Added width (with tests).

Trivial addition for the width of the interval. Implementation also
handles case when type of the width is different from the eltype (eg
ClosedInterval{Date}).

src/IntervalSets.jl

@@ -7,7 +7,7 @@ module IntervalSets
 using Base: @pure
 import Base: eltype, convert, show, in, length, isempty, isequal, issubset, ==, hash, union, intersect, minimum, maximum
 
-export AbstractInterval, ClosedInterval, ⊇, .., ±, ordered
+export AbstractInterval, ClosedInterval, ⊇, .., ±, ordered, width
 
 abstract AbstractInterval{T}
 

src/closed.jl

@@ -71,3 +71,8 @@ end
 issubset(A::ClosedInterval, B::ClosedInterval) = ((A.left in B) && (A.right in B)) || isempty(A)
 
 ⊇(A::ClosedInterval, B::ClosedInterval) = issubset(B, A)
+
+function width{T}(A::ClosedInterval{T})
+    _width = A.right - A.left
+    max(zero(_width), _width)   # this works when T is a Date
+end

test/runtests.jl

@@ -73,5 +73,11 @@ using Base.Test
         @test I ⊇ ClosedInterval(1, 2)
 
         @test hash(1..3) == hash(1.0..3.0)
+
+        let A = Date(1990, 1, 1), B = Date(1990, 3, 1)
+            @test width(ClosedInterval(A, B)) == Base.Dates.Day(59)
+            @test width(ClosedInterval(B, A)) == Base.Dates.Day(0)
+            @test isempty(ClosedInterval(B, A))
+        end
     end
 end