4 files changed, 29 insertions, 13 deletions
diff --git a/dhall_syntax/src/core/expr.rs b/dhall_syntax/src/core/expr.rs
index 750b58b..131f97e 100644
--- a/dhall_syntax/src/core/expr.rs
+++ b/dhall_syntax/src/core/expr.rs
@@ -223,6 +223,8 @@ pub enum ExprF<SubExpr, Embed> {
     Field(SubExpr, Label),
     ///  `e.{ x, y, z }`
     Projection(SubExpr, DupTreeSet<Label>),
+    ///  `e.(t)`
+    ProjectionByExpr(SubExpr, SubExpr),
     /// `./some/path`
     Import(Import<SubExpr>),
     /// Embeds the result of resolving an import
diff --git a/dhall_syntax/src/core/visitor.rs b/dhall_syntax/src/core/visitor.rs
index 39a027f..6189f26 100644
--- a/dhall_syntax/src/core/visitor.rs
+++ b/dhall_syntax/src/core/visitor.rs
@@ -161,6 +161,7 @@ where
         }
         Field(e, l) => Field(v.visit_subexpr(e)?, l.clone()),
         Projection(e, ls) => Projection(v.visit_subexpr(e)?, ls.clone()),
+        ProjectionByExpr(e, x) => ProjectionByExpr(v.visit_subexpr(e)?, v.visit_subexpr(x)?),
         Assert(e) => Assert(v.visit_subexpr(e)?),
         Import(i) => Import(i.traverse_ref(|e| v.visit_subexpr(e))?),
         Embed(a) => Embed(v.visit_embed(a)?),
@@ -274,6 +275,10 @@ where
         }
         Field(e, _) => v.visit_subexpr(e)?,
         Projection(e, _) => v.visit_subexpr(e)?,
+        ProjectionByExpr(e, x) => {
+            v.visit_subexpr(e)?;
+            v.visit_subexpr(x)?;
+        }
         Assert(e) => v.visit_subexpr(e)?,
         Import(i) => i.traverse_mut(|e| v.visit_subexpr(e))?,
         Embed(a) => v.visit_embed(a)?,
diff --git a/dhall_syntax/src/parser.rs b/dhall_syntax/src/parser.rs
index eaded50..f5d161f 100644
--- a/dhall_syntax/src/parser.rs
+++ b/dhall_syntax/src/parser.rs
@@ -24,9 +24,10 @@ pub type ParseError = pest::error::Error<Rule>;
 pub type ParseResult<T> = Result<T, ParseError>;
 
 #[derive(Debug)]
-enum Either<A, B> {
-    Left(A),
-    Right(B),
+enum Selector<E> {
+    Field(Label),
+    Projection(DupTreeSet<Label>),
+    ProjectionByExpr(Expr<E>),
 }
 
 impl crate::Builtin {
@@ -777,8 +778,9 @@ impl DhallParser {
                             acc.span(),
                             e.1,
                             match e.0 {
-                                Either::Left(l) => Field(acc, l),
-                                Either::Right(ls) => Projection(acc, ls),
+                                Selector::Field(l) => Field(acc, l),
+                                Selector::Projection(ls) => Projection(acc, ls),
+                                Selector::ProjectionByExpr(e) => ProjectionByExpr(acc, e)
                             }
                         )
                     }
@@ -787,14 +789,15 @@ impl DhallParser {
         ))
     }
 
-    fn selector(
+    fn selector<E: Clone>(
         input: ParseInput,
-    ) -> ParseResult<(Either<Label, DupTreeSet<Label>>, Span)> {
-        Ok(match_nodes!(input.children();
-            [label(l)] => (Either::Left(l), input_to_span(input)),
-            [labels(ls)] => (Either::Right(ls), input_to_span(input)),
-            // [expression(_e)] => unimplemented!("selection by expression"), // TODO
-        ))
+    ) -> ParseResult<(Selector<E>, Span)> {
+        let stor = match_nodes!(input.children();
+            [label(l)] => Selector::Field(l),
+            [labels(ls)] => Selector::Projection(ls),
+            [expression(e)] => Selector::ProjectionByExpr(e),
+        );
+        Ok((stor, input_to_span(input)))
     }
 
     fn labels(input: ParseInput) -> ParseResult<DupTreeSet<Label>> {
diff --git a/dhall_syntax/src/printer.rs b/dhall_syntax/src/printer.rs
index 19ec781..ce6ff97 100644
--- a/dhall_syntax/src/printer.rs
+++ b/dhall_syntax/src/printer.rs
@@ -66,6 +66,9 @@ impl<SE: Display + Clone, E: Display> Display for ExprF<SE, E> {
                 write!(f, "{}.", e)?;
                 fmt_list("{ ", ", ", " }", ls, f, Display::fmt)?;
             }
+            ProjectionByExpr(a, b) => {
+                write!(f, "{}.({})", a, b)?;
+            }
             Var(a) => a.fmt(f)?,
             Const(k) => k.fmt(f)?,
             Builtin(v) => v.fmt(f)?,
@@ -159,7 +162,9 @@ impl<A: Display + Clone> RawExpr<A> {
             // Precedence is magically handled by the ordering of BinOps.
             ExprF::BinOp(op, _, _) if phase > PrintPhase::BinOp(*op) => true,
             ExprF::App(_, _) if phase > PrintPhase::App => true,
-            Field(_, _) | Projection(_, _) if phase > PrintPhase::Import => {
+            Field(_, _) | Projection(_, _) | ProjectionByExpr(_, _)
+                if phase > PrintPhase::Import =>
+            {
                 true
             }
             _ => false,
@@ -196,6 +201,7 @@ impl<A: Display + Clone> RawExpr<A> {
             ),
             Field(a, b) => Field(a.phase(Primitive), b),
             Projection(e, ls) => Projection(e.phase(Primitive), ls),
+            ProjectionByExpr(a, b) => ProjectionByExpr(a.phase(Primitive), b),
             e => e,
         };