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Remove alerts. Add issue JuliaLang#9498 and example.
Added an example of issue JuliaLang#9498 - since it is not scheduled to be fixed until Julia 2.0.
Kept the example of ambiguous/last-seen default code being run because that is the core
use case for default values.
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taqtiqa-mark authored and taqtiqa-admin committed May 15, 2019
1 parent 923645c commit b173b89
Showing 1 changed file with 44 additions and 4 deletions.
48 changes: 44 additions & 4 deletions doc/src/manual/methods.md
Expand Up @@ -806,15 +806,21 @@ There are two styles for setting optional agruments (a.k.a default values).

Keyword arguments behave differently from ordinary positional arguments.
Specifically, they do not participate in method dispatch.

!!! note Function methods are dispatched based *only* on positional arguments.
Function methods are dispatched based *only* on positional arguments.
Keyword arguments are processed after the matching method is identified.

!!! warning A consequence of the above note on dispatch:
However, an existing issue means that function method dispatch behaves differently when
doing a function call with or without keyword arguments.
This issue is currently scheduled to be fixed in Julia 2.0.
See [Issue #9498](https://github.com/JuliaLang/julia/issues/9498) for further details.

One consequence of keyword arguments not influencing dispatch decisions is that the default
method executed will be the last defined:
If a function, say `f`, has a method that uses position based default values and elsewhere in your
code you have the same function name with a method defined to use keyword based default values;
then the default value returned by `f()` will be the last definition Julia processed.
**There will be no ambiguity warning**
*There will be no ambiguity warning*

```jldoctest
julia> f(a=1,b=2)=5
f (generic function with 3 methods)
Expand All @@ -825,6 +831,40 @@ f (generic function with 3 methods)
julia> f()
10
```
An example of [Issue #9498](https://github.com/JuliaLang/julia/issues/9498), which works when methods are defined in one order:

```jldoctest
julia> bar(x, y; c) = x + y + c
bar (generic function with 1 method)
julia> bar(x, y) = x / y
bar (generic function with 1 method)
julia> bar(3,4)
0.75
julia> bar(3,4, c=5)
12
```

Yet, throws an error if the code is defined in a different order

```jldoctest
julia> bar(x, y) = x / y
bar (generic function with 1 method)
julia> bar(x, y; c) = x + y + c
bar (generic function with 1 method)
julia> bar(3,4,c=5)
12
julia> bar(3,4)
ERROR: UndefKeywordError: keyword argument c not assigned
Stacktrace:
[1] bar(::Int64, ::Int64) at ./REPL[2]:1
[2] top-level scope at none:0
```

**Position** based default/optional arguments are implemented as a shorthand for multiple method
definitions (this was mentioned briefly in [Functions](@ref man-functions)).
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