Whisper is very much work-in-progress and still being designed. It is not in any state for contributions (probably?) except maybe by chatting with me cuz that sounds like fun. You can chat with me on Twitter, on Discord (sleffy#6314), or by email. That last one's a treasure hunt, I'm sure you can find it.

Whisper: a Logic Programming DSL for Rust

Whisper is a small, embeddable logic programming language in pure Rust. The whisper and whisper_ir crates include functionality for easily constructing Whisper syntax and compiling it into bytecode for execution. It supports a simplistic foreign function interface for calling into external code, as well as facilities for supporting backtracking for external goals. This makes it possible to use Whisper for doing a sort of traced proof-search, where if Whisper finds a solution, you can use external goals to track what steps it took to get there, and react accordingly.

Additionally through Serde, Whisper supports converting Rust structs to and from its internal term representation. This means Whisper can be used for doing unification and general reasoning about Rust terms, and then extracting the result back into native Rust.

Due to its structure as a logic programming language, it can also work well as an embedded read-heavy database. Writing would require recompiling a knowledge base, which is currently not terribly performant since emitting a compiled heap means reading through the entire knowledge base. This could be improved through incremental compilation but is not a high priority.

Motivation

Logic programming is a very convenient system for problems which need inference, but mixing multiple programming languages can be very inconvenient. Whisper is intended to serve as a simple, embeddable, and usably performant solution for embedding logic programs in Rust.

Whisper is not intended as:

• A super-fast, feature-rich Pro/Hilog implementation which just happens to have good Rust interop.
• A database capable of streaming to/from disk.
• A super-small interpreter with a tiny dependency footprint.
• A fully-fledged language of its own, which can run without any need for a host program.

Features

• Simple, human-readable syntax, with limited support for scoping constants
• A simple, easy to build and modify intermediate representation (IR)
• Macros for generating Whisper syntax programmatically, with quasiquoting support
• A parser for parsing files containing Whisper syntax
• Simple but powerful interface for external goals written in Rust, with support for properly handling backtracking
• Simple but powerful interface for external datatypes, with support for handling unifying two external values against each other

Example: Typechecker for the Simply Typed Lambda Calculus

Taken from whisper/examples/stlc.rs.

use ::{
    serde::{Deserialize, Serialize},
    whisper::{
        builder::QueryBuilder,
        ir::{IrNode, IrTermGraph},
        session::DebugHandler,
        Heap, Session, Symbol,
    },
};

#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum Type {
    Base(i32),
    Fun(Box<(Type, Type)>),
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum Term {
    Const(i32, Type),
    Var(i32),
    Lam(i32, Type, Box<Term>),
    App(Box<(Term, Term)>),
}

whisper::module! {
    fn stlc();

    context Gamma proves Term is_type Sigma if
        context Gamma proves Term is_type Sigma in extern,
        fail;

    // Rule 1.) Variables can be typed if they are in the context.
    context { X: Sigma | Gamma } proves ("Var" : X) is_type Sigma;
    context { Y: Tau   | Gamma } proves ("Var" : X) is_type Sigma if
        context Gamma proves ("Var" : X) is_type Sigma;

    // Rule 2.) Constants are automatically typed.
    context Gamma proves ("Const" : (C Tau)) is_type Tau;

    // Rule 3.) If adding `X: Sigma` to the context gives `E` some type `Tau`,
    // then the lambda expression `\X. E` can be typed as a function
    // `Sigma -> Tau`.
    context Gamma proves ("Lam" : (X Sigma E)) is_type ("Fun" : (Sigma Tau)) if
        context { X: Sigma | Gamma } proves E is_type Tau;

    // Rule 4.) If `E1` is a function `Sigma -> Tau` and `E2` is `Sigma`, then
    // applying `E1` to `E2` gets us a term of type `Tau`.
    context Gamma proves ("App" : (E1 E2)) is_type Tau if
        context Gamma proves E1 is_type ("Fun" : (Sigma Tau)),
        context Gamma proves E2 is_type Sigma;

    context Gamma proves Term is_type Sigma if
        context Gamma proves Term is_type Sigma failed in extern,
        fail;
}

whisper::query! {
    fn stlc_infer(term: &IrNode);

    // Note that if the line below weren't commented out, #term is
    // parsed as `(#term)` because it is of type `IrNode` and
    // therefore `query!` has to wrap it in a single-arity tuple.
    //
    // where Term is #term,
    context { 1: Ligma } proves #term is_type Tau;
}

#[derive(Debug)]
pub struct Typechecker {
    terms: IrTermGraph,
    root: Symbol,
    session: Session<DebugHandler>,
}

impl Typechecker {
    pub fn new() -> Self {
        use whisper::{ir::IrKnowledgeBase, SymbolTable};
        let symbols = SymbolTable::new();
        let mut terms = IrTermGraph::new(symbols.clone());
        let mut modules = IrKnowledgeBase::new(symbols.clone());

        let stlc_module = modules.new_named_module_with_root(Symbol::PUBLIC);
        stlc(&mut terms, &mut modules, stlc_module);
        modules.link(&mut terms, stlc_module);

        let root = modules[stlc_module].get_root().clone();

        let stlc_kb = whisper::trans::knowledge_base(&terms, &modules);
        println!(
            "Compiled knowledge base:\n{}",
            stlc_kb.get(Symbol::PUBLIC_INDEX).unwrap().display()
        );
        let stlc_session = Session::new(symbols.clone(), stlc_kb.into());

        Typechecker {
            terms,
            root,
            session: stlc_session,
        }
    }

    pub fn infer(&mut self, term: &Term) -> Option<Type> {
        let mut builder = QueryBuilder::new(Heap::new(self.terms.symbol_table().clone()));
        let term_addr = builder.bind(term);
        let ir_query = stlc_infer(&mut self.terms, &self.root, &term_addr);
        let query = builder.finish(&self.terms, &ir_query);

        self.session.load(query.into());

        if self.session.resume() {
            let addr = self.session.query_vars()[&"Tau".into()];
            Some(whisper_schema::serde::de::from_reader(self.session.heap().read_at(addr)).unwrap())
        } else {
            None
        }
    }
}

fn main() {
    let mut typechecker = Typechecker::new();
    let term = Term::App(Box::new((
        Term::Lam(0, Type::Base(132), Box::new(Term::Var(0))),
        Term::Var(1),
    )));
    let ty = typechecker.infer(&term);

    println!("Whisper input: {:?}", term);
    println!("Whisper output: {:?}", ty);
}

Example: Configuration Validator

See whisper_std/tests/foo_schema.rs for the test files as well.

use ::{
    failure::Error,
    im,
    serde::{
        de::{DeserializeOwned, Deserializer},
        Deserialize,
    },
    serde_json::Value,
    std::marker::PhantomData,
    whisper::{prelude::*, session::DebugHandler},
};

#[derive(Debug, Clone)]
pub struct Schema(SharedKnowledgeBase);

impl Schema {
    pub fn from_str<S: AsRef<str> + ?Sized>(string: &S) -> Self {
        let mut terms = IrTermGraph::new(SymbolTable::new());
        let ir_kb = terms.parse_knowledge_base_str(string).expect("oops");
        Self(whisper::trans::knowledge_base(&terms, &ir_kb).into())
    }

    pub fn from_embedded(embedded: fn(&mut IrTermGraph) -> IrKnowledgeBase) -> Self {
        let mut terms = IrTermGraph::new(SymbolTable::new());
        let ir_kb = embedded(&mut terms);
        Self(whisper::trans::knowledge_base(&terms, &ir_kb).into())
    }
}

whisper::query! {
    fn validator_query(input: IrNode);

    valid #input;
}

#[derive(Debug)]
pub struct Validator<T: DeserializeOwned> {
    symbols: SymbolTable,
    session: Session<DebugHandler>,
    _phantom: PhantomData<T>,
}

impl<T: DeserializeOwned> Validator<T> {
    pub fn new(schema: &Schema) -> Self {
        Self {
            symbols: schema.0.symbol_table().clone(),
            session: Session::new(schema.0.symbol_table().clone(), schema.0.clone()),
            _phantom: PhantomData,
        }
    }

    fn build_validator_query(&self, value: &Value) -> SharedQuery {
        let mut terms = IrTermGraph::new(self.symbols.clone());
        let mut builder = QueryBuilder::new(Heap::new(self.symbols.clone()));
        let bound = builder.bind(value);
        let ir_query = validator_query(&mut terms, &Symbol::MOD, bound);
        SharedQuery::from(builder.finish(&terms, &ir_query))
    }

    pub fn deserialize_validated<'de, D: Deserializer<'de>>(
        &mut self,
        deserializer: D,
    ) -> Result<T, Error> {
        let serializer = serde_json::value::Serializer;
        let json_value = serde_transcode::transcode(deserializer, serializer).unwrap();
        self.session.load(self.build_validator_query(&json_value));

        if self.session.resume() {
            Ok(serde_json::from_value(json_value)?)
        } else {
            failure::bail!("Failed to validate!");
        }
    }
}

whisper::knowledge_base! {
    fn foo_schema();

    // If TLS is disabled, we dgaf about the key/cert.
    valid Foo if
        Foo matches { tls_enabled: false | _ } in std::map::match;

    // If TLS is enabled, we want to be sure that key/cert are both `Some`.
    valid Foo if
        Foo matches {
            tls_enabled: true,
            tls: {
                cert: _,
                key: _,
            }
        } in std::map::match;

    // Eventually we'll have some stuff for error reporting in the stdlib.
    // There are a number of ways to get data out of Whisper in a way that's
    // convenient for aggregating errors, but for now imagine