Merge pull request #681 from tweag/rec-let

Recursive let

benches/numeric/fibonacci.ncl

@@ -1,9 +1,9 @@
 {
-  run = {
-      f = fun n =>
-        if n == 0 || n == 1 then
-          1
-        else
-          f (n - 1) + f (n - 2)
-    }.f
+  run =
+    let rec f = fun n =>
+      if n == 0 || n == 1 then
+        1
+      else
+        f (n - 1) + f (n - 2)
+    in f
 }

doc/manual/syntax.md

@@ -328,7 +328,8 @@ Examples:
 ```
 
 ### Let-In
-Let-in allows the binding of an expression. It is used as `let <ident> = <expr> in <expr>`.
+Let-in allows the binding of an expression. It is used as `let <rec?> <ident> = <expr> in <expr>`.
+The `rec` keyword in Let-in constructs allows the let binding to become recursive, enabling the use of the `<ident>` within the first `<expr>`.
 
 Examples:
 ```
@@ -340,6 +341,15 @@ true
 
 > let a = 1 in let b = 2 in a + b
 3
+
+> let rec f = fun n => if n == 0 then n else n + f (n - 1) in f 10
+55
+
+> let rec fib = fun n => if n <= 2 then 1 else fib (n - 1) + fib (n - 2) in fib 9
+34
+
+> let rec repeat = fun n x => if n <= 0 then [] else repeat (n - 1) x @ [x] in repeat 3 "foo"
+["foo", "foo", "foo"]
 ```
 
 ## Functions

examples/fibonacci/README.md

@@ -3,9 +3,6 @@
 This is a basic example of a recursive function: fibonnacci. This is the naive,
 exponential version of fibonacci: don't call it on a big value!
 
-Currently, only record bindings are recursive. To use a recursive function, one
-has to use a record.
-
 ## Run
 
 ```

examples/fibonacci/fibonacci.ncl

@@ -1,15 +1,11 @@
-# Currently, only record bindings are recursive. To use a recursive function,
-# one has to use a record.
-
 # This is the naive, exponential version of fibonacci: don't call it on a big
 # value!
-let fibonacci = {
- f = fun n =>
+let rec fibonacci = fun n =>
   if n == 0 then
     0
   else if n == 1 then
     1
   else
-    f (n - 1) + f (n - 2)
-}.f in
+    fibonacci (n - 1) + fibonacci (n - 2)
+in
 fibonacci 10

lsp/nls/src/linearization/mod.rs

@@ -161,11 +161,11 @@ impl Linearizer for AnalysisHost {
             Term::LetPattern(ident, destruct, ..) | Term::FunPattern(ident, destruct, _) => {
                 if let Some(ident) = ident {
                     let value_ptr = match term {
-                        Term::LetPattern(_, _, _, _) => {
+                        Term::LetPattern(..) => {
                             self.let_binding = Some(id);
                             ValueState::Unknown
                         }
-                        Term::FunPattern(_, _, _) => {
+                        Term::FunPattern(..) => {
                             // stub object
                             lin.push(LinearizationItem {
                                 id: id_gen.get_and_advance(),
@@ -218,13 +218,13 @@ impl Linearizer for AnalysisHost {
                     });
                 }
             }
-            Term::Let(ident, _, _, _) | Term::Fun(ident, _) => {
+            Term::Let(ident, ..) | Term::Fun(ident, ..) => {
                 let value_ptr = match term {
-                    Term::Let(_, _, _, _) => {
+                    Term::Let(..) => {
                         self.let_binding = Some(id);
                         ValueState::Unknown
                     }
-                    Term::Fun(_, _) => {
+                    Term::Fun(..) => {
                         // stub object
                         lin.push(LinearizationItem {
                             id: id_gen.get_and_advance(),

src/error.rs

@@ -306,6 +306,9 @@ pub enum ParseError {
         RawSpan, /* tail position */
         RawSpan, /* whole record position */
     ),
+    /// A recursive let pattern was encountered. They are not currently supported because we
+    /// decided it was too involved to implement them.
+    RecursiveLetPattern(RawSpan),
 }
 
 /// An error occurring during the resolution of an import.
@@ -462,6 +465,9 @@ impl ParseError {
                 InternalParseError::InvalidUniRecord(illegal_pos, tail_pos, pos) => {
                     ParseError::InvalidUniRecord(illegal_pos, tail_pos, pos)
                 }
+                InternalParseError::RecursiveLetPattern(pos) => {
+                    ParseError::RecursiveLetPattern(pos)
+                }
             },
         }
     }
@@ -1220,6 +1226,15 @@ impl ToDiagnostic<FileId> for ParseError {
                     String::from("Using a polymorphic tail in a record `{ ..; a}` requires the rest of the record to be only composed of type annotations, of the form `<field>: <type>`."),
                     String::from("Value assignements, such as `<field> = <expr>`, metadata, etc. are forbidden."),
                 ]),
+            ParseError::RecursiveLetPattern(span) => Diagnostic::error()
+                .with_message(format!("recursive destructuring is not supported"))
+                .with_labels(vec![
+                    primary(span),
+                ])
+                .with_notes(vec![
+                    String::from("A destructuring let-binding can't be recursive. Try removing the `rec` from `let rec`."),
+                    String::from("Note: you can reference other fields of a record recursively from within a field, so you might not need the recursive let."),
+                ]),
         };
 
         vec![diagnostic]

src/eval/mod.rs

@@ -95,8 +95,8 @@ use crate::{
     identifier::Ident,
     match_sharedterm, mk_app,
     term::{
-        make as mk_term, BinaryOp, BindingType, MetaValue, RichTerm, SharedTerm, StrChunk, Term,
-        UnaryOp,
+        make as mk_term, BinaryOp, BindingType, LetAttrs, MetaValue, RichTerm, SharedTerm,
+        StrChunk, Term, UnaryOp,
     },
 };
 
@@ -225,7 +225,7 @@ where
     use crate::transform::fresh_var;
 
     let var = fresh_var();
-    // Desugar to let x = term in deepSeq x x
+    // Desugar to let x = term in %deep_seq% x x
     let wrapper = mk_term::let_in(
         var.clone(),
         t0,
@@ -357,19 +357,25 @@ where
                     env,
                 }
             }
-            Term::Let(x, s, t, btype) => {
+            Term::Let(x, s, t, LetAttrs { binding_type, rec }) => {
                 let closure = Closure {
                     body: s.clone(),
                     env: env.clone(),
                 };
 
-                let thunk = match btype {
+                let mut thunk = match binding_type {
                     BindingType::Normal => Thunk::new(closure, IdentKind::Let),
                     BindingType::Revertible(deps) => {
                         Thunk::new_rev(closure, IdentKind::Let, deps.clone())
                     }
                 };
 
+                // Patch the environment with the (x <- closure) binding
+                if *rec {
+                    let thunk_ = thunk.clone();
+                    thunk.borrow_mut().env.insert(x.clone(), thunk_);
+                }
+
                 env.insert(x.clone(), thunk);
                 Closure {
                     body: t.clone(),
@@ -692,11 +698,11 @@ pub fn subst(rt: RichTerm, global_env: &Environment, env: &Environment) -> RichT
             | v @ Term::Enum(_)
             | v @ Term::Import(_)
             | v @ Term::ResolvedImport(_) => RichTerm::new(v, pos),
-            Term::Let(id, t1, t2, btype) => {
+            Term::Let(id, t1, t2, attrs) => {
                 let t1 = subst_(t1, global_env, env, Cow::Borrowed(bound.as_ref()));
                 let t2 = subst_(t2, global_env, env, bound);
 
-                RichTerm::new(Term::Let(id, t1, t2, btype), pos)
+                RichTerm::new(Term::Let(id, t1, t2, attrs), pos)
             }
             p @ Term::LetPattern(..) => panic!("Pattern {:?} has not been transformed before evaluation", p),
             p @ Term::FunPattern(..) => panic!("Pattern {:?} has not been transformed before evaluation", p),

src/grammar.lalrpop

@@ -166,9 +166,9 @@ UniTerm: UniTerm = {
     InfixExpr,
     AnnotatedInfixExpr,
     AsUniTerm<Forall>,
-    "let" <pat:Pattern> <meta: Annot<FixedType>?>
+    "let" <l: @L> <recursive:"rec"?> <r: @R> <pat:Pattern> <meta: Annot<FixedType>?>
         "=" <t1: Term>
-        "in" <t2: Term> => {
+        "in" <t2: Term> =>? {
         let t1 = if let Some(mut meta) = meta {
             let pos = t1.pos;
             meta.value = Some(t1);
@@ -178,7 +178,7 @@ UniTerm: UniTerm = {
             t1
         };
 
-        UniTerm::from(mk_term::let_pat(pat.0, pat.1, t1, t2))
+        Ok(UniTerm::from(mk_let(recursive.is_some(), pat.0, pat.1, t1, t2, mk_span(src_id, l, r))?))
     },
     <l: @L> "fun" <pats: Pattern+> "=>" <t: Term> <r: @R> => {
         let pos = mk_pos(src_id, l, r);
@@ -781,6 +781,7 @@ extern {
         "forall" => Token::Normal(NormalToken::Forall),
         "in" => Token::Normal(NormalToken::In),
         "let" => Token::Normal(NormalToken::Let),
+	"rec" => Token::Normal(NormalToken::Rec),
         "switch" => Token::Normal(NormalToken::Switch),
 
         "null" => Token::Normal(NormalToken::Null),

src/parser/error.rs

@@ -33,4 +33,7 @@ pub enum ParseError {
         RawSpan, /* tail position */
         RawSpan, /* whole record position */
     ),
+    /// A recursive let pattern was encountered. They are not currently supported because we
+    /// decided it was too involved to implement them.
+    RecursiveLetPattern(RawSpan),
 }

src/parser/lexer.rs

@@ -68,6 +68,8 @@ pub enum NormalToken<'input> {
     In,
     #[token("let")]
     Let,
+    #[token("rec")]
+    Rec,
     #[token("switch")]
     Switch,
 

src/parser/utils.rs

@@ -7,7 +7,10 @@ use std::rc::Rc;
 
 use codespan::FileId;
 
+use super::error::ParseError;
+
 use crate::{
+    destruct::Destruct,
     identifier::Ident,
     label::Label,
     mk_app, mk_fun,
@@ -314,6 +317,41 @@ pub fn mk_label(types: Types, src_id: FileId, l: usize, r: usize) -> Label {
     }
 }
 
+/// Generate a `Let` or a `LetPattern` (depending on `pat` being empty or not) from a the parsing
+/// of a let definition. This function fails if the definition has both a non-empty pattern and
+/// is recursive (`pat != Destruct::Empty && rec`), because recursive let-patterns are currently
+/// not supported.
+pub fn mk_let(
+    rec: bool,
+    id: Option<Ident>,
+    pat: Destruct,
+    t1: RichTerm,
+    t2: RichTerm,
+    span: RawSpan,
+) -> Result<RichTerm, ParseError> {
+    let result = match pat.into() {
+        d @ (Destruct::Record { .. } | Destruct::Array { .. }) => {
+            if rec {
+                return Err(ParseError::RecursiveLetPattern(span));
+            }
+            mk_term::let_pat(id, d, t1, t2)
+        }
+        Destruct::Empty => {
+            if let Some(id) = id {
+                if rec {
+                    mk_term::let_rec_in(id, t1, t2)
+                } else {
+                    mk_term::let_in(id, t1, t2)
+                }
+            } else {
+                panic!("unexpected let-binding without pattern or identifier")
+            }
+        }
+    };
+
+    Ok(result)
+}
+
 /// Determine the minimal level of indentation of a multi-line string.
 ///
 /// The result is determined by computing the minimum indentation level among all lines, where the

src/term.rs

@@ -69,7 +69,7 @@ pub enum Term {
 
     /// A let binding.
     #[serde(skip)]
-    Let(Ident, RichTerm, RichTerm, BindingType),
+    Let(Ident, RichTerm, RichTerm, LetAttrs),
     /// A destructuring let-binding.
     #[serde(skip)]
     LetPattern(Option<Ident>, Destruct, RichTerm, RichTerm),
@@ -185,6 +185,15 @@ pub struct RecordAttrs {
     pub open: bool,
 }
 
+/// The attributes of a let binding.
+#[derive(Debug, Default, Eq, PartialEq, Clone)]
+pub struct LetAttrs {
+    /// The type of a let binding. See the documentation of [`BindingType`].
+    pub binding_type: BindingType,
+    /// A recursive let binding adds its binding to the environment of the expression.
+    pub rec: bool,
+}
+
 impl RecordAttrs {
     pub fn merge(attrs1: RecordAttrs, attrs2: RecordAttrs) -> RecordAttrs {
         RecordAttrs {
@@ -1002,11 +1011,11 @@ impl RichTerm {
                     pos,
                 )
             },
-            Term::Let(id, t1, t2, btype) => {
+            Term::Let(id, t1, t2, attrs) => {
                 let t1 = t1.traverse(f, state, order)?;
                 let t2 = t2.traverse(f, state, order)?;
                 RichTerm::new(
-                    Term::Let(id, t1, t2, btype),
+                    Term::Let(id, t1, t2, attrs),
                     pos,
                 )
             },
@@ -1346,13 +1355,35 @@ pub mod make {
         Term::Var(v.into()).into()
     }
 
+    fn let_in_<I, T1, T2>(rec: bool, id: I, t1: T1, t2: T2) -> RichTerm
+    where
+        T1: Into<RichTerm>,
+        T2: Into<RichTerm>,
+        I: Into<Ident>,
+    {
+        let attrs = LetAttrs {
+            binding_type: BindingType::Normal,
+            rec,
+        };
+        Term::Let(id.into(), t1.into(), t2.into(), attrs).into()
+    }
+
     pub fn let_in<I, T1, T2>(id: I, t1: T1, t2: T2) -> RichTerm
     where
         T1: Into<RichTerm>,
         T2: Into<RichTerm>,
         I: Into<Ident>,
     {
-        Term::Let(id.into(), t1.into(), t2.into(), BindingType::Normal).into()
+        let_in_(false, id, t1, t2)
+    }
+
+    pub fn let_rec_in<I, T1, T2>(id: I, t1: T1, t2: T2) -> RichTerm
+    where
+        T1: Into<RichTerm>,
+        T2: Into<RichTerm>,
+        I: Into<Ident>,
+    {
+        let_in_(true, id, t1, t2)
     }
 
     pub fn let_pat<I, D, T1, T2>(id: Option<I>, pat: D, t1: T1, t2: T2) -> RichTerm
@@ -1362,18 +1393,7 @@ pub mod make {
         D: Into<Destruct>,
         I: Into<Ident>,
     {
-        match pat.into() {
-            d @ (Destruct::Record { .. } | Destruct::Array { .. }) => {
-                Term::LetPattern(id.map(|i| i.into()), d, t1.into(), t2.into()).into()
-            }
-            Destruct::Empty => {
-                if let Some(id) = id {
-                    let_in(id, t1, t2)
-                } else {
-                    Term::Null.into()
-                }
-            }
-        }
+        Term::LetPattern(id.map(|i| i.into()), pat.into(), t1.into(), t2.into()).into()
     }
 
     #[cfg(test)]