match julia

match.jl

module Match

export @match

macro match(item, arms)
    var = gensym(:var)
    quote
        let $var = $(esc(item))
            $(begin
                code = :nothing
                for e in reverse(filter((e) -> e isa Expr, arms.args))
                    code = make_match(var, e, code, item) # need to pass the original expr for a comparison in make_match
                end
                code
              end)
        end
    end
end


function make_match(item, e, code, original)
    (pattern, value) = e.args[2:3]
    value = replace_underscore(value, item)
    test = if pattern == :_ || pattern == original
        :true
    elseif hasproperty(pattern, :args) && pattern.args[1] == :(:)
        (lower, upper) = pattern.args[2:3]
        :($(esc(lower)) <= $(item) && $(item) <= $(esc(upper)))
    else
        :($(item) == $(esc(pattern)))
    end
    quote
        if $test
            $(esc(value))
        else
            $code
        end
    end
end

replace_underscore(value, item) = value == :_ ? item : value
replace_underscore(values::AbstractArray, item) = [replace_underscore(x, item) for x in values]
replace_underscore(value::Expr, item) = begin
    Expr(replace_underscore(value.head, item), replace_underscore(value.args, item)...)
end
end