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A Simple Dynamic Scripting Language for .NET

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README.markdown

Barb is a small dynamic scripting language for .NET with the following properties:

  • Syntax that makes both F# and C# devs feel at home
  • Fully expression oriented (Whitespace is completely ignored)
  • Paren based scoping rules (Lisp-like)
  • Pure functional (No mutation allowed)

This makes Barb an easy to learn language which you can give to your users for safely scripting over data in your application. Best of all, you'll be able to use it without learning a bunch of new syntax.

Why Barb?

I created Barb because I needed a user language for my applications but yet no existing .NET language both hosted nicely inside of my application and met my criteria:

  • Simple/Terse Syntax
  • Expression Oriented (Any program should be able to be represented in a single line of code)
  • No casting necessary and transparent type conversions
  • Native .NET type system (No painful conversions!)
  • Tries really hard to do what a non-technical user would expect
  • Fast Execution (I use it to script over sets of records in the millions range)
  • Safe from accidental data corruption by non-technical users

Dynamic Linq, IronPython and IronRuby all failed for one of these reasons or another.

The current implementation is still young and a bit rough around the edges. It's was originally designed for writing one-line expressions along these lines:

(Name.Contains "John" or Name.Contains "Mary") and (Age > 20 or Weight > 200)

It can do much more now, but still needs work to be friendly enoungh for a real release. In particular, it needs better error messages and a friendly API for hosting. However, I thought it might be useful to others in its current form. If you do play with it I hope you'll let me know what you think.

Current Features

  • Recursive lambda bindings with F# or C# syntax
  • Value name binding with F# or C# syntax
  • Order of operations
  • Static or instance calls and indexers via cached reflection
  • Numerical sequence expressions (Identical to F#'s)
  • Tuples (any IEnumerable is treated as a tuple internally)

Planned Limitations

  • All numbers are converted internally to Int64 or Double. This will change but was necessary for now to keep things simple.
  • Barb may cause unexpected behavior when making calls that mutate things. There is a settings option which fixes this, but it tones down optimization significantly.

Things To Do

  • Lambda interop with .NET
  • Extension method resolution
  • Generalized Sequence Expressions
  • Better Looping Constructs
  • Pattern Matching
  • Optimized Numeric Type Handling

Examples of Use

The best way to think about Barb is over some collection of records (data classes to you C# folks).

type CustRecord =
    {
        Name: string
        Age: int
        Weight: int
        Source: obj
        Locations: string array
    }

As mentioned above, Barb is great for writing queries.

In Barb you can choose either the words "and"/"or" or use "&&"/"||", whichever makes you more comfortable.

let predicate = buildExpr<CustRecord,bool>("Name.Contains 'John' and (Age > 20 or Weight > 200)")

This will will return a predicate which you can then use to filter over large numbers of records.

It can also be used to for user specified reflection.

One case where I use this is for writing out CSV files. Given a set of record field specifying strings, you can easily build functions which will grab the specified data.

let fld1Getter = buildExpr<CustRecord,int>("Weight") 

Quite simple right? Barb will also convert to the correct output type if able.

let fld1Getter = buildExpr<CustRecord,string>("Weight") 

Here the integer will be converted on the fly to string because it's what you statically specified.

let fld1Getter = buildExpr<CustRecord,string>("Source.Url")

Barb can will also reflect into the real type of a given obj, which can be quite handy for taking the hard work out of accessing untyped data.

Barb supports many of the constructs a F# user would expect

buildExpr<CustRecord,string>("Locations.[0]") // The F# style '.' is optional :)
buildExpr<CustRecord,string>("if Age > 40 then 'Old' else 'Young'")
buildExpr<CustRecord,int>("let x = Age + 1 in x") // or..
buildExpr<CustRecord,int>("var x = Age + 1 in x") // (although, I may use var for mutables in the future)

...and some fancier ones from F#

buildExpr<unit,seq<int>>("{ 1 .. 5 }") // Returns 1,2,3,4,5
buildExpr<unit,seq<int>>("{ 1 .. 2 .. 10 }") // Returns 1,3,5,7,9
buildExpr<unit,seq<int>>("(1, 2, 3)") // Returns 1,2,3

...all looping must currently be handled with lambda recursion

buildExpr<unit,seq<int>>("let recfun = fun x -> if x > 1 then x else recfun (x + 1) in recfun 0") 

...but it's still growing and more features are being added all the time.

Language Details

Subexpression Forms

Subscope:               ( <code> )
Tuple:                  ... <code>, <code>, <code> ...
Lambda:                 fun <binding> -> <code> ...
Lambda (alternate):     <binding> => <code> ...
Branching:              if <code> then <code> else <code> ...
Indexing:               <obj>[ <code> ]
Binding:                let <name> = <code> in
Binding (Alternate*):   var <name> = <code> in

The '...' here indicates an unbounded expression and is not actual Barb syntax. Any unbounded expression can be bounded implicitly by a parent expression or explicitly by creating a subscope with parentheses.

*The var keyword may be used in the future to allow for locally mutable variables.

Operators and Keywords

()              Unit
new <name>      Creates a new .NET instance of Class "name"
.               Invocation or Decimal Point within a Number
..              Invoke on each in a set and collect
null            Creates an instance of null
true            Creates a boolean of value true
false           Creates a boolean of value false
== or =         Equals Operator
<> or !=        Not-Equals Operator
>               Greater than
>=              Greater than or equal to
<               Less than
<=              Less than or equal to
! or not        Invert Boolean
&, && or and    Boolean And
|, || or or     Boolean Or
/               Arithmetic Divide
*               Arithmetic Multiply
+               Arithmetic Add
-               Arithmetic Subtract
\/              Set Union
/\              Set Intersection
/?\             Set Has Intersection
=)              Is Superset
(=              Is Subset

Nested Invocation

In Barb you can invoke the same method or property on a set of objects and then get the result quite easily:

Collection..Property

You can also chain this when the properties each return collections:

Collection..MethodReturningCollection()..Property

It also can be used at arbitrary depth for dealing with collections of collections.

CollectionOfCollections...Property

This feature can be mixed interchangeably with standard "0th layer" invocations.

Collection..Property.Items..Item

Items in the collection which do not support the invocation are skipped.

Set Operator Semantics

Any instance can be thought of as a one element set except for null, which indicates an empty set. Any IEnumerable will be considered a set containing the number of unique elements within it.

Null Semantics

Outside of the context of sets or calls into .NET null should always consume. That is, invoking something with a null member name returns null. Similarly, invoking any member on null will return null. In the context of arithmetic (1 * null = null), etc.

Extra Nerdy Stuff Under Here

Compilation is done by n-tuple merging of expression nodes. This is done initially to pre-compute as much as possible and the optimized result is then reduced again with the information given by the passed in data object. Decisions can be made at runtime based on the contents of these tuples and their neighbors. This allows for a huge amount of flexibility in deciding what to do. Function/Value composition also allows for fast code to be built out of existing .NET constructs and eliminates the complexity of a intermediate representation.

It's kind of Scheme-like under the hood, but I've worked hard to make it very C#/F#ish on the surface.

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