Sabre

DEPRECATED: This repository is deprecated in favour much better slurp project and will be archived/removed soon.

Sabre is highly customizable, embeddable LISP engine for Go.

Check out Slang for a tiny LISP written using Sabre.

Features

• Highly Customizable reader/parser through a read table (Inspired by Clojure) (See Reader)
• Built-in data types: nil, bool, string, number, character, keyword, symbol, list, vector, set, hash-map and module.
• Multiple number formats supported: decimal, octal, hexadecimal, radix and scientific notations.
• Full unicode support. Symbols can include unicode characters (Example: find-δ, π etc.) and ?, ? etc. (yes, smileys too).
• Character Literals with support for:
  1. simple literals (e.g., \a for a)
  2. special literals (e.g., \newline, \tab etc.)
  3. unicode literals (e.g., \u00A5 for ¥ etc.)
• Clojure style built-in special forms: fn*, def, if, do, throw, let*
• Simple interface sabre.Value and optional sabre.Invokable, sabre.Seq interfaces for adding custom data types. (See Evaluation)
• A macro system.

Please note that Sabre is NOT an implementation of a particular LISP dialect. It provides pieces that can be used to build a LISP dialect or can be used as a scripting layer.

Usage

What can you use it for?

1. Embedded script engine to provide dynamic behavior without requiring re-compilation of your application.
2. Business rule engine by exposing very specific & composable rule functions.
3. To build your own LISP dialect.

Sabre requires Go 1.13 or higher.

As Embedded Script Engine

Sabre has concept of Scope which is responsible for maintaining bindings. You can bind any Go value and access it using LISP code, which makes it possible to expose parts of your API and make it scriptable or build your own LISP dialect. Also, See Extending for more information on customizing the reader or eval.

package main

import "github.com/spy16/sabre"

func main() {
    scope := sabre.NewScope(nil)
    _ = scope.BindGo("inc", func(v int) int { return v+1 })

    result, _ := sabre.ReadEvalStr(scope, "(inc 10)")
    fmt.Printf("Result: %v\n", result) // should print "Result: 11"
}

Expose through a REPL

Sabre comes with a tiny repl package that is very flexible and easy to setup to expose your LISP through a read-eval-print-loop.

package main

import (
  "context"

  "github.com/spy16/sabre"
  "github.com/spy16/sabre/repl"
)

func main() {
  scope := sabre.NewScope(nil)
  scope.BindGo("inc", func(v int) int { return v+1 })

  repl.New(scope,
    repl.WithBanner("Welcome to my own LISP!"),
    repl.WithPrompts("=>", "|"),
    // many more options available
  ).Loop(context.Background())
}

Standalone

Sabre has a small reference LISP dialect named Slang (short for Sabre Lang) for which a standalone binary is available. Check out Slang for instructions on installing Slang.

Extending

Reader

Sabre reader is inspired by Clojure reader and uses a read table. Reader supports following forms:

• Numbers:
  • Integers use int64 Go representation and can be specified using decimal, binary hexadecimal or radix notations. (e.g., 123, -123, 0b101011, 0xAF, 2r10100, 8r126 etc.)
  • Floating point numbers use float64 Go representation and can be specified using decimal notation or scientific notation. (e.g.: 3.1412, -1.234, 1e-5, 2e3, 1.5e3 etc.)
• Characters: Characters use rune or uint8 Go representation and can be written in 3 ways:
  • Simple: \a, \λ, \β etc.
  • Special: \newline, \tab etc.
  • Unicode: \u1267
• Boolean: true or false are converted to Bool type.
• Nil: nil is represented as a zero-allocation empty struct in Go.
• Keywords: Keywords are like symbols but start with : and evaluate to themselves.
• Symbols: Symbols can be used to name a value and can contain any Unicode symbol.
• Lists: Lists are zero or more forms contained within parenthesis. (e.g., (1 2 3), (1 [])). Evaluating a list leads to an invocation.
• Vectors: Vectors are zero or more forms contained within brackets. (e.g., [], [1 2 3])
• Sets: Set is a container for zero or more unique forms. (e.g. #{1 2 3})
• HashMaps: HashMap is a container for key-value pairs (e.g., {:name "Bob" :age 10})

Reader can be extended to add new syntactical features by adding reader macros to the read table. Reader Macros are implementations of sabre.ReaderMacro function type. Except numbers and symbols, everything else supported by the reader is implemented using reader macros.

Evaluation

• Keyword, String, Int, Float, Character, Bool, nil, MultiFn, Fn, Type and Any evaluate to themselves.
• Symbol is resolved as follows:
  • If symbol has no ., symbol is directly used to lookup in current Scope to find the value.
  • If symbol is qualified (i.e., contains .), symbol is split using . as delimiter and first field is resolved as per previous rule and rest of the fields are recursively resolved as members. (For example, foo.Bar.Baz: foo is resolved from scope, Bar should be member of value of foo. And Baz should be member of value resolved for foo.Bar)
• Evaluating HashMap, Vector & Set simply yields new hashmap, vector and set whose values are evaluated values contained in the original hashmaap, vector and set.
• Evaluating Module evaluates all the forms in the module and returns the result of last evaluation. Any error stops the evaluation process.
• Empty List is returned as is.
• Non empty List is an invocation and evaluated using following rules:
  • If the first argument resolves to a special-form (SpecialForm Go type), it is invoked and return value is cached in the list. This return value is used for evaluating the list.
  • If the first argument resolves to a Macro, macro is invoked with the rest of the list as arguments and return value replaces the list with (do retval) form.
  • If first value resolves to an Invokable value, Invoke() is called. Functions are implemented using MultiFn which implements Invokable. Vector also implements Invokable and provides index access.
  • It is an error.