Design pattern for correctly overwriting abstract methods (interface methods)

General Usage
1

I have something like the following situation:

module Cache

export cache

function cache(data)
  throw(NotImplementedException())
end

end

module FileCache

import Cache

function Cache.cache(data)
  # do cache stuff
end

end

include("Cache.jl")
include("FileCache.jl")

# use caching

However, Julia complains that FileCache.cache overwrites Cache.cache and breaks pre-compilation. Having different method signatures fixes the problem, but I don’t want to invent artificial function signatures for interface methods just for this purpose.

What’s the correct way to do it?

1 Like
2

Going through some code I’ve seen from other authors, I think the solution is to pass a type for dispatching and specializing methods, ie:

module Cache

function cache()
  throw(NotImplementedException())
end

end

module FileCache

function cache(::Cache{FileCache}, data)
  # stuff
end

end

Is this the (only) way?

3

You can define a function with no methods:

function cache end

in the Cache module.

12 Likes
4

Ah, that’s brilliant, thanks!

5

An aside, after working with a lot of code that onces used this kind of NotImplementedException,
we concluded it was an antipattern.
and replaced it with declaring the functin in the

function cache end

form.

  • Plus a docstring saying what the espected signature was.
  • Plus a test-suite function which accepts an implementation of the interface in the docstring and tests that it follows the interface.

Its great.
Strong recommend,.

12 Likes
6

Excellent! It felt too much like Java anyway!

2 Likes
7

the really big downside of the NotImplementedException is that if you do somethig like

function train(::AbstractModel, data) 
    throw(NotImplementedException()) 
end

struct GoodModel <: AbstractModel
...
end

function train(mdl::GoodModel, data::Vector) 
...
end

then call
train(mdl, (1,2,3)) (a tuple rather than a vector)
then you get a NotImplementedException with a uninformative error message.

Rather than a MethodError listing what you called, vs what exists

4 Likes