First pass at pattern exhaustivess checking

compiler/mono/src/pattern.rs

@@ -0,0 +1,482 @@
+use roc_collections::all::MutMap;
+use roc_module::ident::TagName;
+use roc_region::all::{Located, Region};
+
+use self::Pattern::*;
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct Union {
+    alternatives: Vec<Ctor>,
+    num_alts: usize,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct Ctor {
+    name: TagName,
+    arity: usize,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Pattern {
+    Anything,
+    Literal(Literal),
+    Ctor(Union, TagName, std::vec::Vec<Pattern>),
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Literal {
+    Num(i64),
+    Int(i64),
+    Float(f64),
+    Str(Box<str>),
+}
+
+fn simplify(pattern: &roc_can::pattern::Pattern) -> Pattern {
+    let mut errors = Vec::new();
+
+    simplify_help(pattern, &mut errors)
+}
+
+fn simplify_help(pattern: &roc_can::pattern::Pattern, errors: &mut Vec<Error>) -> Pattern {
+    use roc_can::pattern::Pattern::*;
+
+    match pattern {
+        IntLiteral(v) => Literal(Literal::Int(*v)),
+        NumLiteral(_, v) => Literal(Literal::Int(*v)),
+        FloatLiteral(v) => Literal(Literal::Float(*v)),
+        StrLiteral(v) => Literal(Literal::Str(v.clone())),
+
+        Underscore => Anything,
+        Identifier(_) => Anything,
+        RecordDestructure { .. } => {
+            // TODO we must check the guard conditions!
+            Anything
+        }
+
+        Shadowed(_region, _ident) => {
+            // Treat as an Anything
+            // code-gen will make a runtime error out of the branch
+            Anything
+        }
+        UnsupportedPattern(_region) => {
+            // Treat as an Anything
+            // code-gen will make a runtime error out of the branch
+            Anything
+        }
+
+        AppliedTag {
+            tag_name,
+            arguments,
+            ..
+        } => {
+            let union = Union {
+                alternatives: Vec::new(),
+                num_alts: 0,
+            };
+            let simplified_args: std::vec::Vec<_> = arguments
+                .iter()
+                .map(|v| simplify_help(&v.1.value, errors))
+                .collect();
+            Ctor(union, tag_name.clone(), simplified_args)
+        }
+    }
+}
+
+/// Error
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Error {
+    Incomplete(Region, Context, Vec<Pattern>),
+    Redundant(Region, Region, usize),
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Context {
+    BadArg,
+    BadDestruct,
+    BadCase,
+}
+
+/// Check
+
+pub fn check(
+    region: Region,
+    patterns: &[Located<roc_can::pattern::Pattern>],
+) -> Result<(), Vec<Error>> {
+    let mut errors = Vec::new();
+    check_patterns(region, Context::BadArg, patterns, &mut errors);
+
+    if errors.is_empty() {
+        Ok(())
+    } else {
+        Err(errors)
+    }
+}
+
+// pub fn check(module: roc_can::module::ModuleOutput) -> Result<(), Vec<Error>> {
+//     let mut errors = Vec::new();
+//     check_declarations(&module.declarations, &mut errors);
+//
+//     if errors.is_empty() {
+//         Ok(())
+//     } else {
+//         Err(errors)
+//     }
+// }
+//
+// /// CHECK DECLS
+//
+// fn check_declarations(decls: &[roc_can::def::Declaration], errors: &mut Vec<Error>) {
+//     use roc_can::def::Declaration;
+//
+//     for decl in decls {
+//         Declaration::Declare(def) => check_def(def, errors),
+//         Declaration::DeclareRef(defs) => {
+//             for def in defs {
+//                 check_def(def, errors);
+//             }
+//         }
+//         Declaration::InvalidCycle(_) => {}
+//     }
+// }
+//
+// fn check_def(def: &roc_can::def::Def, errors: &mut Vec<Error>) {
+//     check_patttern
+//
+//
+// }
+
+pub fn check_patterns(
+    region: Region,
+    context: Context,
+    patterns: &[Located<roc_can::pattern::Pattern>],
+    errors: &mut Vec<Error>,
+) {
+    match to_nonredundant_rows(region, patterns) {
+        Err(err) => errors.push(err),
+        Ok(matrix) => {
+            let bad_patterns = is_exhaustive(&matrix, 1);
+            if !bad_patterns.is_empty() {
+                // TODO i suspect this is like a concat in in practice? code below can panic
+                let heads = bad_patterns.into_iter().map(|mut v| v.remove(0)).collect();
+                errors.push(Error::Incomplete(region, context, heads));
+            }
+        }
+    }
+}
+
+/// EXHAUSTIVE PATTERNS
+
+/// INVARIANTS:
+///
+///   The initial rows "matrix" are all of length 1
+///   The initial count of items per row "n" is also 1
+///   The resulting rows are examples of missing patterns
+fn is_exhaustive(matrix: &PatternMatrix, n: usize) -> PatternMatrix {
+    if matrix.is_empty() {
+        vec![std::iter::repeat(Anything).take(n).collect()]
+    } else if n == 0 {
+        vec![]
+    } else {
+        let ctors = collect_ctors(matrix);
+        let num_seen = ctors.len();
+
+        if num_seen == 0 {
+            let new_matrix = matrix
+                .iter()
+                .filter_map(specialize_row_by_anything)
+                .collect();
+            let mut rest = is_exhaustive(&new_matrix, n - 1);
+
+            for row in rest.iter_mut() {
+                row.push(Anything);
+            }
+
+            rest
+        } else {
+            let alts = ctors.iter().next().unwrap().1;
+
+            let alt_list = &alts.alternatives;
+            let num_alts = alts.num_alts;
+
+            if num_seen < num_alts {
+                let new_matrix = matrix
+                    .iter()
+                    .filter_map(specialize_row_by_anything)
+                    .collect();
+                let rest: Vec<Vec<Pattern>> = is_exhaustive(&new_matrix, n - 1);
+
+                let last: _ = alt_list
+                    .iter()
+                    .filter_map(|r| is_missing(alts.clone(), ctors.clone(), r));
+
+                let mut result = Vec::new();
+
+                for last_option in last {
+                    for mut row in rest.clone() {
+                        row.push(last_option.clone());
+
+                        result.push(row);
+                    }
+                }
+
+                result
+            } else {
+                let is_alt_exhaustive = |Ctor { name, arity }| {
+                    let new_matrix = matrix
+                        .iter()
+                        .filter_map(|r| specialize_row_by_ctor(&name, arity, r))
+                        .collect();
+                    let rest: Vec<Vec<Pattern>> = is_exhaustive(&new_matrix, arity + n - 1);
+
+                    let mut result = Vec::with_capacity(rest.len());
+                    for row in rest {
+                        result.push(recover_ctor(alts.clone(), name.clone(), arity, row));
+                    }
+
+                    result
+                };
+
+                alt_list
+                    .iter()
+                    .cloned()
+                    .map(is_alt_exhaustive)
+                    .flatten()
+                    .collect()
+            }
+        }
+    }
+}
+
+fn is_missing<T>(union: Union, ctors: MutMap<TagName, T>, ctor: &Ctor) -> Option<Pattern> {
+    let Ctor { name, arity, .. } = ctor;
+
+    if ctors.contains_key(&name) {
+        None
+    } else {
+        let anythings = std::iter::repeat(Anything).take(*arity).collect();
+        Some(Pattern::Ctor(union, name.clone(), anythings))
+    }
+}
+
+fn recover_ctor(
+    union: Union,
+    tag_name: TagName,
+    arity: usize,
+    mut patterns: Vec<Pattern>,
+) -> Vec<Pattern> {
+    // TODO ensure that this behaves the same as haskell's splitAt
+    let mut rest = patterns.split_off(arity);
+    let args = patterns;
+
+    rest.push(Ctor(union, tag_name, args));
+
+    rest
+}
+
+/// REDUNDANT PATTERNS
+
+/// INVARIANT: Produces a list of rows where (forall row. length row == 1)
+fn to_nonredundant_rows(
+    overall_region: Region,
+    patterns: &[Located<roc_can::pattern::Pattern>],
+) -> Result<Vec<Vec<Pattern>>, Error> {
+    let mut checked_rows = Vec::with_capacity(patterns.len());
+
+    for loc_pat in patterns {
+        let region = loc_pat.region;
+
+        let next_row = vec![simplify(&loc_pat.value)];
+
+        if is_useful(&checked_rows, &next_row) {
+            checked_rows.push(next_row);
+        } else {
+            return Err(Error::Redundant(
+                overall_region,
+                region,
+                checked_rows.len() + 1,
+            ));
+        }
+    }
+
+    Ok(checked_rows)
+}
+
+/// Check if a new row "vector" is useful given previous rows "matrix"
+fn is_useful(matrix: &PatternMatrix, vector: &Row) -> bool {
+    if matrix.is_empty() {
+        // No rows are the same as the new vector! The vector is useful!
+        true
+    } else if vector.is_empty() {
+        // There is nothing left in the new vector, but we still have
+        // rows that match the same things. This is not a useful vector!
+        false
+    } else {
+        let mut vector = vector.clone();
+        let first_pattern = vector.remove(0);
+        let patterns = vector;
+
+        match first_pattern {
+            // keep checking rows that start with this Ctor or Anything
+            Ctor(_, name, args) => {
+                let new_matrix: Vec<_> = matrix
+                    .iter()
+                    .filter_map(|r| specialize_row_by_ctor(&name, args.len(), r))
+                    .collect();
+
+                let mut new_row = Vec::new();
+                new_row.extend(args);
+                new_row.extend(patterns);
+
+                is_useful(&new_matrix, &new_row)
+            }
+
+            Anything => {
+                // check if all alts appear in matrix
+                match is_complete(matrix) {
+                    Complete::No => {
+                        // This Anything is useful because some Ctors are missing.
+                        // But what if a previous row has an Anything?
+                        // If so, this one is not useful.
+                        let new_matrix: Vec<_> = matrix
+                            .iter()
+                            .filter_map(|r| specialize_row_by_anything(r))
+                            .collect();
+
+                        is_useful(&new_matrix, &patterns)
+                    }
+                    Complete::Yes(alts) => {
+                        // All Ctors are covered, so this Anything is not needed for any
+                        // of those. But what if some of those Ctors have subpatterns
+                        // that make them less general? If so, this actually is useful!
+                        let is_useful_alt = |Ctor { name, arity, .. }| {
+                            let new_matrix = matrix
+                                .iter()
+                                .filter_map(|r| specialize_row_by_ctor(&name, arity, r))
+                                .collect();
+                            let mut new_row: Vec<Pattern> =
+                                std::iter::repeat(Anything).take(arity).collect::<Vec<_>>();
+
+                            new_row.extend(patterns.clone());
+
+                            is_useful(&new_matrix, &new_row)
+                        };
+
+                        alts.iter().cloned().any(is_useful_alt)
+                    }
+                }
+            }
+
+            Literal(literal) => {
+                // keep checking rows that start with this Literal or Anything
+                let new_matrix = matrix
+                    .iter()
+                    .filter_map(|r| specialize_row_by_literal(&literal, r))
+                    .collect();
+                is_useful(&new_matrix, &patterns)
+            }
+        }
+    }
+}
+
+/// INVARIANT: (length row == N) ==> (length result == arity + N - 1)
+fn specialize_row_by_ctor(tag_name: &TagName, arity: usize, row: &Row) -> Option<Row> {
+    let mut row = row.clone();
+
+    let head = row.pop();
+    let patterns = row;
+
+    match head {
+        Some(Ctor(_,name, args)) =>
+            if &name == tag_name {
+                // TODO order!
+                let mut new_patterns = Vec::new();
+                new_patterns.extend(args);
+                new_patterns.extend(patterns);
+                Some(new_patterns)
+            } else {
+                None
+            }
+        Some(Anything) => {
+            // TODO order!
+            let new_patterns =
+                    std::iter::repeat(Anything).take(arity).chain(patterns).collect();
+            Some(new_patterns)
+            }
+        Some(Literal(_)) => panic!( "Compiler bug! After type checking, constructors and literal should never align in pattern match exhaustiveness checks."),
+        None => panic!("Compiler error! Empty matrices should not get specialized."),
+    }
+}
+
+/// INVARIANT: (length row == N) ==> (length result == N-1)
+fn specialize_row_by_literal(literal: &Literal, row: &Row) -> Option<Row> {
+    let mut row = row.clone();
+
+    let head = row.pop();
+    let patterns = row;
+
+    match head {
+        Some(Literal(lit)) => if &lit == literal { Some(patterns) } else{  None } ,
+        Some(Anything) => Some(patterns),
+
+      Some(Ctor(_,_,_)) => panic!( "Compiler bug! After type checking, constructors and literals should never align in pattern match exhaustiveness checks."),
+
+      None => panic!("Compiler error! Empty matrices should not get specialized."),
+        }
+}
+
+/// INVARIANT: (length row == N) ==> (length result == N-1)
+fn specialize_row_by_anything(row: &Row) -> Option<Row> {
+    let mut row = row.clone();
+
+    match row.pop() {
+        Some(Anything) => Some(row),
+        _ => None,
+    }
+}
+
+/// ALL CONSTRUCTORS ARE PRESENT?
+
+pub enum Complete {
+    Yes(Vec<Ctor>),
+    No,
+}
+
+fn is_complete(matrix: &PatternMatrix) -> Complete {
+    let ctors = collect_ctors(matrix);
+
+    let mut it = ctors.values();
+
+    match it.next() {
+        None => Complete::No,
+        Some(Union {
+            alternatives,
+            num_alts,
+            ..
+        }) => {
+            if ctors.len() == *num_alts {
+                Complete::Yes(alternatives.to_vec())
+            } else {
+                Complete::No
+            }
+        }
+    }
+}
+
+/// COLLECT CTORS
+
+type RefPatternMatrix = [Vec<Pattern>];
+type PatternMatrix = Vec<Vec<Pattern>>;
+type Row = Vec<Pattern>;
+
+fn collect_ctors(matrix: &RefPatternMatrix) -> MutMap<TagName, Union> {
+    let mut ctors = MutMap::default();
+
+    for row in matrix {
+        if let Some(Ctor(union, name, _)) = row.get(0) {
+            ctors.insert(name.clone(), union.clone());
+        }
+    }
+
+    ctors
+}