represent the error info with singleton types instead of NamedTuple

src/CheckedSizeProduct.jl

@@ -1,6 +1,28 @@
 module CheckedSizeProduct
     export checked_size_product
 
+    """
+        StatusInvalidValue::DataType
+
+    Singleton type. Returned by [`checked_size_product`](@ref) in case its input
+    contained an invalid value. An invalid value is either:
+    * less than zero
+    * equal to `typemax(T)`
+
+    See also: [`StatusOverflow`](@ref)
+    """
+    struct StatusInvalidValue end
+
+    """
+        StatusOverflow::DataType
+
+    Singleton type. Returned by [`checked_size_product`](@ref) in case the product
+    is not representable in the element type of the input.
+
+    See also: [`StatusInvalidValue`](@ref)
+    """
+    struct StatusOverflow end
+
     """
         checked_size_product(size_tuple)
 
@@ -22,12 +44,11 @@ module CheckedSizeProduct
         * `==`
     2. Calculate the product. In case no element is negative, no element is
        `typemax(T)` and the product is representable as `T`, return the product.
-       Otherwise, return a `NamedTuple` containing two Boolean properties:
-        * `any_is_negative`: at least one element is negative
-        * `any_is_typemax`: at least one element is the maximum value representable
-          in the given type, as given by `typemax`
-       If a `NamedTuple` is returned with both fields `false`, the product is not
-       representable as `T`.
+       Otherwise return:
+        * a value of type [`StatusInvalidValue`]: the input contains a negative
+          element, or an element equal to `typemax(T)`
+        * a value of type [`StatusOverflow`]: the product is not representable as
+          `T`
 
     Throws when given an empty tuple to avoid having to choose a default return
     type arbitrarily.
@@ -106,7 +127,11 @@ module CheckedSizeProduct
         if !(any_is_negative | any_is_typemax | is_not_representable)
             product
         else
-            (; any_is_negative = any_is_negative, any_is_typemax = any_is_typemax)
+            if any_is_negative | any_is_typemax
+                StatusInvalidValue()
+            else
+                StatusOverflow()
+            end
         end
     end
 end

test/runtests.jl

@@ -72,9 +72,9 @@ using .ExampleInts: ExampleInt
             (-1, typemax(Int)), (typemax(Int), -1),
             (0, -4, -4), (-4, 1, 0), (-4, -4, 1),
         )
-            @test (checked_size_product(t)).any_is_negative
+            @test checked_size_product(t) isa CheckedSizeProduct.StatusInvalidValue
             s = map(ExampleInt, t)
-            @test (checked_size_product(s)).any_is_negative
+            @test checked_size_product(s) isa CheckedSizeProduct.StatusInvalidValue
         end
     end
     @testset "input includes `typemax(T)`" begin
@@ -89,9 +89,9 @@ using .ExampleInts: ExampleInt
         for t ∈ (
             (m,), (m, m), (1, m), (m, 1), (0, m), (m, 0), (-1, m), (m, -1),
         )
-            @test (checked_size_product(t)).any_is_typemax
+            @test checked_size_product(t) isa CheckedSizeProduct.StatusInvalidValue
             s = map(ExampleInt, t)
-            @test (checked_size_product(s)).any_is_typemax
+            @test checked_size_product(s) isa CheckedSizeProduct.StatusInvalidValue
         end
     end
     @testset "overflows" begin
@@ -101,11 +101,9 @@ using .ExampleInts: ExampleInt
             (m, m), (15, m), (m, 15), (m, m, m), (1, m, m), (m, 1, m), (m, m, 1),
             (b, b), (1, b, b),
         )
-            @test !(checked_size_product(t)).any_is_negative
-            @test !(checked_size_product(t)).any_is_typemax
+            @test checked_size_product(t) isa CheckedSizeProduct.StatusOverflow
             s = map(ExampleInt, t)
-            @test !(checked_size_product(s)).any_is_negative
-            @test !(checked_size_product(s)).any_is_typemax
+            @test checked_size_product(s) isa CheckedSizeProduct.StatusOverflow
         end
     end
     @testset "overflows, but OK because of multiplication with zero" begin