Add support for `with` keyword

3 files changed, 44 insertions, 1 deletions

diff --git a/dhall/src/syntax/ast/span.rs b/dhall/src/syntax/ast/span.rs
index 7c004c5..50985e1 100644
--- a/dhall/src/syntax/ast/span.rs
+++ b/dhall/src/syntax/ast/span.rs
@@ -21,6 +21,7 @@ pub(crate) enum Span {
     /// Desugarings
     DuplicateRecordFieldsSugar,
     DottedFieldSugar,
+    WithSugar,
     /// For expressions obtained from decoding binary
     Decoded,
     /// For expressions constructed during normalization/typecheck
diff --git a/dhall/src/syntax/text/dhall.pest.visibility b/dhall/src/syntax/text/dhall.pest.visibility
index 03a000b..0ff1bfe 100644
--- a/dhall/src/syntax/text/dhall.pest.visibility
+++ b/dhall/src/syntax/text/dhall.pest.visibility
@@ -95,6 +95,7 @@ equivalent
 prefer
 lambda
 forall
+# with
 arrow
 # complete
 # exponent
@@ -164,6 +165,8 @@ times_expression
 equal_expression
 not_equal_expression
 equivalent_expression
+with_expression
+with_clause
 application_expression
 first_application_expression
 # import_expression
diff --git a/dhall/src/syntax/text/parser.rs b/dhall/src/syntax/text/parser.rs
index 7140332..f59cf06 100644
--- a/dhall/src/syntax/text/parser.rs
+++ b/dhall/src/syntax/text/parser.rs
@@ -2,6 +2,7 @@ use itertools::Itertools;
 use pest::prec_climber as pcl;
 use pest::prec_climber::PrecClimber;
 use std::collections::BTreeMap;
+use std::iter::once;
 use std::rc::Rc;
 
 use pest_consume::{match_nodes, Parser};
@@ -144,6 +145,23 @@ fn insert_recordlit_entry(map: &mut BTreeMap<Label, Expr>, l: Label, e: Expr) {
     }
 }
 
+fn desugar_with_expr(x: Expr, labels: &[Label], y: Expr) -> Expr {
+    use crate::syntax::BinOp::RightBiasedRecordMerge;
+    let expr = |k| Expr::new(k, Span::WithSugar);
+    match labels {
+        [] => y,
+        [l, rest @ ..] => {
+            let res =
+                desugar_with_expr(expr(Field(x.clone(), l.clone())), rest, y);
+            expr(BinOp(
+                RightBiasedRecordMerge,
+                x,
+                expr(RecordLit(once((l.clone(), res)).collect())),
+            ))
+        }
+    }
+}
+
 lazy_static::lazy_static! {
     static ref PRECCLIMBER: PrecClimber<Rule> = {
         use Rule::*;
@@ -760,6 +778,27 @@ impl DhallParser {
     }
 
     #[alias(expression, shortcut = true)]
+    fn with_expression(input: ParseInput) -> ParseResult<Expr> {
+        Ok(match_nodes!(input.children();
+            [expression(e)] => e,
+            [expression(first), with_clause(clauses)..] => {
+                clauses.fold(
+                    first,
+                    |acc, (labels, e)| {
+                        desugar_with_expr(acc, &labels, e)
+                    }
+                )
+            },
+        ))
+    }
+
+    fn with_clause(input: ParseInput) -> ParseResult<(Vec<Label>, Expr)> {
+        Ok(match_nodes!(input.children();
+            [label(labels).., expression(e)] => (labels.collect(), e),
+        ))
+    }
+
+    #[alias(expression, shortcut = true)]
     fn application_expression(input: ParseInput) -> ParseResult<Expr> {
         Ok(match_nodes!(input.children();
             [expression(e)] => e,
@@ -919,7 +958,7 @@ impl DhallParser {
             [label(first_name), label(names).., expression(expr)] => {
                 // Desugar dotted field syntax into nested records
                 let expr = names.rev().fold(expr, |e, l| {
-                    let map = Some((l, e)).into_iter().collect();
+                    let map = once((l, e)).collect();
                     Expr::new(
                         RecordLit(map),
                         Span::DottedFieldSugar,