# Can typeof() return an alias?

**URL:** <https://discourse.julialang.org/t/can-typeof-return-an-alias/89412>\
**Category:** General Usage\
**Tags:** type\
**Created:** [October 28, 2022, 12:56am UTC](https://discourse.julialang.org/t/can-typeof-return-an-alias/89412 "2022-10-28T00:56:35Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Lincoln\_Hannah](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lincoln_hannah/32/19198_2.png) [@Lincoln\_Hannah](https://discourse.julialang.org/u/Lincoln_Hannah)\
**Post date:** [October 28, 2022, 12:56am UTC](https://discourse.julialang.org/t/can-typeof-return-an-alias/89412/1 "2022-10-28T00:56:35Z")

</div>

Is it possible to have a type alias where `typeof()` returns the alias rather than the base type  
e.g.

```julia
XString = String
x::XString = "ab"
typeof(x) # gives XString not String

```

**Use Case** The below code produces a string from the `::String` elements of a struct.

```julia

using NamedTupleTools, Dictionaries, Chain, Lazy, Parameters

◀join(a,b) = join(b,a)
◀filter(a,b) = filter(b,a)
◀map(a,b) = map(b,a)

@with_kw struct X
    a::String
    b::String
    c::Int
end

XtoString(x::X) = @chain x begin
    ntfromstruct
    Dictionary
    ◀filter(x->typeof(x)==String)
    pairs
    ◀map(( (k,v),) -> "$k=$v" )
    join(',')
end

@> X( a= "blue", b="green", c = 1) XtoString #"a=blue,b=green"

```

I’d like to instead filter on `typof(x)==XString`. e.g.

```julia
@with_kw struct X2
    a::XString
    b::XString
    c::String
end

@> X( a= "blue", b="green", c ="red") X2toString #"a=blue,b=green"

```

If there’s a better way to build this function would be interested as well.

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [October 28, 2022, 2:12am UTC](https://discourse.julialang.org/t/can-typeof-return-an-alias/89412/2 "2022-10-28T02:12:30Z")

</div>

no

```julia
XString = String

```

this is just the same type, it’s like `a=3` and you can’t see “a” no matter what you do when variable `a` is passed to a function.

The minimal way to custom display is to make a new type

```julia
struct XString
    data::String
end

```

---

<div class="post-metadata">

**Author:** ![Lincoln\_Hannah](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lincoln_hannah/32/19198_2.png) [@Lincoln\_Hannah](https://discourse.julialang.org/u/Lincoln_Hannah)\
**Post date:** [October 28, 2022, 2:17am UTC](https://discourse.julialang.org/t/can-typeof-return-an-alias/89412/3 "2022-10-28T02:17:31Z")

</div>

unfortunately that makes it a structure not a String. So i can’t do

```julia
x = XString("abc")
x * "def"

```

to get the string, you’d always have to write x.data

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [October 28, 2022, 2:21am UTC](https://discourse.julialang.org/t/can-typeof-return-an-alias/89412/4 "2022-10-28T02:21:36Z")

</div>

well you have to teach it how to `*`

---

<div class="post-metadata">

**Author:** ![bertschi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bertschi/32/33462_2.png) [@bertschi](https://discourse.julialang.org/u/bertschi)\
**Post date:** [October 28, 2022, 6:58am UTC](https://discourse.julialang.org/t/can-typeof-return-an-alias/89412/5 "2022-10-28T06:58:11Z")

</div>

`XString` is just an alias of `String`, i.e., it denotes the same type for all practical purposes. In particular,

```julia
julia> XString = String
String

julia> XString == String
true

```

and accordingly you can simply replace `String` with `XString` in your filter condition:

```julia
julia> filter(x -> typeof(x) == String, [1, "a", 2, "bc"])
2-element Vector{Any}:
 "a"
 "bc"

julia> filter(x -> typeof(x) == XString, [1, "a", 2, "bc"])
2-element Vector{Any}:
 "a"
 "bc"

```

I.e., it does not matter that `typeof(x)` will return (and print) `String` as its just the same an `XString` per the assignment `XString = String`. If there is no need to ever reassign the type alias, I would probably use `const XString = String` which makes it even more clear that `XString` is just an alternative name for `String`.  
Note that this also means that your code will work with regular strings as there is no way to ever distinguish them from `XString`s. If it is required that they are separate types, you will need to create a new type as suggested by @jling and define all methods required on that type, e.g., by forwarding to the underlying `data` field:

```julia
struct MyXString
    data::String
end

Base.:*(u::MyXString, v::MyXString) = MyXString(u.data * v.data)

```