Custom Macros

Extending @set and @lens

This code demonstrates how to extend the @set and @lens mechanism with custom lenses. As a demo, we want to implement @mylens! and @myset!, which work much like @lens and @set, but mutate objects instead of returning modified copies.

using Setfield
using Setfield: IndexLens, PropertyLens, ComposedLens

struct Lens!{L <:Lens} <: Lens
    pure::L
end

Setfield.get(o, l::Lens!) = Setfield.get(o, l.pure)
function Setfield.set(o, l::Lens!{<: ComposedLens}, val)
    o_inner = get(o, l.pure.outer)
    set(o_inner, Lens!(l.pure.inner), val)
end
function Setfield.set(o, l::Lens!{PropertyLens{prop}}, val) where {prop}
    setproperty!(o, prop, val)
    o
end
function Setfield.set(o, l::Lens!{<:IndexLens}, val) where {prop}
    o[l.pure.indices...] = val
    o
end

Now this implements the kind of lens the new macros should use. Of course there are more variants like Lens!(<:DynamicIndexLens), for which we might want to overload set, but lets ignore that. Instead we want to check, that everything works so far:

using Test
mutable struct M
    a
    b
end

o = M(1,2)
l = Lens!(@lens _.b)
set(o, l, 20)
@test o.b == 20

l = Lens!(@lens _.foo[1])
o = (foo=[1,2,3], bar=:bar)
set(o, l, 100)
@test o == (foo=[100,2,3], bar=:bar)
Test Passed

Now we can implement the syntax macros

using Setfield: setmacro, lensmacro

macro myset!(ex)
    setmacro(Lens!, ex)
end

macro mylens!(ex)
    lensmacro(Lens!, ex)
end

o = M(1,2)
@myset! o.a = :hi
@myset! o.b += 98
@test o.a == :hi
@test o.b == 100

deep = [[[[1]]]]
@myset! deep[1][1][1][1] = 2
@test deep[1][1][1][1] === 2

l = @mylens! _.foo[1]
o = (foo=[1,2,3], bar=:bar)
set(o, l, 100)
@test o == (foo=[100,2,3], bar=:bar)
Test Passed

Everything works, we can do arbitrary nesting and also use += syntax etc.

This page was generated using Literate.jl.