interpreting a script through F#

Question

I really like F# but I feel like it's not succint and short enough. I want to go further. I do have an idea of how I'd like to improve it but I have no experience in making compilers so I thought I'd make it a scripting language. Then I realized that I could make it a scripting language and interpret it using F# but still get pretty much 100% performance thanks to F# having the inline option. Am I right? Is it really possible to make a script interpreter in F# that would go through my script and turn it into lots of functors and stuff and so get really good performance?

Answer 1

I really like F# but I feel like it's not succinct and short enough. I want to go further. I do have an idea of how I'd like to improve it but I have no experience in making compilers so I thought I'd make it a scripting language.

F# supports scripting scenarios via F# Interactive, so I'd recommend considering an internal DSL first, or suggesting features on the F# Language UserVoice page.

Then I realized that I could make it a scripting language and interpret it using F# but still get pretty much 100% performance thanks to F# having the inline option. Am I right?

Depending on the scenario, interpreted code may be fast enough, for example if 99% of your application's time is spent waiting on network, database or graphics rendering, the overall cost of interpreting the code may be negligible. This is less true for compute based operations. F#'s inline functions can help with performance tuning but are unlikely to provide a global panacea.

Is it really possible to make a script interpreter in F#

As a starting point, it is possible to write an interpreter for vanilla F# code. You could for example use F#'s quotation mechanism to get an abstract syntax tree (AST) for a code fragment or entire module and then evaluate it. Here's a small F# snippet that evaluates a small subset of F# code quotations: http://fssnip.net/h1

Alternatively you could design your own language from scratch...

Is it really possible to make a script interpreter in F# that would go through my script and turn it into lots of functors and stuff and so get really good performance?

Yes, you could design your own scripting language, defining an AST using the F# type system, then writing a parser that transforms script code into the AST representation, and finally interpreting the AST.

Parser

There are a number of options for parsing including:

• active patterns & regex, for example evaluating cells in a spreadsheet
• FsLex & FsYacc, for example to parse SQL
• FParsec, a parser combinator library, for example to parse Small Basic

I'd recommend starting with FParsec, it's got a good tutorial, plenty of samples and gives basic error messages for free based on your code.

Small Examples

Here's a few simple example interpreters using FParsec to get you started:

• Turtle - http://fssnip.net/nM
• Minimal Logo language - http://fssnip.net/nN
• Small Basic - http://fssnip.net/le

Fun Basic

A while back I wrote my own simple programming language with F#, based on Microsoft's Small Basic with interesting extensions like support for tuples and pattern matching. It's called Fun Basic, has an IDE with code completion and is available free on the Windows Store. The Windows Store version is interpreted (due to restrictions on emitting code) and the performance is adequate. There is also a compiler version for the desktop which runs on Windows, Mac and Linux.

Is it really possible to make a script interpreter in F#

So I guess, the answer is YES, if you'd like to learn more there's a free recording of a talk I did at NDC London last year on how to Write Your Own Compiler in 24 Hours

I'd also recommend picking up Peter Sestoft's Programming Language Concepts book which has a chapter on building your own functional language.