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// Simple bidirectional type checking
#![allow(unused_variables)]
use crate::ast::*;
pub fn check(context: Context, expression: Expression, target: Type) -> bool {
match expression {
// should never happen
Expression::Annotation { expr, kind } => (kind == target) && check(context, *expr, kind),
Expression::Constant { term } => term.kind == target,
Expression::Variable { id } => {
// in the future: extend to closures? nah probably not
match context.get(&id) {
Some(term) if term.kind == target => true,
_ => false
}
},
Expression::Abstraction { param, func } => {
match target {
Type::Function { from, to } => {
let mut context = context;
context.insert(param, Term { val: 0, kind: *from }); // hack
return check(context, *func, *to);
},
_ => false
}
},
// should never happen
Expression::Application { func, arg } => check(context, *func, target),
Expression::Conditional { if_cond, if_then, if_else } => {
check(context.clone(), *if_cond, Type::Boolean) &&
check(context.clone(), *if_then, target.clone()) &&
check(context.clone(), *if_else, target.clone())
}
}
}
// empty's gonna cause problems
pub fn infer(context: Context, expression: Expression) -> Option<Type> {
match expression {
Expression::Annotation { expr, kind } => {
match check(context, *expr, kind.clone()) {
true => Some(kind),
false => None
}
},
Expression::Constant { term } => Some(term.kind),
Expression::Variable { id } => {
match context.get(&id) {
Some(term) => Some(term.clone().kind),
None => None
}
},
Expression::Application { func, arg } => {
// Bt-App:
// \Gamma \derives t_1 \infer \tau_1 \to \tau_2, \Gamma \derives t_2 \check \tau_1
// -------------------------------------------
// \Gamma \derives t_1 t_2 \infer \tau_2
match infer(context.clone(), *func) {
Some(Type::Function { from, to }) => {
match check(context, *arg, *from) {
true => Some(*to),
false => None
}
},
_ => None
}
},
// should not happen
Expression::Abstraction { param, func } => infer(context, *func),
// idk
Expression::Conditional { if_cond, if_then, if_else } => {
if check(context.clone(), *if_cond, Type::Boolean) {
let kind = infer(context.clone(), *if_then);
if kind == infer(context, *if_else) {
return kind;
}
}
return None;
}
}
}
/// Evaluates an expression given a context (of variables) to a term.
/// Panics on non-evaluatable code.
pub fn execute(context: Context, expression: Expression) -> Result<Term, &'static str> {
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("no such variable in context")
}
},
Expression::Abstraction { .. } => Err("attempting to execute an abstraction"),
Expression::Application { func, arg } => {
match *func {
Expression::Abstraction { param, func } => {
let result = execute(context.clone(), *arg);
match result {
Ok(value) => {
let mut context = context;
context.insert(param, value);
return execute(context, *func);
},
Err(e) => Err(e)
}
},
_ => Err("attempting to execute an application to nothing")
}
},
Expression::Conditional { if_cond, if_then, if_else } => {
match execute(context.clone(), *if_cond) {
Ok(Term { val: 1, .. }) => execute(context, *if_then),
Ok(Term { val: 0, .. }) => execute(context, *if_else),
_ => Err("invalid type for a conditional")
}
}
}
}
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