Is there an operator for function composition in Julia?

Question

Say I have two functions:

f(x) = x^2
g(x) = x + 2

Their composition is the function

h(x) = f(g(x))

Is there an operator for function composition in Julia? For example, if * was an operator for function composition (which it isn't), we could write:

h = f * g

P.S. I know I can define it if I want to,

*(f::Function, g::Function) = x -> f(g(x))

Just asking if there is an operator form already in Julia.

Perhaps using the `∘` operator (enter using `\circ` TAB at REPL) will serve for a better math-y look. — Dan Getz, Mar 30 '16 at 16:17
The route for getting to default would likely be: Define in your code -> In a package -> In a popular package -> In Base. — Dan Getz, Mar 30 '16 at 16:29

Imanol Luengo · Accepted Answer · 2016-03-31T13:03:10.467

16

It is currently an open issue to create such operator, but as now you can keep to the syntax:

julia> h(x) = f(g(x))

or a bit more clearer (for more complex functions):

julia> h(x) = x |> g |> f

It seems as for now you would need to keep the x for making it a composite function.

Another option, is to create your own operator (as you suggest):

julia> ∘(f::Function, g::Function) = x->f(g(x))
julia> h = f ∘ g

This works perfectly fine, however, it introduces a lambda function, and I cannot think a way of performing such operation without lambdas.

NOTE: ∘ operator can be written as \circ as @DanGetz suggested.

EDIT: seems fast closures are coming in future releases and will probably be easy to implement an efficient version of the composite operator.

edited Mar 31 '16 at 13:03

answered Mar 30 '16 at 21:32

Imanol Luengo

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fast closures are now in Julia 0.5. Any updates on a composition operator? – a06e Nov 24 '16 at 15:46
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This is now in Julia 0.6 (as ∘ ) – Frames Catherine White Nov 15 '17 at 09:00
@Imanol As yours is the accepted answer, perhaps you would consider updating it with a note that `\circ` is now the composition operator? Thanks. – PatrickT May 21 '21 at 10:44

Is there an operator for function composition in Julia?

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