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// Bidirectional type checking, simple types for effects (or perhaps subtyping?) and typeclasses
use core::fmt;
use std::collections::HashMap;
pub type Identifier = String;
pub type Context = HashMap<Identifier, Term>;
// note: when comes the time, we'll put effects in here (i think)
#[derive(Clone, PartialEq, Eq)]
pub enum Expression {
Annotation{expr: Box<Expression>, kind: Type},
Constant{term: Term},
Variable{id: Identifier},
// note: we keep parameters as an Identifier because we annotate the WHOLE Abstraction
Abstraction{param: Identifier, func: Box<Expression>},
Application{func: Box<Expression>, arg: Box<Expression>},
Conditional{if_cond: Box<Expression>, if_then: Box<Expression>, if_else: Box<Expression>}
}
// _every_ type in our language is represented as this and interpreted as a type.
// how to store more data than fits... hmm... a problem for later
pub type Value = u64;
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Type {
Empty,
Unit,
Boolean,
Natural,
Integer,
// Float,
// String,
// Enum(Vec<Type>),
// Record(Vec<Type>),
Function{from: Box<Type>, to: Box<Type>},
}
// this means that functions cannot have types? unless we put them as empty values ig
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Term {
pub val: Value,
pub kind: Type, // currently useless / redundant: will be useful for casting
}
impl fmt::Debug for Expression {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
Expression::Annotation { expr, kind } => write!(f, "({:?}: {:?})", expr, kind),
Expression::Constant { term } => write!(f, "'{}", term.val),
Expression::Variable { id } => write!(f, "{}", id),
Expression::Abstraction { param, func } => write!(f, "(λ{}.{:?})", param, func),
Expression::Application { func, arg } => write!(f, "({:?} {:?})", func, arg),
Expression::Conditional { if_cond, if_then, if_else } => write!(f, "(if {:?} then {:?} else {:?})", if_cond, if_then, if_else),
}
}
}
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