From e24f155e9768198d29432f8f1c86023c9dc6750c Mon Sep 17 00:00:00 2001
From: Erik Demaine <edemaine@mit.edu>
Date: Wed, 30 Oct 2024 15:36:23 -0400
Subject: [PATCH 1/2] Inequality slicing without `..`: `x[<i]` etc.

---
 civet.dev/reference.md |  8 ++++++++
 source/parser.hera     | 33 +++++++++++++++++++++++++++++++--
 test/slice.civet       | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 76 insertions(+), 2 deletions(-)

diff --git a/civet.dev/reference.md b/civet.dev/reference.md
index bd6ff151..16bc63f5 100644
--- a/civet.dev/reference.md
+++ b/civet.dev/reference.md
@@ -457,6 +457,14 @@ Slices are increasing by default, but you can reverse them with `>` or `>=`:
 reversed := x[..>=]
 </Playground>
 
+If you just want to specify one endpoint of an increasing slice,
+you can avoid `..` altogether:
+
+<Playground>
+x is x[<=i] + x[>i]
+x is x[<i] + x[>=i]
+</Playground>
+
 ## Strings
 
 Strings can span multiple lines:
diff --git a/source/parser.hera b/source/parser.hera
index b2f1c70b..d68bd6ce 100644
--- a/source/parser.hera
+++ b/source/parser.hera
@@ -1553,6 +1553,32 @@ SliceParameters
       children,
     }
 
+  Loc:l RangeEnd:rend Expression:e ->
+    let start, end, children
+    if (rend.increasing) { // right end
+      end = e
+      if (rend.inclusive) {
+        end = [makeLeftHandSideExpression(end), ` + 1`]
+      }
+      start = {
+        $loc: l.$loc,
+        token: "0",
+      }
+      children = [start, ", ", end]
+    } else { // left end
+      start = e
+      if (!rend.inclusive) {
+        start = [makeLeftHandSideExpression(start), ` + 1`]
+      }
+      children = [start]
+    }
+    return {
+      type: "SliceParameters",
+      start,
+      end,
+      children,
+    }
+
 AccessStart
   PropertyAccessModifier?:modifier Dot:dot ![.\s] ->
     return {
@@ -2951,7 +2977,7 @@ RangeDots
       triple: true,
       children: [],
     }
-  RangeEnd:left _?:ws1 DotDot:dots _?:ws2 RangeEnd:right ->
+  OptionalRangeEnd:left _?:ws1 DotDot:dots _?:ws2 OptionalRangeEnd:right ->
     // Inherit increasing flag from either side
     const increasing = left.increasing ?? right.increasing
     if (left.increasing != null && right.increasing != null &&
@@ -2971,6 +2997,10 @@ RangeDots
       children: [ws1, ws2]
     }
 
+OptionalRangeEnd
+  RangeEnd
+  "" -> { increasing: undefined, inclusive: true, raw: "" }
+
 RangeEnd
   /([<>])(=?)|([≤≥])/ ->
     let dir = $1, equal = $2, unicode = $3
@@ -2987,7 +3017,6 @@ RangeEnd
       inclusive: equal === "=",
       raw: $0,
     }
-  "" -> { increasing: undefined, inclusive: true, raw: "" }
 
 RangeExpression
   Expression:start __:ws RangeDots:range Expression:end ->
diff --git a/test/slice.civet b/test/slice.civet
index a2f54e01..60a3e276 100644
--- a/test/slice.civet
+++ b/test/slice.civet
@@ -251,6 +251,43 @@ describe "slice", ->
       unknown:1:9 Slice range cannot be decreasing in assignment
     """
 
+  describe "inequality without ..", ->
+    testCase """
+      simple
+      ---
+      x[>=a]
+      x[≥a]
+      x[>a]
+      x[<=b]
+      x[≤b]
+      x[<b]
+      ---
+      x.slice(a)
+      x.slice(a)
+      x.slice(a + 1)
+      x.slice(0, b + 1)
+      x.slice(0, b + 1)
+      x.slice(0, b)
+    """
+
+    testCase """
+      complex
+      ---
+      x[>=a ?? a0]
+      x[≥a ?? a0]
+      x[>a ?? a0]
+      x[<=b ?? b0]
+      x[≤b ?? b0]
+      x[<b ?? b0]
+      ---
+      x.slice(a ?? a0)
+      x.slice(a ?? a0)
+      x.slice((a ?? a0) + 1)
+      x.slice(0, (b ?? b0) + 1)
+      x.slice(0, (b ?? b0) + 1)
+      x.slice(0, b ?? b0)
+    """
+
   describe "assignment", ->
     testCase """
       everything

From 5be8f5334285b494e250d0fcd52bf54d8fe7923c Mon Sep 17 00:00:00 2001
From: Erik Demaine <edemaine@mit.edu>
Date: Wed, 30 Oct 2024 16:03:50 -0400
Subject: [PATCH 2/2] Improve slice example

---
 civet.dev/reference.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/civet.dev/reference.md b/civet.dev/reference.md
index 16bc63f5..868e764f 100644
--- a/civet.dev/reference.md
+++ b/civet.dev/reference.md
@@ -461,8 +461,8 @@ If you just want to specify one endpoint of an increasing slice,
 you can avoid `..` altogether:
 
 <Playground>
-x is x[<=i] + x[>i]
-x is x[<i] + x[>=i]
+[left, right] = [x[<=i], x[>i]]
+[left, right] = [x[<i], x[>=i]]
 </Playground>
 
 ## Strings