[Squiggol] Add some fun tests

Author: shingarov

src/PreSmalltalks-Tests/FoldlIsInjectTest.class.st

@@ -38,3 +38,17 @@ FoldlIsInjectTest >> test4 [
 FoldlIsInjectTest >> test5 [
 	self assert: ({1 . 2 . 3} inject: 4 into: [ :x :y | 2*x + y ]) equals: 43
 ]
+
+{ #category : #tests }
+FoldlIsInjectTest >> testFactorial0 [
+	self
+		assert: 0 factorialHylomorphically
+		equals: 1
+]
+
+{ #category : #tests }
+FoldlIsInjectTest >> testFactorial5 [
+	self
+		assert: 5 factorialHylomorphically
+		equals: 120
+]

src/PreSmalltalks-Tests/Integer.extension.st

@@ -0,0 +1,8 @@
+Extension { #name : #Integer }
+
+{ #category : #'*PreSmalltalks-Tests' }
+Integer >> factorialHylomorphically [
+	self < 0 ifTrue: [self error: 'Not valid for negative integers'].
+	^(1 to: self) "anamorphism: unfold scalar to an inductive datatype"
+		inject: 1 into: #* "catamorphism: fold inductive datatype to scalar"
+]