allow a dangling comma after last formal arg of an arrow function (#174)

modules/septima-lang/change-log.md

@@ -29,3 +29,13 @@
 - supoort `JSON.parse(s)`
 - supoort sorting an array: `[97, 100, 50].sort()`, `['the', 'quick', 'brown', 'fox'].sort((a, b) => a.length - b.length)`
 - fix object comparsion
+
+### PR/173
+
+- support block comments `4 + /* now five */ 5`
+- support default values for function args: `(a, b = 1000) => a + b`
+- inspect the runtime type of a value via `constructor.name`
+
+### PR/174
+
+- allow a dangling comma after last formal arg of an arrow function `(a,b,) => a + b`

modules/septima-lang/src/parser.ts

@@ -200,6 +200,9 @@ export class Parser {
         }
 
         this.scanner.consume(',')
+        if (this.scanner.consumeIf(')')) {
+          break
+        }
       }
 
       this.scanner.consume('=>')

modules/septima-lang/tests/septima.spec.ts

@@ -541,6 +541,10 @@ describe('septima', () => {
         expect(run(`let five = () => { let two = 2; let three = 3; return three+two }; five()`)).toEqual(5)
         expect(run(`let concat = (a,b) => { let u = '_'; return u+a+b+u }; concat('a', 'b')`)).toEqual('_ab_')
       })
+      test('allows a dangling comma after last formal arg', () => {
+        expect(run(`let f = (a,) => a+1000; f(3)`)).toEqual(1003)
+        expect(run(`let f = (a,b,) => a+b+1000; f(5,900)`)).toEqual(1905)
+      })
     })
     test('can have no args', () => {
       expect(run(`let pi = fun() 3.14; 2*pi()`)).toEqual(6.28)
@@ -613,6 +617,10 @@ describe('septima', () => {
       test('when undefined is passed to an arg with a default value, the default value is used', () => {
         expect(run(`let f = (a, b = 2000, c = 3) => a+b+c; f(1, undefined, 5)`)).toEqual(2006)
       })
+      test('a dangling comma is allowed after last default value', () => {
+        expect(run(`let f = (a, b = 2000,) => a+b; f(5)`)).toEqual(2005)
+        expect(run(`let f = (a, b = 2000,) => a+b; f(5,20)`)).toEqual(25)
+      })
     })
   })
   describe('array methods', () => {