On August 23, 2015 at 21:08:16, Kevin Squire (kevin.squ...@gmail.com) wrote: If the type hierarchy is implemented well, functions for `Dict` would ideally be written to work with `super(Dict)` (i.e., `Associative`). And in fact, some are--e.g., `in`, `haskey`, `show`, `keys`, `values`, etc. See `base/dict.jl` for the implementation.
This had actually occurred to me when I was first considering how one might implement variations on `Dict`. I think it’s somewhat natural when writing a function that deals with integers to used `myfn(foo::Int)` instead of `myfn(foo::Int64)`, but without digging into the innards of dict.jl I wouldn’t have thought to write `myfn(foodict::Associative)` instead of `myfn(foodict::Dict)`. I opened https://github.com/JuliaLang/julia/issues/12774 to gather any discussion/opinions around adding a warning to help out making sure that library code is written as generally as possible. To work around that, the implementation of `DefaultDict` (and other dictionary types) in `DataStructures.jl` wraps a `Dict` (as in your implementation), and uses a `@delegate` macro to delegate function calls to the wrapped `Dict`. This has been copied a few other places, but it's hard to say this is the best or most Julian way to do this. Thanks for the pointer! This is an interesting solution…it seems that sometimes Julia recycles OOP patterns, sometimes it goes more in the direction of ML-style type algebra, but here we have good ol’ LISP macros. This certainly should work for extending standard-library behavior, but still doesn’t do much for third-party libs (though assuming they properly specialize on abstract types, it shouldn’t be an issue). I am still a bit confused about: > foo = DefaultDict(42) > foo['a'] = 1 ERROR: MethodError: `setindex!` has no method matching setindex!(::DefaultDict{Any,Any}, ::Int64, ::Char) Closest candidates are: setindex!(::Associative{K,V}, ::Any, ::Any, ::Any, ::Any…) After all, I’ve defined `DefaultDict` as a subtype of `Associative`. Here I’m being given a method error, but the suggested “closest candidate” has `Associative` in the signature. Reading the actual implementation of `setindex!` it’s clear why this method won’t work for my `DefaultDict`, but the error message is more than a bit confusing. Actually (and this may be getting a bit off-in-the-weeds, but…), it seems to me that the signature for `setindex!` is wrong. Yes, it’s been defined with that signature, but type inference on the method body would reveal that this method in fact *cannot* accept any `Associative`.
