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