able-script/src/interpret.rs

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//! Expression evaluator and statement interpreter.
//!
//! To interpret a piece of AbleScript code, you first need to
//! construct a [Scope], which is responsible for storing the list of
//! variable and function definitions accessible from an AbleScript
//! snippet. You can then call [Scope::eval_items] to evaluate or
//! execute any number of expressions or statements.
#[deny(missing_docs)]
use std::collections::HashMap;
use std::convert::TryFrom;
use crate::{
error::{Error, ErrorKind},
parser::item::{Expr, Iden, Item, Stmt},
variables::{Value, Variable},
};
/// A set of visible variable and function definitions, which serves
/// as a context in which expressions can be evaluated.
pub struct Scope {
/// The mapping from variable names to values.
variables: HashMap<String, Variable>,
// In the future, this will store functio definitions, a link to a
// parent scope (so we can have nested scopes), and possibly other
// information.
}
impl Scope {
/// Create a new Scope with no predefined variable definitions or
/// other information.
pub fn new() -> Self {
Self {
variables: HashMap::new(),
}
}
/// Evaluate a set of Items. Returns the value of the last Item
/// evaluated, or an error if one or more of the Items failed to
/// evaluate.
pub fn eval_items(&mut self, items: &[Item]) -> Result<Value, Error> {
items
.iter()
.map(|item| self.eval_item(item))
.try_fold(Value::Nul, |_, result| result)
}
/// Evaluate a single Item, returning its value or an error.
fn eval_item(&mut self, item: &Item) -> Result<Value, Error> {
match item {
Item::Expr(expr) => self.eval_expr(expr),
Item::Stmt(stmt) => self.eval_stmt(stmt).map(|_| Value::Nul),
}
}
/// Evaluate an Expr, returning its value or an error.
fn eval_expr(&self, expr: &Expr) -> Result<Value, Error> {
use Expr::*;
use Value::*;
// NOTE(Alex): This is quite nasty, and should probably be
// re-done using macros or something.
Ok(match expr {
Add { left, right } => {
Int(i32::try_from(self.eval_expr(left)?)? + i32::try_from(self.eval_expr(right)?)?)
}
Subtract { left, right } => {
Int(i32::try_from(self.eval_expr(left)?)? - i32::try_from(self.eval_expr(right)?)?)
}
Multiply { left, right } => {
Int(i32::try_from(self.eval_expr(left)?)? * i32::try_from(self.eval_expr(right)?)?)
}
Divide { left, right } => {
Int(i32::try_from(self.eval_expr(left)?)? / i32::try_from(self.eval_expr(right)?)?)
}
Lt { left, right } => {
Bool(i32::try_from(self.eval_expr(left)?)? < i32::try_from(self.eval_expr(right)?)?)
}
Gt { left, right } => {
Bool(i32::try_from(self.eval_expr(left)?)? > i32::try_from(self.eval_expr(right)?)?)
}
Eq { left, right } => Bool(self.eval_expr(left)? == self.eval_expr(right)?),
Neq { left, right } => Bool(self.eval_expr(left)? != self.eval_expr(right)?),
And { left, right } => Bool(
bool::try_from(self.eval_expr(left)?)? && bool::try_from(self.eval_expr(right)?)?,
),
Or { left, right } => Bool(
bool::try_from(self.eval_expr(left)?)? || bool::try_from(self.eval_expr(right)?)?,
),
Not(expr) => Bool(!bool::try_from(self.eval_expr(expr)?)?),
Literal(value) => value.clone(),
Identifier(Iden(name)) => self
.variables
.get(name)
.ok_or_else(|| Error {
kind: ErrorKind::UnknownVariable(name.to_owned()),
// TODO: figure out some way to avoid this 0..0
// dumbness
position: 0..0,
})
.and_then(|var| {
if !var.melo {
Ok(var.value.clone())
} else {
Err(Error {
kind: ErrorKind::MeloVariable(name.to_owned()),
position: 0..0,
})
}
})?,
})
}
/// Perform the action indicated by a statement.
fn eval_stmt(&mut self, stmt: &Stmt) -> Result<(), Error> {
match stmt {
Stmt::Print(expr) => {
println!("{}", self.eval_expr(expr)?);
Ok(())
}
_ => {
todo!()
}
}
}
}