Add integer type and fix natural_number

Strings which merely contained digits were assignable to
`natural_number`. Oops!

Author: mkantor

src/language/semantics/type-system.test.ts

@@ -2,6 +2,7 @@ import { testCases } from '../../test-utilities.test.js'
 import {
   boolean,
   functionType,
+  integer,
   naturalNumber,
   nothing,
   nullType,
@@ -90,6 +91,7 @@ typeAssignabilitySuite('prelude types (assignable)', [
   [[nullType, nullType], true],
   [[boolean, boolean], true],
   [[naturalNumber, naturalNumber], true],
+  [[integer, integer], true],
   [[string, string], true],
   [[object, object], true],
   [[functionType, functionType], true],
@@ -104,8 +106,11 @@ typeAssignabilitySuite('prelude types (assignable)', [
   [[nullType, something], true],
   [[boolean, string], true],
   [[boolean, something], true],
+  [[naturalNumber, integer], true],
   [[naturalNumber, string], true],
   [[naturalNumber, something], true],
+  [[integer, string], true],
+  [[integer, something], true],
   [[string, something], true],
   [[object, something], true],
   [[functionType, something], true],
@@ -115,35 +120,52 @@ typeAssignabilitySuite('prelude types (not assignable)', [
   [[nullType, nothing], false],
   [[nullType, boolean], false],
   [[nullType, naturalNumber], false],
+  [[nullType, integer], false],
   [[nullType, object], false],
   [[nullType, functionType], false],
   [[boolean, nothing], false],
   [[boolean, nullType], false],
   [[boolean, naturalNumber], false],
+  [[boolean, integer], false],
   [[boolean, object], false],
   [[boolean, functionType], false],
+  [[naturalNumber, nothing], false],
+  [[naturalNumber, nullType], false],
+  [[naturalNumber, boolean], false],
+  [[naturalNumber, object], false],
+  [[naturalNumber, functionType], false],
+  [[integer, nothing], false],
+  [[integer, nullType], false],
+  [[integer, boolean], false],
+  [[integer, naturalNumber], false],
+  [[integer, object], false],
+  [[integer, functionType], false],
   [[string, nothing], false],
   [[string, nullType], false],
   [[string, boolean], false],
   [[string, naturalNumber], false],
+  [[string, integer], false],
   [[string, object], false],
   [[string, functionType], false],
   [[object, nothing], false],
   [[object, nullType], false],
   [[object, boolean], false],
   [[object, naturalNumber], false],
+  [[object, integer], false],
   [[object, string], false],
   [[object, functionType], false],
   [[functionType, nothing], false],
   [[functionType, nullType], false],
   [[functionType, boolean], false],
   [[functionType, naturalNumber], false],
+  [[functionType, integer], false],
   [[functionType, string], false],
   [[functionType, object], false],
   [[something, nothing], false],
   [[something, nullType], false],
   [[something, boolean], false],
   [[something, naturalNumber], false],
+  [[something, integer], false],
   [[something, string], false],
   [[something, object], false],
   [[something, functionType], false],
@@ -154,7 +176,6 @@ typeAssignabilitySuite('custom types (assignable)', [
   [[makeUnionType('', ['a']), string], true],
   [[makeUnionType('', ['a', 'b']), string], true],
   [[makeUnionType('', ['a']), makeUnionType('', [string, 'b'])], true],
-
   [[makeUnionType('', ['1']), naturalNumber], true],
   [[makeUnionType('', ['0', '1']), naturalNumber], true],
   [
@@ -164,6 +185,7 @@ typeAssignabilitySuite('custom types (assignable)', [
     ],
     true,
   ],
+  [[makeUnionType('', ['0', '-1']), integer], true],
   [
     [
       makeObjectType('', {
@@ -537,6 +559,8 @@ typeAssignabilitySuite('custom types (not assignable)', [
     [makeUnionType('', ['a', 'b', 'c']), makeUnionType('', ['b', 'c', 'd'])],
     false,
   ],
+  [[makeUnionType('', ['-1']), naturalNumber], false],
+  [[makeUnionType('', ['-0']), integer], false],
   [
     [
       makeObjectType('', {

src/language/semantics/type-system/prelude-types.ts

@@ -16,17 +16,29 @@ export const nullType = makeUnionType('null', ['null'])
 
 export const boolean = makeUnionType('boolean', ['false', 'true'])
 
+// The current type hierarchy for opaque types is:
+//  - string
+//    - integer
+//      - natural_number
+
 export const string = makeOpaqueStringType('string', {
   isAssignableFromLiteralType: (_literalType: string) => true,
-  nearestOpaqueAssignableFrom: () => optionADT.makeSome(naturalNumber),
+  nearestOpaqueAssignableFrom: () => optionADT.makeSome(integer),
   nearestOpaqueAssignableTo: () => optionADT.none,
 })
 
+export const integer = makeOpaqueStringType('natural_number', {
+  isAssignableFromLiteralType: literalType =>
+    /^(?:0|-?[1-9](?:[0-9])*)+$/.test(literalType),
+  nearestOpaqueAssignableFrom: () => optionADT.makeSome(naturalNumber),
+  nearestOpaqueAssignableTo: () => optionADT.makeSome(string),
+})
+
 export const naturalNumber = makeOpaqueStringType('natural_number', {
   isAssignableFromLiteralType: literalType =>
-    /(?:0|[1-9](?:[0-9])*)+/.test(literalType),
+    /^(?:0|[1-9](?:[0-9])*)+$/.test(literalType),
   nearestOpaqueAssignableFrom: () => optionADT.none,
-  nearestOpaqueAssignableTo: () => optionADT.makeSome(string),
+  nearestOpaqueAssignableTo: () => optionADT.makeSome(integer),
 })
 
 export const object = makeObjectType('object', {})