# Wrong dispatch due to kwargs?

**URL:** <https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977>\
**Category:** New to Julia\
**Created:** [April 2, 2023, 5:42pm UTC](https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977 "2023-04-02T17:42:24Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![freeman](https://avatars.discourse-cdn.com/v4/letter/f/ec9cab/32.png) [@freeman](https://discourse.julialang.org/u/freeman)\
**Post date:** [April 2, 2023, 5:42pm UTC](https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977/1 "2023-04-02T17:42:24Z")

</div>

It seems to me that the dispatch system is doing something wrong. Apologies ahead of time if there’s something blindingly obvious that I’m missing here.

See the examples below:

This is as expected:

```julia
function func(
    x::Vector{Char};
    convert::Bool=true,
)
    println("x=$x")
end

x = "julia" |> collect # typeof Vector{Char}
func(x) # this line prints: x=['j', 'u', 'l', 'i', 'a']

```

But what’s going on here?!

```julia
function func(
    x::Vector{Char};
    convert::Bool=true,
    other::Vector{String}=[],
)
    println("x=$x")
end

x = "julia" |> collect
func(x)
# ERROR: MethodError: no method matching var"#func#44"(::Bool, ::Vector{Any}, ::typeof(func), ::Vector{Char})
# Closest candidates are:
# var"#func#44"(::Bool, ::Vector{String}, ::typeof(func), ::Vector{Char}) at ~/repos/test_arrow_compression.jl:30

```

It tells me no method matching `func(::Bool,...` which I’m not trying to call and suggests that the closest candidate is `func(::Bool...` which doesn’t exist. I suspect this Bool comes from the `convert` kwarg.

In the process of building this minimal example I actually found a way of making it work:

```julia
function func(
    x::Vector{Char};
    convert::Bool=true,
    other::Vector{String}=String[],
)
    println("x=$x")
end

x = "julia" |> collect
func(x) # this line prints: x=['j', 'u', 'l', 'i', 'a']

```

If you’ll make experiments with this, make sure you clear the function each time so that the experiments are independent:

```julia
for m in Base.methods(func)
    Base.delete_method(m)
end

```

I’ve checked that this happens with both `1.8.5` and `1.9.0-rc1`:

```julia
julia> versioninfo()
Julia Version 1.8.5
Commit 17cfb8e65ea (2023-01-08 06:45 UTC)
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × Intel(R) Core(TM) i7-8665U CPU @ 1.90GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-13.0.1 (ORCJIT, skylake)
  Threads: 1 on 8 virtual cores
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 

```

---

<div class="post-metadata">

**Author:** ![tomerarnon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomerarnon/32/3170_2.png) [@tomerarnon](https://discourse.julialang.org/u/tomerarnon)\
**Post date:** [April 2, 2023, 10:28pm UTC](https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977/2 "2023-04-02T22:28:58Z")

</div>

The error message is misleading, and someone more qualified to comment on what can be done should chime in there.  
~~But this is happening because you’re naming a keyword argument `convert`, which is the built-in function required for turning an `Any[]` into a `String[]`. So when you type it correctly and don’t need the convert operation, the error magically goes away.~~  
^ this was incorrect. See correct answer below.

---

<div class="post-metadata">

**Author:** ![freeman](https://avatars.discourse-cdn.com/v4/letter/f/ec9cab/32.png) [@freeman](https://discourse.julialang.org/u/freeman)\
**Post date:** [April 3, 2023, 2:01am UTC](https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977/3 "2023-04-03T02:01:20Z")

</div>

I’m not sure I understood. Even giving it another name, it still fails the same way:

```julia
function func(
    x::Vector{Char};
    convert_::Bool=true,
    other::Vector{String}=[],
)
    println("x=$x")
end

```

---

<div class="post-metadata">

**Author:** ![torrance](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/torrance/32/38990_2.png) [@torrance](https://discourse.julialang.org/u/torrance)\
**Post date:** [April 3, 2023, 2:14am UTC](https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977/4 "2023-04-03T02:14:27Z")

</div>

Your error is because you have typed `other` as a `Vector{String}` but have provided a default value `[]` whose type is `Vector{Any}`.

(Since no type information is given, `[]` must be typed as a container to accept values of any type. You can check this as `typeof([]) == Vector{Any}`.)

The multiple dispatch is then failing because when you don’t provide an explicit value for `other`, you are in fact calling it as `f(x::Vector{Car}, true::Bool, []::Vector{Any})`, which isn’t defined.

This will work if you provide the default value as `String[]`.

---

<div class="post-metadata">

**Author:** ![freeman](https://avatars.discourse-cdn.com/v4/letter/f/ec9cab/32.png) [@freeman](https://discourse.julialang.org/u/freeman)\
**Post date:** [April 3, 2023, 4:43am UTC](https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977/5 "2023-04-03T04:43:54Z")

</div>

> [@torrance](#):
>
> you are in fact calling it as `f(x::Vector{Car}, true::Bool, []::Vector{Any})`, which isn’t defined.

The error message says something completely different.

```julia
MethodError: no method matching var"#func#44"(::Bool, ::Vector{Any}, ::typeof(func), ::Vector{Char})

```

---

<div class="post-metadata">

**Author:** ![sylvaticus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sylvaticus/32/203883_2.png) [@sylvaticus](https://discourse.julialang.org/u/sylvaticus)\
**Post date:** [April 3, 2023, 4:49am UTC](https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977/6 "2023-04-03T04:49:43Z")

</div>

Just a reminder: in Julia (multiple) dispatch operates only on positional arguments, not on keyword based ones.

---

<div class="post-metadata">

**Author:** ![torrance](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/torrance/32/38990_2.png) [@torrance](https://discourse.julialang.org/u/torrance)\
**Post date:** [April 3, 2023, 5:28am UTC](https://discourse.julialang.org/t/wrong-dispatch-due-to-kwargs/96977/7 "2023-04-03T05:28:38Z")

</div>

> [@freeman](#):
>
> The error message says something completely different.

Yeah it looks like that type signature is the result of using the keyword arguments. It’s created a number of wrapped function depending on your arguments which I’ll admit makes the error message confusing.

It’s simpler if we provide default values to positional arguments - we can generate the same error and the type signature is more straightforward:

```julia
func(x::Vector{Char}, convert_::Bool=true, other::Vector{String}=[]) = println("x=$x")

func(Char['h', 'i']) # LoadError: MethodError: no method matching func(::Vector{Char}, ::Bool, ::Vector{Any})

```

(Note I’ve removed the semicolon from the argument list.)

The same thing is going on in the case where you’ve used keyword arguments: ultimately your calling types don’t match your declared types.