support undefined as a first-class value (#164)

modules/septima-lang/change-log.md

@@ -6,3 +6,9 @@
 ### PR/163
 
 - retire `sink` - it was too confusing + there are some things that undefined (in JS) can do but sink could not
+
+### PR/164
+
+- undefined is now a first class value: `let x = undefined`
+- `{a: undefined, n: 42}` is identical to `{n: 42}`
+- `undefined ?? 42` is `42`

modules/septima-lang/src/ast-node.ts

@@ -16,7 +16,7 @@ export type Import = {
 
 export type Literal = {
   tag: 'literal'
-  type: 'str' | 'bool' | 'num'
+  type: 'str' | 'bool' | 'num' | 'undef'
   t: Token
   unitId: UnitId
 }
@@ -70,7 +70,7 @@ export type AstNode =
     }
   | {
       tag: 'binaryOperator'
-      operator: '+' | '-' | '*' | '/' | '**' | '%' | '&&' | '||' | '>' | '<' | '>=' | '<=' | '==' | '!='
+      operator: '+' | '-' | '*' | '/' | '**' | '%' | '&&' | '||' | '>' | '<' | '>=' | '<=' | '==' | '!=' | '??'
       lhs: AstNode
       rhs: AstNode
       unitId: UnitId
@@ -181,6 +181,7 @@ export function show(ast: AstNode | AstNode[]): string {
     return switchOn(ast.type, {
       bool: () => ast.t.text,
       num: () => ast.t.text,
+      undef: () => 'undefined',
       str: () => `'${ast.t.text}'`,
     })
   }

modules/septima-lang/src/parser.ts

@@ -46,6 +46,7 @@ export class Parser {
       switchOn(pathToImportFrom.type, {
         bool: notString,
         num: notString,
+        undef: notString,
         str: () => {},
       })
       ret.push({ start, ident, pathToImportFrom: pathToImportFrom.t, unitId: this.unitId })
@@ -210,7 +211,7 @@ export class Parser {
   }
 
   ternary(): AstNode {
-    const condition = this.or()
+    const condition = this.undefinedCoallesing()
     if (this.scanner.headMatches('??')) {
       return condition
     }
@@ -226,6 +227,14 @@ export class Parser {
     return { tag: 'ternary', condition, positive, negative, unitId: this.unitId }
   }
 
+  undefinedCoallesing(): AstNode {
+    const lhs = this.or()
+    if (this.scanner.consumeIf('??')) {
+      return { tag: 'binaryOperator', operator: '??', lhs, rhs: this.undefinedCoallesing(), unitId: this.unitId }
+    }
+    return lhs
+  }
+
   or(): AstNode {
     const lhs = this.and()
     if (this.scanner.consumeIf('||')) {
@@ -402,7 +411,11 @@ export class Parser {
   }
 
   maybePrimitiveLiteral(): Literal | undefined {
-    let t = this.scanner.consumeIf('true')
+    let t = this.scanner.consumeIf('undefined')
+    if (t) {
+      return { tag: 'literal', type: 'undef', t, unitId: this.unitId }
+    }
+    t = this.scanner.consumeIf('true')
     if (t) {
       return { tag: 'literal', type: 'bool', t, unitId: this.unitId }
     }

modules/septima-lang/src/runtime.ts

@@ -250,6 +250,10 @@ export class Runtime {
         return lhs.over(rhs)
       }
 
+      if (ast.operator === '??') {
+        return lhs.coalesce(() => this.evalNode(ast.rhs, table))
+      }
+
       shouldNeverHappen(ast.operator)
     }
 
@@ -284,6 +288,10 @@ export class Runtime {
       if (ast.type === 'str') {
         return Value.str(ast.t.text)
       }
+
+      if (ast.type === 'undef') {
+        return Value.undef()
+      }
       shouldNeverHappen(ast.type)
     }
 
@@ -323,7 +331,7 @@ export class Runtime {
       })
 
       // TODO(imaman): verify type of all keys (strings, maybe also numbers)
-      return Value.obj(Object.fromEntries(entries))
+      return Value.obj(Object.fromEntries(entries.filter(([_, v]) => !v.isUndefined())))
     }
 
     if (ast.tag === 'lambda') {