Merge pull request #18533 from TotalVerb/fw/take-take

Make take and drop type-idempotent, and canonicalize their order

Author: JeffBezanson

base/iterator.jl

@@ -338,6 +338,7 @@ julia> collect(take(a,3))
 ```
 """
 take(xs, n::Int) = Take(xs, n)
+take(xs::Take, n::Int) = Take(xs.xs, min(n, xs.n))
 
 eltype{I}(::Type{Take{I}}) = eltype(I)
 iteratoreltype{I}(::Type{Take{I}}) = iteratoreltype(I)
@@ -391,6 +392,8 @@ julia> collect(drop(a,4))
 ```
 """
 drop(xs, n::Int) = Drop(xs, n)
+drop(xs::Take, n::Int) = Take(drop(xs.xs, n), max(0, xs.n - n))
+drop(xs::Drop, n::Int) = Drop(xs.xs, n + xs.n)
 
 eltype{I}(::Type{Drop{I}}) = eltype(I)
 iteratoreltype{I}(::Type{Drop{I}}) = iteratoreltype(I)

test/functional.jl

@@ -155,6 +155,21 @@ end
 @test Base.iteratorsize(drop(countfrom(1),3)) == Base.IsInfinite()
 @test_throws MethodError length(drop(countfrom(1), 3))
 
+# double take
+# and take/drop canonicalization
+# -----------
+
+for xs in Any["abc", [1, 2, 3]]
+    @test take(take(xs, 2), 3) === take(xs, 2)
+    @test take(take(xs, 4), 2) === take(xs, 2)
+    @test drop(drop(xs, 1), 1) === drop(xs, 2)
+    @test take(drop(xs, 1), 1) === drop(take(xs, 2), 1)
+    @test take(drop(xs, 3), 0) === drop(take(xs, 2), 3)
+    @test isempty(drop(drop(xs, 2), 2))
+    @test drop(take(drop(xs, 1), 2), 1) === take(drop(xs, 2), 1)
+    @test take(drop(take(xs, 3), 1), 1) === take(drop(xs, 1), 1)
+end
+
 # cycle
 # -----