:59:45Z")
Docs say that type annotations on variables and return types of functions facilitate type inference and performance. But does the opposite hold? If I have a function:
```julia
function foo(x)::Number
if isa(x, Number)
return x
else
return 0
end
end
```
Will the `convert(Number, ...)` method be called on the return value of `foo`, or it will be a no-op, for it is clear that both `x` and `0` are `Number`s?
In general, if I have a variable with type annotation, is `convert` called at every assignment to the variable or it optimized away whenever type inference is sure about the type of RHS?
---
### Author:  [@sdanisch](https://discourse.julialang.org/u/sdanisch)
#### Post date: [January 4, 2018, 11:11am UTC](https://discourse.julialang.org/t/performance-of-type-annotations/8152/2 "2018-01-04T11:11:04Z")
Julia has excellent tools to answer those questions 🙂
E.g. use `@code_warntype` and `@code_lowered` to figure out what happens (you can use `?@code_lowered` in the REPL to get the doc string for those macros):
```julia
julia> function foo(x)::Number
if isa(x, Number)
return x
else
return 0
end
end
foo (generic function with 1 method)
julia> @code_lowered foo(1)
CodeInfo(:(begin
nothing
SSAValue(0) = Main.Number
unless x isa Main.Number goto 6 # line 3:
return (Core.typeassert)((Base.convert)(SSAValue(0), x), SSAValue(0))
6: # line 5:
return (Core.typeassert)((Base.convert)(SSAValue(0), 0), SSAValue(0))
end))
julia> @code_warntype foo(1)
Variables:
#self#
x::Int64
Body:
begin # line 3:
return x::Int64 # line 5:
end::Int64
julia> @code_warntype foo(1.0)
Variables:
#self#
x::Float64
Body:
begin # line 3:
return x::Float64 # line 5:
end::Float64
julia> @code_warntype foo("1")
Variables:
#self#
x
Body:
begin
goto 3 # line 3:
3: # line 5:
return 0
end::Int64
```
So Julia seems to turn each `return` into `convert(Number, x)::Number`. The type assert and convert get inlined and removed.
---
### Author:  [@julbinb](https://discourse.julialang.org/u/julbinb)
#### Post date: [January 4, 2018, 11:35am UTC](https://discourse.julialang.org/t/performance-of-type-annotations/8152/3 "2018-01-04T11:35:51Z")
@sdanisch Indeed, thanks a lot!