# Functions in an input file-- how?

**URL:** <https://discourse.julialang.org/t/functions-in-an-input-file-how/3707>\
**Category:** General Usage\
**Created:** [May 14, 2017, 4:14pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707 "2017-05-14T16:14:20Z")\
**Posts on this page:** 16\
**Page:** 2

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 15, 2017, 2:29pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/21 "2017-05-15T14:29:00Z")

</div>

The problem `FunctionWrappers.jl` tries to solve is explained in the [issue](https://github.com/JuliaLang/julia/issues/13984) linked in the package [readme](https://github.com/yuyichao/FunctionWrappers.jl). It doesn’t make calling anonymous function faster, in fact it makes it a little slower since there’ll be no inlining. It does eliminate the dispatch if you have a set of user defined functions that will be called with identical signatures.

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<div class="post-metadata">

**Author:** ![PetrKryslUCSD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/petrkryslucsd/32/215825_2.png) [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)\
**Post date:** [May 15, 2017, 3:03pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/22 "2017-05-15T15:03:16Z")

</div>

Types for functions? Well, Julia has come a long way since I last got to play with it.  
But I can’t see in the documentation how you write the type for a function to pass  
as a parameter. What is F?

```julia
immutable Neumann{F}
    f::F
end

```

For instance, when I write

```julia
g(i::Int)=i

```

I get back

```julia
typeof(g)
#g

```

???

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<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [May 15, 2017, 3:06pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/23 "2017-05-15T15:06:49Z")

</div>

```julia
immutable Neumann{F}
    f::F
end

```

`F` is a type parameter and the type will be “specialized” on that type parameter. You can read more at [https://docs.julialang.org/en/release-0.4/manual/types/#man-parametric-types](https://docs.julialang.org/en/release-0.4/manual/types/#man-parametric-types).

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 15, 2017, 3:09pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/24 "2017-05-15T15:09:45Z")

</div>

> [@PetrKryslUCSD](#):
>
> how you write the type for a function to pass  
> as a parameter.

Just write `typeof(g)` if you have to.

---

<div class="post-metadata">

**Author:** ![PetrKryslUCSD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/petrkryslucsd/32/215825_2.png) [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)\
**Post date:** [May 15, 2017, 6:07pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/25 "2017-05-15T18:07:42Z")

</div>

But what if I want to define a type, say:

type A  
f::Function  
end

where I would now like to replace ::Function with the type of this function

function f(a::Array{Float64,2})::Float64  
…

How do I do this please?

Thanks.

P

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 15, 2017, 6:16pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/26 "2017-05-15T18:16:25Z")

</div>

```julia
type A
    f::typeof(f)
end

```

A non-parametric type like this is pretty useless though. Especially since it’s not even a closure…

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [May 15, 2017, 7:17pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/27 "2017-05-15T19:17:40Z")

</div>

There is not really any advantage over what you are trying to do compared to my version with just `F`.

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<div class="post-metadata">

**Author:** ![PetrKryslUCSD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/petrkryslucsd/32/215825_2.png) [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)\
**Post date:** [May 15, 2017, 7:20pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/28 "2017-05-15T19:20:41Z")

</div>

Where do you get F I wonder? Do you _have_ to use typeof()?

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [May 15, 2017, 7:23pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/29 "2017-05-15T19:23:09Z")

</div>

You don’t need to specify `F`, it is done automatically when the type is created:

```julia
julia> immutable Neumann{F}
           f::F
       end

julia> f(x) = x^2
f (generic function with 1 method)

julia> n = Neumann(f)
Neumann{#f}(f)

julia> n.f(3)
9

julia> assemble(n::Neumann, x) = n.f(x) # Usage is optimized
assemble (generic function with 1 method)

julia> @code_llvm assemble(n, 2.0)

define double @julia_assemble_60683(double) #0 !dbg !5 {
top:
  %1 = fmul double %0, %0
  ret double %1
}

```

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<div class="post-metadata">

**Author:** ![PetrKryslUCSD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/petrkryslucsd/32/215825_2.png) [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)\
**Post date:** [May 15, 2017, 7:24pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/30 "2017-05-15T19:24:10Z")

</div>

I don’t think it is useless: say it is in a library, and the lib will set up the arguments and expects a certain type on return. What is useless about matching what is expected anyway?

But: Does one have to use typeof() or can the type be constructed in any other way?

Thanks.

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 15, 2017, 7:44pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/31 "2017-05-15T19:44:44Z")

</div>

It is more useful to parametrize the object shown in @kristoffer.carlsson’s reply. It’s useless when it’s not parametrized since you’ve limited the type to a single function. Basically, if you know that the user have to pass in a `sin` function, why don’t you just not letting the user passing it in at all.

The “other way” is exactly using a parametrized type/function and the dispatch will figure out the type parameter for you automatically.

For the few cases I’ve seen function type or type of callable are useful are,

- Specialization on a certain function:

- Use as a field type, see @kristoffer.carlsson’s code above.

- Use in a container, this is where `FunctionWrappers.jl` is useful. It allows you to store multiple different functions in a leaftype container and load/call them without any dispatch overhead as long as you specify the type behavior of the function.

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<div class="post-metadata">

**Author:** ![PetrKryslUCSD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/petrkryslucsd/32/215825_2.png) [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)\
**Post date:** [May 15, 2017, 8:06pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/32 "2017-05-15T20:06:36Z")

</div>

@yuyichao:

Perhaps I’m misunderstanding how this thing works. If I write  
double (\*)(double, double)  
in the C language, any number of functions are going to match the signature. Are you saying that if I have

type A  
f::F  
end

in Julia, the type F is literally going to match only a single function when F= typeof(somefun)?

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<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 15, 2017, 8:54pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/33 "2017-05-15T20:54:14Z")

</div>

Correct. And `double (*)(double, double)` (or more generally `std::function<double(double,double)>`) is exactly what `FunctionWrappers.jl` does. It was actually the first sentence in the issue I linked above [https://github.com/JuliaLang/julia/issues/13984#issue-116879024](https://github.com/JuliaLang/julia/issues/13984#issue-116879024) . Function in julia are all functors, making anonymous functions, callable objects, and functions the same thing afa calling them is concerned. You get the same behavior in C++ if you do `decltype` on a lambda. This behavior is very useful to allow the maximum amount of optimizations, e.g. the callback can be inlined in to the function you are passing it to. It is bad if you want to have a list of them with different types or want to reduce specialization on them. That’s exactly why c++ have `std::function` AFAIK and is also what [https://github.com/JuliaLang/julia/issues/13984#issue-116879024](https://github.com/JuliaLang/julia/issues/13984#issue-116879024) mentions in the second sentence…

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**Author:** ![PetrKryslUCSD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/petrkryslucsd/32/215825_2.png) [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)\
**Post date:** [May 16, 2017, 1:52pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/34 "2017-05-16T13:52:30Z")

</div>

@yuyichao:

Thank you very much for the thorough explanation. By the way, something like this should be somewhere in the documentation. It is either well hidden or absent at this point. I couldn’t find any mention of declaring the type of a function in the way described in this thread anywhere.

Petr

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**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 16, 2017, 3:55pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/35 "2017-05-16T15:55:36Z")

</div>

I’m 80% sure the use of `typeof(f)` and function parameter are both described in the doc. Improvements to the doc are always welcome…

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**Author:** ![PetrKryslUCSD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/petrkryslucsd/32/215825_2.png) [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)\
**Post date:** [May 16, 2017, 9:43pm UTC](https://discourse.julialang.org/t/functions-in-an-input-file-how/3707/36 "2017-05-16T21:43:59Z")

</div>

@yuyichao:

I think the documentation needs some work. The only mention of the type of a function is in the section “Julia Functions”:

> For example the name of Base.sin is sin. By convention, the name of the created type is the same as the function name, with a # prepended. So typeof(sin) is Base.#sin.

There is no mention of it in the section on “Functions” which would be the first place I would go looking for it.

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