Simplify cache and add more iteration tests

src/purse.jl

@@ -90,8 +90,8 @@ julia> Purses.value(Purse(2.0))
 """
     cache(x[, idx])
 
-If `x` is a purse, return its stored cache as a tuple, otherwise return an empty tuple.  If
-the optional argument `idx` is supplied, return the cache item stored at index `idx`.
+Return the cache associated with the purse `x`.  If the optional argument `idx` is supplied,
+return the cache item stored at index `idx`.
 
 # Examples
 ```jldoctest
@@ -101,6 +101,5 @@ julia> Purses.cache(purse, 1)
 -1.0
 ```
 """
-@inline cache(x) = ()
 @inline cache(x::Purse) = x.cache
 @inline cache(x, idx) = cache(x)[idx]

test/basic.jl

@@ -8,9 +8,8 @@ using Purses
     x = 1.5
     purse = Purse(x)
     @test Purses.value(purse) === Purses.value(x) === x
-    @test Purses.cache(purse) === Purses.cache(x) === ()
+    @test Purses.cache(purse) === ()
     @test_throws BoundsError Purses.cache(purse, 1)
-    @test_throws BoundsError Purses.cache(x, 1)
     @test Purses.cache_signature(typeof(purse)) === Tuple{}
 
     x = 1.5

test/operators.jl

@@ -82,13 +82,21 @@ end
 end
 
 @testset "Iteration protocol" begin
-    purse = Purse(1:10, sum)
+    purse = Purse(1:5, sum)
     @test map(x -> x^2, purse) == map(x -> x^2, value(purse))
-    begin
-        for idx in eachindex(purse)
-            @test purse[idx] == value(purse)[idx]
-        end
+
+    @test iterate(purse) === (1, 1)
+    @test iterate(purse, 1) === (2, 2)
+    @test iterate(purse, 5) === nothing
+
+    for idx in eachindex(purse)
+        @test purse[idx] == value(purse)[idx]
     end
+
+    for (x1, x2) in zip(purse, value(purse))
+        @test x1 == x2
+    end
+
     collected_purse = Purse(collect(purse), sum)
     @test_throws ErrorException purse[1] = 2
 end