Added exacty parser per discussions in francisrstokes#46

Readme.md

@@ -41,6 +41,7 @@ Arcsecond is a zero-dependency, Fantasy Land compliant JavaScript [Parser Combin
       - [lookAhead](#lookAhead)
       - [sepBy](#sepBy)
       - [sepBy1](#sepBy1)
+      - [exactly](#exactly)
       - [many](#many)
       - [many1](#many1)
       - [between](#between)
@@ -1002,6 +1003,41 @@ newParser.run('1,2,3')
 //    }
 ```
 
+#### exactly
+
+`exactly :: (Integer, Parser e s a) -> Parser e s [a]`
+
+`exactly` takes a positive number and parser, and returns a new parser which matches that parser the given number of times.
+
+**Example**
+```JavaScript
+const newParser = exactly (4, letter)
+
+newParser.run('abcdef')
+// -> {
+//      isError: false,
+//      result: [ "a", "b", "c", "d" ],
+//      index: 4,
+//      data: null
+//    }
+
+newParser.run('abc')
+// -> {
+//      isError: true,
+//      error: 'ParseError (position 0): Expecting 4 letter, but got end of input.',
+//      index: 0,
+//      data: null
+//    }
+
+newParser.run('12345')
+// -> {
+//      isError: true,
+//      error: 'ParseError (position 0): Expecting 4 letter, got '1'',
+//      index: 0,
+//      data: null
+//    }
+```
+
 #### many
 
 `many :: Parser e s a -> Parser e s [a]`

index.mjs

@@ -9,6 +9,7 @@ const reDigits = /^[0-9]+/;
 const reLetter = /[a-zA-Z]/;
 const reLetters = /^[a-zA-Z]+/;
 const reWhitespaces = /^\s+/;
+const reErrorExpectation = /ParseError.+Expecting/;
 
 //    createParserState :: x -> s -> ParserState e a s
 const createParserState = (target, data = null) => ({
@@ -291,6 +292,31 @@ export const coroutine = function coroutine(g) {
   });
 };
 
+//           exactly :: (Integer, Parser e s a) -> Parser e s [a]
+export const exactly = function exactly(n, parser) {
+  if (typeof n !== 'number' || n <= 0) {
+    throw new TypeError (`exactly must be called with a number > 0, but got ${n}`);
+  }
+  return new Parser(function exactly$state(state) {
+    if (state.isError) return state;
+
+    const results = [];
+    let nextState = state;
+
+    for (let i = 0; i < n; i++) {
+      const out = parser.p(nextState);
+      if(out.isError) {
+        return out;
+      } else {
+        nextState = out;
+        results.push(nextState.result);
+      }
+    }
+
+    return updateResult(nextState, results);
+  }).errorMap((_, index) => `ParseError (position ${index}): Expecting ${n}${_.replace(reErrorExpectation, '')}`);
+}
+
 //           many :: Parser e s a -> Parser e s [a]
 export const many = function many(parser) {
   return new Parser(function many$state(state) {

test/main.test.js

@@ -5,6 +5,7 @@ const {
   str,
   digit,
   fail,
+  exactly,
   many,
   many1,
   digits,
@@ -44,7 +45,7 @@ const {
   mapData,
   endOfInput,
   withData
-} = require('../index')
+} = require('../index');
 
 const f = x => ({ f: x });
 const g = x => ({ g: x });
@@ -267,6 +268,15 @@ testMany(
   ]
 );
 
+testMany(
+  'exactly', [
+    expectedSuccessTest(exactly (3, char('*')), '***'.split(''), '***'),
+    expectedSuccessTest(exactly (4, digit), '1234'.split(''), '1234abc'),
+    expectedFailTest(exactly (4, digit), 'abc'),
+    expectedThrowTest(() => exactly ('a', digit), '123abc', `exactly must be called with a number > 0, but got a`)
+  ]
+);
+
 testMany(
   'many', [
     expectedSuccessTest(many (digit), '1234'.split(''), '1234abc'),