Split-off normalization into its own crate

2 files changed, 291 insertions, 0 deletions

diff --git a/dhall_normalize/src/lib.rs b/dhall_normalize/src/lib.rs
new file mode 100644
index 0000000..a2f7041
--- /dev/null
+++ b/dhall_normalize/src/lib.rs
@@ -0,0 +1,5 @@
+#![feature(box_patterns)]
+#![feature(trace_macros)]
+
+mod normalize;
+pub use crate::normalize::*;
diff --git a/dhall_normalize/src/normalize.rs b/dhall_normalize/src/normalize.rs
new file mode 100644
index 0000000..1fa5524
--- /dev/null
+++ b/dhall_normalize/src/normalize.rs
@@ -0,0 +1,286 @@
+#![allow(non_snake_case)]
+use std::fmt;
+use dhall_core::core::*;
+
+/// Reduce an expression to its normal form, performing beta reduction
+///
+/// `normalize` does not type-check the expression.  You may want to type-check
+/// expressions before normalizing them since normalization can convert an
+/// ill-typed expression into a well-typed expression.
+///
+/// However, `normalize` will not fail if the expression is ill-typed and will
+/// leave ill-typed sub-expressions unevaluated.
+///
+pub fn normalize<'i, S, T, A>(e: &Expr<'i, S, A>) -> Expr<'i, T, A>
+where
+    S: Clone + fmt::Debug,
+    T: Clone + fmt::Debug,
+    A: Clone + fmt::Debug,
+{
+    use dhall_core::BinOp::*;
+    use dhall_core::Builtin::*;
+    use dhall_core::Expr::*;
+    match *e {
+        Const(k) => Const(k),
+        Var(v) => Var(v),
+        Lam(x, ref tA, ref b) => {
+            let tA2 = normalize(tA);
+            let b2 = normalize(b);
+            Lam(x, bx(tA2), bx(b2))
+        }
+        Pi(x, ref tA, ref tB) => {
+            let tA2 = normalize(tA);
+            let tB2 = normalize(tB);
+            pi(x, tA2, tB2)
+        }
+        App(ref f, ref a) => match normalize::<S, T, A>(f) {
+            Lam(x, _A, b) => {
+                // Beta reduce
+                let vx0 = V(x, 0);
+                let a2 = shift::<S, S, A>(1, vx0, a);
+                let b2 = subst::<S, T, A>(vx0, &a2, &b);
+                let b3 = shift::<S, T, A>(-1, vx0, &b2);
+                normalize(&b3)
+            }
+            f2 => match (f2, normalize::<S, T, A>(a)) {
+                // fold/build fusion for `List`
+                (App(box Builtin(ListBuild), _), App(box App(box Builtin(ListFold), _), box e2)) |
+                (App(box Builtin(ListFold), _), App(box App(box Builtin(ListBuild), _), box e2)) |
+
+                // fold/build fusion for `Natural`
+                (Builtin(NaturalBuild), App(box Builtin(NaturalFold), box e2)) |
+                (Builtin(NaturalFold), App(box Builtin(NaturalBuild), box e2)) => normalize(&e2),
+
+            /*
+                App (App (App (App NaturalFold (NaturalLit n0)) _) succ') zero ->
+                    normalize (go n0)
+                  where
+                    go !0 = zero
+                    go !n = App succ' (go (n - 1))
+                App NaturalBuild k
+                    | check     -> NaturalLit n
+                    | otherwise -> App f' a'
+                  where
+                    labeled =
+                        normalize (App (App (App k Natural) "Succ") "Zero")
+
+                    n = go 0 labeled
+                      where
+                        go !m (App (Var "Succ") e') = go (m + 1) e'
+                        go !m (Var "Zero")          = m
+                        go !_  _                    = internalError text
+                    check = go labeled
+                      where
+                        go (App (Var "Succ") e') = go e'
+                        go (Var "Zero")          = True
+                        go  _                    = False
+                        */
+                (Builtin(NaturalIsZero), NaturalLit(n)) => BoolLit(n == 0),
+                (Builtin(NaturalEven), NaturalLit(n)) => BoolLit(n % 2 == 0),
+                (Builtin(NaturalOdd), NaturalLit(n)) => BoolLit(n % 2 != 0),
+                (Builtin(NaturalToInteger), NaturalLit(n)) => IntegerLit(n as isize),
+                (Builtin(NaturalShow), NaturalLit(n)) => TextLit(n.to_string()),
+                (App(f@box Builtin(ListBuild), box t), k) => {
+                        let labeled =
+                            normalize::<_, T, _>(&app(app(app(k.clone(), app(
+                                Builtin(self::Builtin::List), t.clone())), "Cons"), "Nil"));
+
+                        fn list_to_vector<'i, S, A>(v: &mut Vec<Expr<'i, S, A>>, e: Expr<'i, S, A>)
+                            where S: Clone, A: Clone
+                        {
+                            match e {
+                                App(box App(box Var(V("Cons", _)), box x), box e2) => {
+                                    v.push(x);
+                                    list_to_vector(v, e2)
+                                }
+                                Var(V("Nil", _)) => {}
+                                _ => panic!("internalError list_to_vector"),
+                            }
+                        }
+                        fn check<S, A>(e: &Expr<S, A>) -> bool {
+                            match *e {
+                                App(box App(box Var(V("Cons", _)), _), ref e2) => check(e2),
+                                Var(V("Nil", _)) => true,
+                                _ => false,
+                            }
+                        }
+
+                        if check(&labeled) {
+                            let mut v = vec![];
+                            list_to_vector(&mut v, labeled);
+                            ListLit(Some(bx(t)), v)
+                        } else {
+                            app(App(f, bx(t)), k)
+                        }
+                    }
+                (App(box App(box App(box App(box Builtin(ListFold), _), box ListLit(_, xs)), _), cons), nil) => {
+                    let e2: Expr<_, _> = xs.into_iter().rev().fold(nil, |y, ys| // foldr
+                        App(bx(App(cons.clone(), bx(y))), bx(ys))
+                    );
+                    normalize(&e2)
+                }
+                (App(f, x_), ListLit(t, ys)) => match *f {
+                    Builtin(ListLength) =>
+                        NaturalLit(ys.len()),
+                    Builtin(ListHead) =>
+                        normalize(&OptionalLit(t, ys.into_iter().take(1).collect())),
+                    Builtin(ListLast) =>
+                        normalize(&OptionalLit(t, ys.into_iter().last().into_iter().collect())),
+                    Builtin(ListReverse) => {
+                        let mut xs = ys;
+                        xs.reverse();
+                        normalize(&ListLit(t, xs))
+                    }
+                    _ => app(App(f, x_), ListLit(t, ys)),
+                },
+                /*
+                App (App ListIndexed _) (ListLit t xs) ->
+                    normalize (ListLit t' (fmap adapt (Data.Vector.indexed xs)))
+                  where
+                    t' = Record (Data.Map.fromList kts)
+                      where
+                        kts = [ ("index", Natural)
+                              , ("value", t)
+                              ]
+                    adapt (n, x) = RecordLit (Data.Map.fromList kvs)
+                      where
+                        kvs = [ ("index", NaturalLit (fromIntegral n))
+                              , ("value", x)
+                              ]
+            */
+                (App(box App(box App(box App(box Builtin(OptionalFold), _), box OptionalLit(_, xs)), _), just), nothing) => {
+                    let e2: Expr<_, _> = xs.into_iter().fold(nothing, |y, _|
+                        App(just.clone(), bx(y))
+                    );
+                    normalize(&e2)
+                }
+                (App(box Builtin(OptionalBuild), _), App(box App(box Builtin(OptionalFold), _), b)) => {
+                    normalize(&b)
+                }
+                (App(box Builtin(OptionalBuild), a0), g) => {
+                    let e2: Expr<_, _> = app(app(app(g,
+                        App(bx(Builtin(Optional)), a0.clone())),
+                            Lam("x", a0.clone(),
+                                bx(OptionalLit(Some(a0.clone()), vec![Var(V("x", 0))])))),
+                            OptionalLit(Some(a0), vec![]));
+                    normalize(&e2)
+                }
+                (f2, a2) => app(f2, a2),
+            },
+        },
+        Let(f, _, ref r, ref b) => {
+            let r2 = shift::<_, S, _>(1, V(f, 0), r);
+            let b2 = subst(V(f, 0), &r2, b);
+            let b3 = shift::<_, T, _>(-1, V(f, 0), &b2);
+            normalize(&b3)
+        }
+        Annot(ref x, _) => normalize(x),
+        Builtin(v) => Builtin(v),
+        BoolLit(b) => BoolLit(b),
+        BinOp(BoolAnd, ref x, ref y) => with_binop(
+            BoolAnd,
+            Expr::bool_lit,
+            |xn, yn| BoolLit(xn && yn),
+            normalize(x),
+            normalize(y),
+        ),
+        BinOp(BoolOr, ref x, ref y) => with_binop(
+            BoolOr,
+            Expr::bool_lit,
+            |xn, yn| BoolLit(xn || yn),
+            normalize(x),
+            normalize(y),
+        ),
+        BinOp(BoolEQ, ref x, ref y) => with_binop(
+            BoolEQ,
+            Expr::bool_lit,
+            |xn, yn| BoolLit(xn == yn),
+            normalize(x),
+            normalize(y),
+        ),
+        BinOp(BoolNE, ref x, ref y) => with_binop(
+            BoolNE,
+            Expr::bool_lit,
+            |xn, yn| BoolLit(xn != yn),
+            normalize(x),
+            normalize(y),
+        ),
+        BoolIf(ref b, ref t, ref f) => match normalize(b) {
+            BoolLit(true) => normalize(t),
+            BoolLit(false) => normalize(f),
+            b2 => BoolIf(bx(b2), bx(normalize(t)), bx(normalize(f))),
+        },
+        NaturalLit(n) => NaturalLit(n),
+        BinOp(NaturalPlus, ref x, ref y) => with_binop(
+            NaturalPlus,
+            Expr::natural_lit,
+            |xn, yn| NaturalLit(xn + yn),
+            normalize(x),
+            normalize(y),
+        ),
+        BinOp(NaturalTimes, ref x, ref y) => with_binop(
+            NaturalTimes,
+            Expr::natural_lit,
+            |xn, yn| NaturalLit(xn * yn),
+            normalize(x),
+            normalize(y),
+        ),
+        IntegerLit(n) => IntegerLit(n),
+        DoubleLit(n) => DoubleLit(n),
+        TextLit(ref t) => TextLit(t.clone()),
+        BinOp(TextAppend, ref x, ref y) => with_binop(
+            TextAppend,
+            Expr::text_lit,
+            |xt, yt| TextLit(xt + &yt),
+            normalize(x),
+            normalize(y),
+        ),
+        ListLit(ref t, ref es) => {
+            let t2 = t.as_ref().map(|x| x.as_ref()).map(normalize).map(bx);
+            let es2 = es.iter().map(normalize).collect();
+            ListLit(t2, es2)
+        }
+        OptionalLit(ref t, ref es) => {
+            let t2 = t.as_ref().map(|x| x.as_ref()).map(normalize).map(bx);
+            let es2 = es.iter().map(normalize).collect();
+            OptionalLit(t2, es2)
+        }
+        Record(ref kts) => Record(map_record_value(kts, normalize)),
+        RecordLit(ref kvs) => RecordLit(map_record_value(kvs, normalize)),
+        Union(ref kts) => Union(map_record_value(kts, normalize)),
+        UnionLit(k, ref v, ref kvs) => {
+            UnionLit(k, bx(normalize(v)), map_record_value(kvs, normalize))
+        }
+        Merge(ref _x, ref _y, ref _t) => unimplemented!(),
+        Field(ref r, x) => match normalize(r) {
+            RecordLit(kvs) => match kvs.get(x) {
+                Some(r2) => normalize(r2),
+                None => {
+                    Field(bx(RecordLit(map_record_value(&kvs, normalize))), x)
+                }
+            },
+            r2 => Field(bx(r2), x),
+        },
+        Note(_, ref e) => normalize(e),
+        Embed(ref a) => Embed(a.clone()),
+        _ => unimplemented!(),
+    }
+}
+
+fn with_binop<'a, S, A, U, Get, Set>(
+    op: BinOp,
+    get: Get,
+    set: Set,
+    x: Expr<'a, S, A>,
+    y: Expr<'a, S, A>,
+) -> Expr<'a, S, A>
+where
+    Get: Fn(&Expr<'a, S, A>) -> Option<U>,
+    Set: FnOnce(U, U) -> Expr<'a, S, A>,
+{
+    if let (Some(xv), Some(yv)) = (get(&x), get(&y)) {
+        set(xv, yv)
+    } else {
+        Expr::BinOp(op, bx(x), bx(y))
+    }
+}