FParsec.CSharp 0.1.9

FParsec.CSharp

This is a C# wrapper for the parser combinator library FParsec which is written in optimized F#.

Why Parser Combinators?

Easy: because they are the most elegant way to define parsers.

Check out the tests and see for yourself.

Why FParsec.CSharp?

While using FParsec from C# is entirely possible in theory, it is very awkward in practice. Most of FParsec's elegance is lost in translation due to C#'s inferior type inference and its lack of custom operators.

FParsec.CSharp tries to alleviate that by wrapping FParsec's operators as extension functions.

FParsec.CSharp does not try to hide any types from FParsec or FSharp.Core--the wrapper is thin and also avoids name collisions. That way you can always fallback to FParsec anytime you need some functionality not (yet) implemented by FParsec.CSharp.

Based on the current implementation it should be easy to extend the wrapper yourself. Pull requests are always welcome!

Example

This is what a basic JSON parser looks like with FParsec.CSharp:

FSharpFunc<CharStream<Unit>, Reply<object>> jvalue = null;

var jnull = StringCI("null").Return((object)null);
var jnum = Int.Map(i => (object)i);
var jbool = StringCI("true").Or(StringCI("false"))
    .Map(b => (object)bool.Parse(b));
var quotedString = Skip('"').And(Many(CharP(c => c != '"'))).And(Skip('"'))
    .Map(string.Concat);
var jstring = quotedString.Map(s => (object)s);
var jarray = Skip('[').And(WS).And(Many(Rec(() => jvalue), sep: CharP(',').And(WS))).And(Skip(']'))
    .Map(elems => (object)new JArray(elems));
var jidentifier = quotedString;
var jprop = jidentifier.And(WS).And(Skip(':')).And(WS).And(Rec(() => jvalue))
    .Map(x => new JProperty(x.Item1, x.Item2));
var jobject = Skip('{').And(WS).And(Many(jprop, sep: CharP(',').And(WS))).And(Skip('}'))
    .Map(props => (object)new JObject(props));
jvalue = OneOf(jnum, jbool, jnull, jstring, jarray, jobject).And(WS);

var simpleJsonParser = WS.And(jobject).And(WS).And(EOF).Map(o => (JObject)o);

Run it like so:

[Fact] public void CanParseJson() => simpleJsonParser
    .ParseString(@"{
        ""prop1"" : ""val"",
        ""prop2"" : [false, 13, null],
        ""prop3"" : { }
    }")
    .ShouldBe(new JObject(
        new JProperty("prop1", "val"),
        new JProperty("prop2", new JArray(false, 13, null)),
        new JProperty("prop3", new JObject())));

You can find more examples in the test project.

Hints

In order to simplify the types shown in IntelliSense you can use type aliases:

using Chars = FParsec.CharStream<Microsoft.FSharp.Core.Unit>;

Unfortunately C# does not support aliasing types with unbound generics, hence if you want to simplify the type of parser you will have to do it for each of the possible Reply<T>s you are using:

using StringParser = FSharpFunc<CharStream<Unit>, Reply<string>>;
using JsonParser = FSharpFunc<CharStream<Unit>, Reply<JObject>>;
// ...

Alternatives

FParsec.CSharp does not wrap all of FParsec's features yet. If you need an all-in-one solution then have a look at the following alternatives:

• Pidgin
  • Can also parse binary data.
  • Not as fast as FParsec.
• Sprache
  • Is not as optimized as Pidgin.