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use crate::ast::*;
impl Context {
/// Evaluates an expression given a context (of variables) to a term, or fails.
pub fn execute(&self, expression: Expression) -> Result<Term> {
match expression {
Expression::Annotation { expr, .. } => self.execute(*expr),
Expression::Constant { term } => Ok(term),
Expression::Variable { id } => match self.get_term(&id) {
Some(term) => Ok(term.clone()),
None => Err(format!("no such variable in context {self:?}").into())
},
Expression::Abstraction { param, func } =>
Err(format!("attempting to execute an abstraction ({}){}", param, func).into()),
Expression::Application { func, arg } => match *func {
Expression::Abstraction { param, func } => {
let value = self.execute(*arg)?;
let mut context = self.clone();
context.insert_term(param, value);
return context.execute(*func);
}
_ => Err(format!("attempting to execute an application to non-abstraction {}", *func).into())
},
Expression::Conditional { if_cond, if_then, if_else } => {
match self.execute(*if_cond)? {
Term::Boolean(true) => self.execute(*if_then),
Term::Boolean(false) => self.execute(*if_else),
term => Err(format!("invalid type {} for a conditional", &term.convert()?).into())
}
}
}
}
}
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