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Diffstat (limited to 'src/bidirectional.rs')
-rw-r--r-- | src/bidirectional.rs | 120 |
1 files changed, 120 insertions, 0 deletions
diff --git a/src/bidirectional.rs b/src/bidirectional.rs new file mode 100644 index 0000000..493ae2f --- /dev/null +++ b/src/bidirectional.rs @@ -0,0 +1,120 @@ +// Simple bidirectional type checking + +use crate::ast::*; + +/// Checking judgement: takes an expression and a type to check against and calls out to `infer` as needed. +pub fn check(context: &Context, expression: Expression, target: &Type) -> Result<(), String> { + match expression { + // fall through to inference mode + Expression::Annotation { expr, kind } => { + let result = infer(context, Expression::Annotation { expr, kind })?; + return match subtype(&result, &target) { + true => Ok(()), + false => Err(format!("inferred type {result} does not match target {target}")) + } + }, + // Bt-CheckInfer + Expression::Constant { term } => match subtype(&convert(&term)?, &target) { + true => Ok(()), + false => Err(format!("constant is of wrong type, expected {target}")) + // false => Ok(()) // all our constants are Empty for now + }, + // Bt-CheckInfer + Expression::Variable { id } => match context.get(&id) { + Some(term) if subtype(&convert(term)?, &target) => Ok(()), + Some(_) => Err(format!("variable {id} is of wrong type")), + None => Err(format!("failed to find variable {id} in context")) + }, + // Bt-Abs + Expression::Abstraction { param, func } => match target { + Type::Function { from, to } => { + let mut context = context.clone(); + context.insert(param, default(from)?); + return check(&context, *func, &to); + }, + _ => Err(format!("attempting to check an abstraction with a non-function type {target}")) + }, + // fall through to inference mode + Expression::Application { func, arg } => { + let result = &infer(context, Expression::Application { func, arg })?; + return match subtype(&result, &target) { + true => Ok(()), + false => Err(format!("inferred type {result} does not match {target}")) + } + }, + // T-If + Expression::Conditional { if_cond, if_then, if_else } => { + check(context, *if_cond, &Type::Boolean)?; + check(context, *if_then, &target)?; + check(context, *if_else, &target)?; + return Ok(()); + } + } +} + +/// Inference judgement: takes an expression and attempts to infer the associated type. +pub fn infer(context: &Context, expression: Expression) -> Result<Type, String> { + match expression { + // Bt-Ann + Expression::Annotation { expr, kind } => check(context, *expr, &kind).map(|x| kind), + // Bt-True / Bt-False / etc + Expression::Constant { term } => convert(&term), + // Bt-Var + Expression::Variable { id } => match context.get(&id) { + Some(term) => infer(&Context::new(), Expression::Constant { term: term.clone() }), + None => Err(format!("failed to find variable in context {context:?}")) + }, + // Bt-App + Expression::Application { func, arg } => match infer(context, *func)? { + Type::Function { from, to } => check(context, *arg, &*from).map(|x| *to), + _ => Err(format!("application abstraction is not a function type")) + }, + // inference from an abstraction is always an error + // we could try and infer the func without adding the parameter to scope: + // but this is overwhelmingly likely to be an error, so just report it now. + Expression::Abstraction { param, func } => + Err(format!("attempting to infer from an abstraction")), + // idk + Expression::Conditional { if_cond, if_then, if_else } => { + check(context, *if_cond, &Type::Boolean)?; + let if_then = infer(context, *if_then)?; + let if_else = infer(context, *if_else)?; + if subtype(&if_then, &if_else) && subtype(&if_else, &if_then) { + Ok(if_then) // fixme: should be the join + } else { + Err(format!("if clauses of different types: {if_then} and {if_else}")) + } + } + } +} + +/// The subtyping relation between any two types. +pub fn subtype(is: &Type, of: &Type) -> bool { + match (is, of) { + (Type::Record(is_fields), Type::Record(of_fields)) => { + // width, depth, and permutation + for (key, of_value) in of_fields { + match is_fields.get(key) { + Some(is_value) => { + if !subtype(is_value, of_value) { + return false; + } + } + None => return false + } + } + return true; + }, + (Type::Enum(is_variants), Type::Enum(of_variants)) => false, // fixme + (Type::Function { from: is_from, to: is_to }, + Type::Function { from: of_from, to: of_to }) => { + subtype(of_from, is_from) && subtype(is_to, of_to) + }, + (Type::Natural, Type::Integer) => true, // obviously not, but let's pretend + (_, Type::Empty) => true, + (Type::Error, _) => true, + (_, _) if is == of => true, + (_, _) => false + } +} + |