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/// A lightweight interpreter interface.
pub trait Interpreter: 'static {
/// The value type the interpreter uses and returns.
///
/// It is usually a trait object that allows supporting different kinds
/// of actual values.
///
/// Refer to the `value` module for example implementations.
type Value: Clone;
/// The error type that the interpreter raises.
///
/// Could be a string, or the same as the `Value` type, or a dedicated
/// error type.
type Error: From<String> + From<&'static String>;
/// The environment type that is used to evaluate code.
///
/// Environments are complementary (mutable) structures that helps
/// resolving static expressions.
///
/// For example, environments might include these kinds of information:
/// - The state (registers, heap and the stack). Tracking local and outer
/// variables.
/// - What to do next (the program counter). Making it possible to suspend
/// and resume execution. This would require explicit support from the
/// stdlib so execution can be suspended and resumed.
///
/// Refer to the `env` module for example implementations.
type Env;
/// The expression type.
///
/// The type that can represent parsed code of this language.
/// If the language wants to operate code as data, then there
/// needs a way to convert `Expr` to `Value`.
/// For interpreters that use bytecode, this might also include
/// the bytecode.
///
/// Refer to the `expr` module for example implementations.
type Expr;
/// Evaluate code in string. This is a shortcut for calling `parse`, and
/// `evaluate` with `global_env()`.
fn interpret(code: &str) -> Result<Self::Value, Self::Error> {
let env = Self::global_env();
let expr = Self::parse(code)?;
Self::evaluate(&env, &expr)
}
/// Parse code into an `Expr`.
///
/// For interpreters that use bytecode, consider compiling the code into
/// bytecode in this function.
fn parse(code: &str) -> Result<Self::Expr, Self::Error>;
/// Define the default global environment.
///
/// Usually this defines the standard library.
fn global_env() -> Self::Env;
/// Evaluate an expression in a given environment.
fn evaluate(env: &Self::Env, expr: &Self::Expr) -> Result<Self::Value, Self::Error>;
}