10 files changed, 69 insertions, 4 deletions

diff --git a/README.md b/README.md
index 1d77e95..072dd10 100644
--- a/README.md
+++ b/README.md
@@ -160,6 +160,7 @@ same name as the corresponding test.
 
 [???]
 
+- Add support for `with` keyword
 - Implement remote imports with conservative sanity checking
 - Implement `missing` and `env:VAR` imports
 - Implement `as Text` and `as Location` imports
diff --git a/dhall-lang b/dhall-lang
-Subproject 1a20024421d4e0574e7d9c85a7b0e751b0534dc
+Subproject 527f391fc8e48b3c8847b6982b76ac959808c0d
diff --git a/dhall/build.rs b/dhall/build.rs
index 4b6e52b..3d062da 100644
--- a/dhall/build.rs
+++ b/dhall/build.rs
@@ -376,6 +376,19 @@ fn convert_abnf_to_pest() -> std::io::Result<()> {
     rules.remove("url_path");
     writeln!(&mut file, "url_path = _{{ path }}")?;
 
+    // TODO: upstream this tweak
+    rules.remove("with_expression");
+    writeln!(
+        &mut file,
+        r#"
+            with_expression = {{ application_expression ~ (whsp1 ~ with ~ whsp1 ~ with_clause)* }}
+            with_clause = {{
+                any_label_or_some ~ (whsp ~ "." ~ whsp ~ any_label_or_some)*
+                    ~ whsp ~ "=" ~ whsp ~ application_expression
+            }}
+        "#
+    )?;
+
     // Work around some greediness issue in the grammar.
     rules.remove("missing");
     writeln!(
@@ -439,7 +452,7 @@ fn convert_abnf_to_pest() -> std::io::Result<()> {
             bool_or |
             import_alt
         }}
-        operator_expression = {{ application_expression ~ (whsp ~ operator ~ whsp ~ application_expression)* }}
+        operator_expression = {{ with_expression ~ (whsp ~ operator ~ whsp ~ with_expression)* }}
     "##)?;
 
     writeln!(
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,
diff --git a/dhall/tests/parser/failure/spacing/MergeNoSpace2.txt b/dhall/tests/parser/failure/spacing/MergeNoSpace2.txt
index 96d937b..307d873 100644
--- a/dhall/tests/parser/failure/spacing/MergeNoSpace2.txt
+++ b/dhall/tests/parser/failure/spacing/MergeNoSpace2.txt
@@ -3,4 +3,4 @@
 1 | merge x(y)␊
   |       ^---
   |
-  = expected missing, double_quote_literal, single_quote_literal, if_, merge, non_empty_list_literal, NaN, Some_, toMap, assert, forall, numeric_double_literal, minus_infinity_literal, plus_infinity_literal, natural_literal, integer_literal, or import_hashed
+  = expected missing, double_quote_literal, single_quote_literal, if_, merge, NaN, Some_, toMap, assert, forall, numeric_double_literal, minus_infinity_literal, plus_infinity_literal, natural_literal, integer_literal, import_hashed, or non_empty_list_literal
diff --git a/dhall/tests/parser/failure/spacing/RecordTypeNoSpace.txt b/dhall/tests/parser/failure/spacing/RecordTypeNoSpace.txt
index 9c1eaef..f6427a4 100644
--- a/dhall/tests/parser/failure/spacing/RecordTypeNoSpace.txt
+++ b/dhall/tests/parser/failure/spacing/RecordTypeNoSpace.txt
@@ -3,4 +3,4 @@
 1 | { x :T }␊
   |   ^---
   |
-  = expected empty_record_literal or non_empty_record_type_or_literal
+  = expected non_empty_record_type_or_literal or empty_record_literal
diff --git a/dhall/tests/type-inference/failure/unit/WithInvalidOverrideA.txt b/dhall/tests/type-inference/failure/unit/WithInvalidOverrideA.txt
new file mode 100644
index 0000000..de101e0
--- /dev/null
+++ b/dhall/tests/type-inference/failure/unit/WithInvalidOverrideA.txt
@@ -0,0 +1 @@
+Type error: error: MustCombineRecord
diff --git a/dhall/tests/type-inference/failure/unit/WithUnderscore.txt b/dhall/tests/type-inference/failure/unit/WithUnderscore.txt
new file mode 100644
index 0000000..77b0777
--- /dev/null
+++ b/dhall/tests/type-inference/failure/unit/WithUnderscore.txt
@@ -0,0 +1,7 @@
+Type error: error: unbound variable ``_``
+ --> <current file>:1:23
+  |
+...
+5 | { a.b = 1 } with a.c = _
+  |                        ^ not found in this scope
+  |