path: root/src/checker_set.rs

use crate::ast::*;
use crate::checker_state::*;

use std::iter::zip;
use tracing::instrument;

impl CheckerState {
    #[instrument(skip(self), level = "debug", fields(%set))]
    pub fn check_set(&self, set: &Set) -> Result<Set, CheckerError> {
        match set {
            Set::BuiltIn(_) => Ok(set.clone()),
            Set::Record(fields) => {
                let fields = fields
                    .iter()
                    .map(|RecordField { name, set }| {
                        let set = self.check_set(set)?;
                        Ok(RecordField {
                            name: name.clone(),
                            set,
                        })
                    })
                    .collect::<Result<Vec<_>, _>>()?;
                Ok(Set::Record(fields))
            }
            Set::Variant(fields) => {
                let fields = fields
                    .iter()
                    .map(|VariantField { name, set }| {
                        let set = self.check_set(set)?;
                        Ok(VariantField {
                            name: name.clone(),
                            set,
                        })
                    })
                    .collect::<Result<Vec<_>, _>>()?;
                Ok(Set::Variant(fields))
            }
            Set::ClaimedSet(_) => Err(CheckerError::Unimplemented("instances as sets".to_string())),
            Set::Var(v) => {
                let deref = self.lookup_set(v)?;
                Ok(deref.clone())
            }
        }
    }

    fn _check_literal_set_helper(&self, claimed: &Set, should_be: Set) -> Result<(), CheckerError> {
        if !self.set_equal(claimed, &should_be) {
            Err(CheckerError::WrongSetForElement(claimed.clone(), should_be))
        } else {
            Ok(())
        }
    }

    #[instrument(skip(self), level = "debug", fields(%element, %set))]
    pub fn check_element(&self, element: &Element, set: &Set) -> Result<Element, CheckerError> {
        match element {
            Element::Literal(lit) => {
                // we may infer the type from the element
                match lit {
                    Literal::Int(_) => {
                        self._check_literal_set_helper(set, Set::BuiltIn(BuiltIn::Int))?;
                    }
                    Literal::Nat(_) => {
                        self._check_literal_set_helper(set, Set::BuiltIn(BuiltIn::Nat))?;
                    }
                    Literal::Str(_) => {
                        self._check_literal_set_helper(set, Set::BuiltIn(BuiltIn::Str))?;
                    }
                    Literal::Bool(_) => {
                        self._check_literal_set_helper(set, Set::BuiltIn(BuiltIn::Bool))?;
                    }
                    Literal::Float(_) => {
                        self._check_literal_set_helper(set, Set::BuiltIn(BuiltIn::Float))?;
                    }
                }
                Ok(element.clone())
            }
            Element::Var(v) => {
                let lookup = self.lookup_element(v)?;
                // we have previously done the work to discover the type of
                // this element, so what we're claiming now must match!
                if !self.set_equal(set, &lookup.set) {
                    return Err(CheckerError::WrongSetForElement(
                        set.clone(),
                        lookup.set.clone(),
                    ));
                }
                Ok(lookup.element.clone())
            }
            Element::Record(assignations) => {
                let rej = |reason| CheckerError::ElementDoesNotBelong {
                    element: element.clone(),
                    claimed: set.clone(),
                    reason,
                };

                // make sure we are filling a record
                let fields = if let Set::Record(fields) = set {
                    Ok(fields)
                } else {
                    Err(rej("element is a record instance".to_string()))
                }?;

                let (set_fnames, set_fsets): (Vec<String>, Vec<Set>) = fields
                    .iter()
                    .map(|RecordField { name, set }| (name.clone(), set.clone()))
                    .unzip();
                let mut set_fnames_sorted = set_fnames.clone();
                set_fnames_sorted.sort();

                let (element_fnames, element_felements): (Vec<String>, Vec<&Element>) =
                    assignations
                        .iter()
                        .map(|ElemAssign { name, element }| (name.clone(), element))
                        .unzip();

                let mut element_fnames_sorted = element_fnames.clone();
                element_fnames_sorted.sort();

                if set_fnames_sorted != element_fnames_sorted {
                    return Err(rej(format!(
                        "expected [{}] but found [{}]",
                        set_fnames.join(", "),
                        element_fnames.join(", "),
                    )));
                }

                // recurse, sets have already been completely expanded
                let sub_els = zip(element_felements, set_fsets)
                    .map(|(e_f, e_s)| self.check_element(e_f, &e_s))
                    .collect::<Result<Vec<_>, _>>()?;
                // rebuild
                let assignations = zip(element_fnames, sub_els)
                    .map(|(name, element)| ElemAssign { name, element })
                    .collect();
                // resign?
                Ok(Element::Record(assignations))
            }
            Element::Project {
                element: inner,
                field,
            } => {
                // globally unique projections mean we know what the sets going
                // in and out must be
                let SetField {
                    field_set,
                    owner_set,
                } = self.lookup_record_field(field)?;

                // enforce the correct typing of the claimed result
                if !self.set_equal(set, field_set) {
                    return Err(CheckerError::WrongSetForElement(
                        set.clone(),
                        field_set.clone(),
                    ));
                }

                // enforce the correct typing of the element
                let inner = self.check_element(inner, owner_set)?;

                // Unfortunately we still have to do something nasty here to obtain the data
                let Element::Record(assignations) = inner else {
                    panic!("invariant violation: check_element returned non-record for record set");
                };
                let sub_element = assignations
                    .into_iter()
                    .find(|a| a.name == *field)
                    .expect("invariant violation: record missing field that was type-checked")
                    .element
                    .clone();

                Ok(sub_element)
            }
            Element::Inject {
                element: inner,
                field,
            } => {
                // globally unique injections mean that we know what the sets
                // going in and out must be, but compared to projections their
                // roles are here interchanged
                let SetField {
                    field_set,
                    owner_set,
                } = self.lookup_variant_field(field)?;

                // enforce the correct typing of the claimed result
                if !self.set_equal(set, owner_set) {
                    return Err(CheckerError::WrongSetForElement(
                        set.clone(),
                        owner_set.clone(),
                    ));
                }

                // enforce the correct typing of the element
                let element = self.check_element(inner, field_set)?;

                Ok(Element::Inject {
                    element: Box::new(element),
                    field: field.clone(),
                })
            }
            Element::Case { arms, scrutinee } => {
                // TODO: do we allow mapping out of bottom?
                if arms.is_empty() {
                    return Err(CheckerError::Unimplemented(
                        "mapping out of bottom types".to_string(),
                    ));
                }

                // 1. Syntactic checks
                // -------------------
                // arms agree on the set to which the scrutinee should belong
                let arm_owners = arms
                    .iter()
                    .map(|ca| self.lookup_variant_field(&ca.tag).map(|sf| &sf.owner_set))
                    .collect::<Result<Vec<_>, _>>()?;
                let owner = arm_owners[0]; // safe because of the above decision about bottom
                if !arm_owners.iter().all(|o| self.set_equal(owner, o)) {
                    return Err(CheckerError::IncosistentCaseScrutineeSet(element.clone()));
                }

                // all cases are handled
                let Set::Variant(fields) = owner else {
                    panic!(
                        "invariant violation: looking up the owner of a variant field resulted in a non-variant set",
                    )
                };
                let mut required_field_names_sorted: Vec<String> =
                    fields.iter().map(|vf| vf.name.clone()).collect();
                required_field_names_sorted.sort();
                let mut covered_field_names_sorted: Vec<String> =
                    arms.iter().map(|ca| ca.tag.clone()).collect();
                covered_field_names_sorted.sort();
                if required_field_names_sorted != covered_field_names_sorted {
                    return Err(CheckerError::IncompleteCaseAnalysis {
                        found: covered_field_names_sorted,
                        required: required_field_names_sorted,
                    });
                }

                // 2. semantic checks
                // ------------------

                // scrutinee must be of the same set that all the arms are
                // implying, in particular this implies that the following holds
                // `inner : self.lookup_variant_field(field).field_set`
                let scrutinee = self.check_element(scrutinee, owner)?;

                // which variant are we?
                let Element::Inject {
                    field,
                    element: inner,
                } = scrutinee
                else {
                    panic!(
                        "invariant violation: we believe element is of a variant set but it's not an injection"
                    );
                };

                // TODO: we would like to check that each arm is correct, but
                // there's no easy way to do this? we can insert hypothetical
                // elements of the correct type into the checkerstate, but if
                // the body exacts non-trivial computation we won't be to pass
                // further checks. In the future would could build first class
                // support for hypothetical elements and do proper bi-di
                // checking, but for now we only check the branch that matters.

                let CaseArm { tag, bound, body } = arms.iter().find(|ca| ca.tag == field).expect("invariant violation: we know that all cases are covered and that the element is of the valid type");
                let SetField { field_set, .. } = self.lookup_variant_field(tag)?;
                // TODO: if we were worried about overhead we'd have a separate
                // locals stack, though truly if we were worried about overhead
                // we'd not have NNN instances of clone elsewhere in the
                // codebase and we wouldn't be eagerly evaluating all
                // expressions fully.
                let mut new_context = self.clone();
                new_context.add_element(bound.clone(), *inner.clone(), field_set.clone())?;
                let computed = new_context.check_element(body, set)?;

                Ok(computed)
            }
        }
    }
}