Utility macros and functions for Julia

macros.jl

## Some useful and/or interesting Julia macros. I haven't seen many
## examples online, so I hope this might be useful for people trying
## to understand macros. Caveat Emptor; I don't understand them that
## well myself. Additions and improvements are more than welcome.
##
## Everything here is available under the MIT license (see bottom of
## file)
##
## - Gray Calhoun (@grayclhn) 10/17/2014

using Base.Meta

## Basic functions for working with loops

isloop(e) = false
isloop(e::Expr) = isexpr(e, [:comprehension, :for, :while])

function loophead!(e::Expr, s::Expr)
    if isexpr(e, :comprehension)
        e.args[2] = s
    elseif isexpr(e, [:for, :while])
        e.args[1] = s
    else
        stop("$(e.head) is not supported yet.")
    end
end

function loopbody!(e::Expr, s::Expr)
    if isexpr(e, :comprehension)
        e.args[1] = s
    elseif isexpr(e, [:for, :while])
        return e.args[2] = s
    else
        stop("$(e.head) is not supported yet.")
    end
end

function loophead(e::Expr)
    isexpr(e, :comprehension) && return e.args[2]
    isexpr(e, [:for, :while]) && return e.args[1]
    stop("$(e.head) is not supported yet.")
end

function loopbody(e::Expr)
    isexpr(e, :comprehension) && return e.args[1]
    isexpr(e, [:for, :while]) && return e.args[2]
    stop("$(e.head) is not supported yet.")
end

## Helper function for the @any and @all macros; this function takes a
## list of expressions and chains them together with the symbol s, so
##     chain_exprs(:&&, [:(x == 1), :(y == 2), (z == 3)])
## becomes
##     :(x == 1 && y == 2 && z == 3
function chain_exprs(s::Symbol, e...)
    length(e) <= 1 && return e[1]
    length(e) == 2 && return Expr(s, e[1], e[2])
    return make_chained(s, Expr(s, e[1], e[2]), e[3:end]...)
end

## Helper function for the @any and @all macros; this function turns
## an array comprehension or a for loop into a loop that breaks
## _immediately_ when its body returns `value`.
shortcircuit_loop(loop::Expr, value::Bool) =
    Expr(:block, :(v = !$value), Expr(:for, loophead(loop), quote
        if ($value == $(loopbody(loop)))
            v = $value
            break
        end end), :v)

## Short-circuit `any` macro that ends when the first element is
## true. This is designed to be used in array comprehensions:
##     @any [x < 2 for x in 1:1000000000000000000000000000000000]
## should immediately return `true`
macro any(e...)
    length(e) >= 2 && return chain_exprs(:||, e...)
    isexpr(e[1], :comparison) && return e[1]
    isexpr(e[1], :comprehension) && return shortcircuit_loop(e[1], true)
    error("@any doesn't support $(e[1].head) yet")
end

## Short-circuit `any` macro that ends when the first element is
## true. This is designed to be used in array comprehensions:
##     @all [x < 2 for x in 1:1000000000000000000000000000000000]
## should return `false` in its second check
macro all(e...)
    isexpr(e, :comparison) && return e
    length(e) >= 2 && return chain_exprs(:&&, e...)
    isexpr(e[1], :comprehension) && return shortcircuit_loop(e[1], false)
    error("@all doesn't support $(e[1].head) yet")
end

## Copyright (c) 2014 Gray Calhoun
##
## Permission is hereby granted, free of charge, to any person
## obtaining a copy of this software and associated documentation
## files (the "Software"), to deal in the Software without
## restriction, including without limitation the rights to use, copy,
## modify, merge, publish, distribute, sublicense, and/or sell copies
## of the Software, and to permit persons to whom the Software is
## furnished to do so, subject to the following conditions:
##
## The above copyright notice and this permission notice shall be
## included in all copies or substantial portions of the Software.
##
## THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
## EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
## MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
## NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
## BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
## ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
## CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
## SOFTWARE.