diff --git a/parsimmon/parsimmon-tests.ts b/parsimmon/parsimmon-tests.ts
index 3f8019891..569328296 100644
--- a/parsimmon/parsimmon-tests.ts
+++ b/parsimmon/parsimmon-tests.ts
@@ -110,6 +110,9 @@ fooPar = P.succeed(foo);
 fooArrPar = P.seq(fooPar, fooPar);
 anyArrPar = P.seq(barPar, fooPar, numPar);
 
+fooPar = P.custom<Foo>((success, failure) => (stream, i) => { str = stream; num = i; return success(num, foo); });
+fooPar = P.custom<Foo>((success, failure) => (stream, i) => failure(num, str));
+
 fooPar = P.alt(fooPar, fooPar);
 anyPar = P.alt(barPar, fooPar, numPar);
 
diff --git a/parsimmon/parsimmon.d.ts b/parsimmon/parsimmon.d.ts
index 719e9c937..94b1b3be1 100644
--- a/parsimmon/parsimmon.d.ts
+++ b/parsimmon/parsimmon.d.ts
@@ -1,12 +1,14 @@
 // Type definitions for Parsimmon 0.5.0
 // Project: https://github.com/jneen/parsimmon
-// Definitions by: Bart van der Schoor <https://github.com/Bartvds>
+// Definitions by: Bart van der Schoor <https://github.com/Bartvds>, Mizunashi Mana <https://github.com/mizunashi-mana>
 // Definitions: https://github.com/borisyankov/DefinitelyTyped
 
 // TODO convert to generics
 
 declare module 'parsimmon' {
 	module Parsimmon {
+		
+		export type StreamType = string;
 
 		export interface Mark<T> {
 			start: number;
@@ -103,6 +105,14 @@ declare module 'parsimmon' {
 		export function seq<U>(...parsers: Parser<U>[]): Parser<U[]>;
 		export function seq(...parsers: Parser<any>[]): Parser<any[]>;
 
+		export type SuccessFunctionType<U> = (index: number, result: U) => Result<U>;
+		export type FailureFunctionType<U> = (index: number, msg: string) => Result<U>;
+		export type ParseFunctionType<U> = (stream: StreamType, index: number) => Result<U>;
+		/*
+		 allows to add custom primitive parsers.
+		 */
+		export function custom<U>(parsingFunction: (success: SuccessFunctionType<U>, failure: FailureFunctionType<U>) => ParseFunctionType<U>): Parser<U>;
+
 		/*
 		 accepts a variable number of parsers, and yields the value of the first one that succeeds, backtracking in between.
 		 */