alist

alist.jl

immutable AListWrapper{Name}; end
immutable AList{Keys<:(AListWrapper...),Types} <: Associative
    v::Types
end

macro alist(args...)
    keys = Array(Any, length(args))
    values = Array(Any, length(args))
    for i = 1:length(args)
        if !Base.Meta.isexpr(args[i], :(=>))
            error("unexpected non-pair in @alist")
        else
            key = args[i].args[1]
            if isa(key, Expr) && key.head == :quote && length(key.args) == 1
                key = key.args[1]
            end
            if isa(key, Expr)
                xdump(args[i].args[1])
                error("key $(args[i].args[1]) is not static")
            end
        end

        keys[i] = AListWrapper{key}
        values[i] = args[i].args[2]
    end
    quote
        AList{$(tuple(keys...)),$(esc(Expr(:tuple, [:(typeof($x)) for x in values]...)))}($(esc(Expr(:tuple, values...))))
    end
end

macro aidx(ref)
    Base.Meta.isexpr(ref, :ref) || error("not an indexing expression")
    Expr(:ref, esc(ref.args[1]), :(AListWrapper{$(esc(ref.args[2]))}))
end

Base.getindex(x::AList, idx) = x[AListWrapper{idx}]
stagedfunction getindex{T}(A::AList, x::Type{AListWrapper{T}})
    keys = A.parameters[1]
    types = A.parameters[2]
    key = x.parameters[1]
    for i = 1:length(keys)
        if keys[i] === key
            return :(A.v[$i])
        end
    end
    errstr = "key not found: $(key)"
    :(throw($errstr))
end

function f()
    a = @alist(:a => 1, :b => 2)
    @aidx a[:a]
end