StructEquality

install like

using Pkg
pkg"add StructEquality"

load like

using StructEquality

which let's you easily define hash and == for your custom struct.

@struct_hash_equal struct MyStruct
  one
  two
end

MyStruct("1", [2]) == MyStruct("1", [2])  # true

API Overview

If you don't like macros, you can directly use the underlying generated functions and implement the definitions yourself.

Motivation & Usage

Struct types have an == implementation by default which uses ===, i.e. object identity, on the underlying components, in order to compare structs. (The same holds true for hash, which should always follow the implementation of ==)

Let's define a struct

struct MyStruct
  a::Int
  b::Vector
end

The default == fails to compare two structs with the same content

MyStruct(1, [2,3]) == MyStruct(1, [2,3])  # false

To fix this use the supplied macro @struct_hash_equal

@struct_hash_equal MyStruct
MyStruct(1, [2,3]) == MyStruct(1, [2,3])  # true

Alternatively you can use the macro right on struct definition

@struct_hash_equal struct MyStruct2
  a::Int
  b::Vector
end
MyStruct2(1, [2,3]) == MyStruct2(1, [2,3])  # true

You could also merely use @struct_equal instead of @struct_hash_equal, however it is recommended to always implement hash and == together.

Implementation

The implementation uses generated functions, which generate optimal code, specified to your custom struct type.

Inspecting the macro with

@macroexpand @struct_hash_equal MyStruct

returns the following

quote
    Base.hash(a::MyStruct, h::UInt) = begin
        StructEquality.struct_hash(a, h)
    end
    Base.:(==)(a::MyStruct, b::MyStruct) = begin
        StructEquality.struct_equal(a, b)
    end
end

In order to inspect generated functions, the @code_lowered macro is best.

struct MyStruct
  a::Int
  b::Vector
end

@code_lowered struct_equal(MyStruct(1, [2,3]), MyStruct(1, [2,3]))

which returns

    @ /path/to/StructEquality/src/StructEquality.jl:15 within `struct_equal`
   ┌ @ /path/to/StructEquality/src/StructEquality.jl within `macro expansion`
1 ─│ %1 = Base.getproperty(e1, :a)
│  │ %2 = Base.getproperty(e2, :a)
│  │ %3 = %1 == %2
└──│      goto #3 if not %3
2 ─│ %5 = Base.getproperty(e1, :b)
│  │ %6 = Base.getproperty(e2, :b)
│  │ %7 = %5 == %6
└──│      return %7
3 ─│      return false
   └
)

It is like you would expect. the generated function extracts the field names and defines == by referring to == comparison of the fields.

References

For more details to this topic, please see this discourse thread https://discourse.julialang.org/t/surprising-struct-equality-test/4890/9 and this issue JuliaLang/julia#4648