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// Bidirectional type checking, subtyping, and typeclasses
use core::fmt;
use std::collections::HashMap;
pub type Identifier = String;
pub type Context = HashMap<Identifier, Term>;
// note: built-in functions do NOT go here!
#[derive(Debug, Clone, PartialEq)]
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>}
}
#[derive(Debug, Clone, PartialEq)]
pub enum Type {
Empty,
Error,
Unit,
Boolean,
Natural,
Integer,
Float,
String,
Enum(Vec<Type>),
Record(HashMap<Identifier, Type>),
Function{from: Box<Type>, to: Box<Type>},
}
#[derive(Debug, Clone, PartialEq)]
pub enum Term {
Unit(),
Boolean(bool),
Natural(usize),
Integer(isize),
Float(f32),
String{len: usize, cap: usize, data: Vec<usize>},
Enum{val: usize, data: Vec<Type>}, // is this right?
Record(HashMap<Identifier, Term>), // is this right?
Function(Box<Expression>) // this should allow us to bind functions
}
impl fmt::Display 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),
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),
}
}
}
impl fmt::Display for Type {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
Type::Empty => write!(f, "⊤"),
Type::Error => write!(f, "⊥"),
Type::Unit => write!(f, "unit"),
Type::Boolean => write!(f, "bool"),
Type::Natural => write!(f, "nat"),
Type::Integer => write!(f, "int"),
Type::Float => write!(f, "float"),
Type::String => write!(f, "str"),
Type::Enum(data) => write!(f, "({:?})", data),
Type::Record(data) => write!(f, "{{{:?}}}", data),
Type::Function { from, to } => write!(f, "{}->{}", from, to),
}
}
}
impl fmt::Display for Term {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
Term::Unit() => write!(f, "∅"),
Term::Boolean(term) => write!(f, "{}", term),
Term::Natural(term) => write!(f, "{}", term),
Term::Integer(term) => write!(f, "{}", term),
Term::Float(term) => write!(f, "{}", term),
Term::String { len, cap, data } => write!(f, "\"{:?}\"", data),
Term::Enum { val, data } => write!(f, "{:?}", data.get(*val)),
Term::Record(term) => write!(f, "{:?}", term),
Term::Function(expr) => write!(f, "{}", *expr),
}
}
}
// hatehatehate that you can't implement a trait for foreign types
// impl<T> fmt::Display for Vec<T> where T: fmt::Display {
// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
// for (i, val) in self.enumerate() {
// if i == 0 {
// write!(f, "{}", val);
// } else {
// write!(f, ",{}", val);
// }
// }
// return Ok(());
// }
// }
// impl<T, U> fmt::Display for HashMap<T, U> where T: fmt::Display, U: fmt::Display {
// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
// for (i, (key, val)) in self.enumerate() {
// if i == 0 {
// write!(f, "{}={}", key, val);
// } else {
// write!(f, ",{}={}", key, val);
// }
// }
// return Ok(());
// }
// }
/// Convert a term into its corresponding type.
pub fn convert(term: &Term) -> Result<Type, String> {
match term {
Term::Unit() => Ok(Type::Unit),
Term::Boolean(_) => Ok(Type::Boolean),
Term::Natural(_) => Ok(Type::Natural),
Term::Integer(_) => Ok(Type::Integer),
Term::Float(_) => Ok(Type::Float),
Term::String { len, cap, data } => Ok(Type::String),
Term::Enum { val, data } => data.get(*val)
.ok_or_else(|| "enum value out of range!".to_string()).cloned(),
Term::Record(data) => {
let mut result = HashMap::new();
for (key, val) in data {
result.insert(key.clone(), convert(val)?);
}
return Ok(Type::Record(result));
},
Term::Function(func) => match *func.clone() {
Expression::Annotation { expr, kind } => match kind {
Type::Function { from, to } => Ok(Type::Function { from, to }),
_ => Err("function term value not a function!".to_string())
}
_ => Err("function term value does not have an annotation!".to_string())
}
}
}
/// Get the default value of a type. Throws an error if it doesn't exist.
pub fn default(kind: &Type) -> Result<Term, String> {
match kind {
Type::Empty => Err("attempting to take the default term for empty".to_string()),
Type::Error => Err("attempting to take the default term for error".to_string()),
Type::Unit => Ok(Term::Unit()),
Type::Boolean => Ok(Term::Boolean(false)),
Type::Natural => Ok(Term::Natural(0)),
Type::Integer => Ok(Term::Integer(0)),
Type::Float => Ok(Term::Float(0.0)),
Type::String => Ok(Term::String { len: 0, cap: 0, data: vec!()}),
Type::Enum(data) => match data.len() {
0 => Err("attempting to get a default term of an enum with no variants!".to_string()),
_ => Ok(Term::Enum { val: 0, data: data.clone() })
},
Type::Record(data) => {
let mut result = HashMap::new();
for (key, val) in data {
result.insert(key.clone(), default(&val)?);
}
return Ok(Term::Record(result));
},
Type::Function { from, to } =>
Err("attempting to take the default term of a function type".to_string()),
}
}
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