# Outer constructor method is failing

**URL:** https://discourse.julialang.org/t/outer-constructor-method-is-failing/78979
**Category:** General Usage
**Created:** [April 4, 2022, 4:08am UTC](https://discourse.julialang.org/t/outer-constructor-method-is-failing/78979 "2022-04-04T04:08:29Z")
**Posts on this page:** 4
**Page:** 1

<div class="post-metadata">

### Author: ![Sijun](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@Sijun](https://discourse.julialang.org/u/Sijun)
#### Post date: [April 4, 2022, 4:08am UTC](https://discourse.julialang.org/t/outer-constructor-method-is-failing/78979/1 "2022-04-04T04:08:29Z")

</div>

I don’t really understand **why** in the following code `Payload(;a, b) = Payload(a=a, b=b, c=Float64[])` fails. It looks a legitimate expression to me.

```julia
module Test
Base.@kwdef mutable struct Payload
    a::Int
    b::String
    c::Vector{Float64}
end

Payload(;a, b) = Payload(a=a, b=b, c=Float64[])

@show Payload(a=1, b="apple")
end

WARNING: replacing module Test.
ERROR: MethodError: no method matching Main.Test.Payload(; a=1, b="apple", c=Float64[])
Closest candidates are:
  Main.Test.Payload(; a, b) at E:\work\julia\test2.jl:8 got unsupported keyword argument "c"        
  Main.Test.Payload(::Int64, ::String, ::Vector{Float64}) at E:\work\julia\test2.jl:3 got unsupported keyword arguments "a", "b", "c"
  Main.Test.Payload(::Any, ::Any, ::Any) at E:\work\julia\test2.jl:3 got unsupported keyword arguments "a", "b", "c"
Stacktrace:
 [1] kwerr(kw::NamedTuple{(:a, :b, :c), Tuple{Int64, String, Vector{Float64}}}, args::Type)
   @ Base .\error.jl:165
 [2] Main.Test.Payload(; a::Int64, b::String)
   @ Main.Test E:\work\julia\test2.jl:8
 [3] top-level scope
   @ show.jl:1045
 [4] eval
   @ .\boot.jl:368 [inlined]

```

This works fine:

```julia
module Test
Base.@kwdef mutable struct Payload
    a::Int
    b::String
    c::Vector{Float64}
end

Payload(;a, b) = Payload(a, b, Float64[])

@show Payload(a=1, b="apple")
end

```

---

<div class="post-metadata">

### Author: ![Sijun](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@Sijun](https://discourse.julialang.org/u/Sijun)
#### Post date: [April 4, 2022, 4:30am UTC](https://discourse.julialang.org/t/outer-constructor-method-is-failing/78979/2 "2022-04-04T04:30:05Z")

</div>

Ah, it looks like that default keyword outer constructor method is not provided once the user defines one for himself.

```julia
module Test
Base.@kwdef mutable struct Payload
    a::Int
    b::String
    c::Vector{Float64}
end

@show methods(Payload)

Payload(;a, b) = Payload(a, b, Float64[])

@show methods(Payload)

@show Payload(a=1, b="apple")
end

WARNING: replacing module Test.
methods(Payload) = # 3 methods for type constructor:
[1] Main.Test.Payload(; a, b, c) in Main.Test at util.jl:489
[2] Main.Test.Payload(a::Int64, b::String, c::Vector{Float64}) in Main.Test at E:\work\julia\test2.jl:3
[3] Main.Test.Payload(a, b, c) in Main.Test at E:\work\julia\test2.jl:3
methods(Payload) = # 3 methods for type constructor:
[1] Main.Test.Payload(; a, b) in Main.Test at E:\work\julia\test2.jl:10
[2] Main.Test.Payload(a::Int64, b::String, c::Vector{Float64}) in Main.Test at E:\work\julia\test2.jl:3
[3] Main.Test.Payload(a, b, c) in Main.Test at E:\work\julia\test2.jl:3
Payload(a = 1, b = "apple") = Main.Test.Payload(1, "apple", Float64[])

```

Is this something similar in spirit to:

> If any inner constructor method is defined, no default constructor method is provided: it is presumed that you  
> have supplied yourself with all the inner constructors you need.

---

<div class="post-metadata">

### Author: ![gustaphe](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gustaphe/32/18174_2.png) [@gustaphe](https://discourse.julialang.org/u/gustaphe)
#### Post date: [April 4, 2022, 4:33am UTC](https://discourse.julialang.org/t/outer-constructor-method-is-failing/78979/3 "2022-04-04T04:33:40Z")

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No, it’s not that. But we don’t dispatch on kwargs, so when you define the `(;a, b)` constructor you overwrite the automatic `(; a, b, c)` constructor. They are both equivalent to a constructor with no arguments to dispatch.

The solution is to make `c` an optional kwarg.

---

<div class="post-metadata">

### Author: ![Sijun](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@Sijun](https://discourse.julialang.org/u/Sijun)
#### Post date: [April 4, 2022, 4:51am UTC](https://discourse.julialang.org/t/outer-constructor-method-is-failing/78979/5 "2022-04-04T04:51:19Z")

</div>

Thank you so much for reminding me of we don’t dispatch on kwargs!