From 761cf9cf775eb7cdd9eab0c346c34e5e780f53bf Mon Sep 17 00:00:00 2001
From: Tim Holy <tim.holy@gmail.com>
Date: Wed, 1 Feb 2017 10:04:12 -0600
Subject: [PATCH] RFC: exportable tuple-manipulation utilities

---
 base/exports.jl |  2 ++
 base/tuple.jl   | 36 ++++++++++++++++++++++++++++++++++++
 test/tuple.jl   |  3 +++
 3 files changed, 41 insertions(+)

diff --git a/base/exports.jl b/base/exports.jl
index 736038da9e5cc..9265b4bef97d1 100644
--- a/base/exports.jl
+++ b/base/exports.jl
@@ -86,6 +86,8 @@ export
     Range,
     RangeIndex,
     Rational,
+    ReferenceBack,
+    ReferenceFront,
     Regex,
     RegexMatch,
     RemoteChannel,
diff --git a/base/tuple.jl b/base/tuple.jl
index 5a6a1ba5b2d7e..6cda152d48f3e 100644
--- a/base/tuple.jl
+++ b/base/tuple.jl
@@ -76,6 +76,42 @@ function _front(out, v, t...)
     _front((out..., v), t...)
 end
 
+abstract ReferenceTuple
+immutable ReferenceFront{TT<:Tuple} <: ReferenceTuple
+    t::TT
+end
+immutable ReferenceBack{TT<:Tuple} <: ReferenceTuple
+    t::TT
+end
+
+"""
+    split(t, ReferenceFront(ref)) -> front, back
+
+Splits a tuple or CartesianIndex `t` into two pieces, `front` and
+`back`, such that `t == (front..., back...)`. `front` has the same
+length as the reference `ref`.
+"""
+split(t::Tuple, ref::ReferenceFront) = _splitfront((), t, ref.t)
+_splitfront{N}(out::NTuple{N}, t, ref::NTuple{N}) = out, t
+_splitfront(out, ::Tuple{}, ref) =
+    (@_noinline_meta; throw(DimensionMismatch("front reference had $(length(ref)) elements, but input had only $(length(out))")))
+_splitfront(out, t, ref) =
+    (@_inline_meta; _splitfront((out..., t[1]), tail(t), ref))
+
+"""
+    split(t, ReferenceBack(ref)) -> front, back
+
+Splits a tuple or CartesianIndex `t` into two pieces, `front` and
+`back`, such that `t == (front..., back...)`. `front` has the same
+length as the reference `ref`.
+"""
+split(t::Tuple, ref::ReferenceBack) = _splitback((), t, ref.t)
+_splitback{N}(out, t::NTuple{N}, ref::NTuple{N}) = out, t
+_splitback(out, ::Tuple{}, ref) =
+    (@_noinline_meta; throw(DimensionMismatch("back reference had $(length(ref)) elements, but input had only $(length(out))")))
+_splitback(out, t, ref) =
+    (@_inline_meta; _splitback((out..., t[1]), tail(t), ref))
+
 ## mapping ##
 
 """
diff --git a/test/tuple.jl b/test/tuple.jl
index 4470ac6992257..6ccef0261f53b 100644
--- a/test/tuple.jl
+++ b/test/tuple.jl
@@ -231,3 +231,6 @@ end
 @test Tuple{Int,Vararg{Any}}(Float64[1,2,3]) === (1, 2.0, 3.0)
 @test Tuple(ones(5)) === (1.0,1.0,1.0,1.0,1.0)
 @test_throws MethodError convert(Tuple, ones(5))
+
+@test @inferred(split((1,2,3), ReferenceFront((5,5)))) === ((1,2), (3,))
+@test @inferred(split((1,2,3), ReferenceBack((5,5))))  === ((1,), (2,3))