Object comprehensions via loops in braces

civet.dev/reference.md

@@ -1937,6 +1937,40 @@ min := for min item of array
 max := for max item of array
 </Playground>
 
+### Object Comprehensions
+
+Loops can also accumulate their body values into an object.
+When any loop is found is found within a braced object expression,
+its body value is spread into the containing object.
+
+<Playground>
+object  := {a: 1, b: 2, c: 3}
+doubled := {
+  for key in object
+    [key]: 2 * object[key]
+}
+</Playground>
+
+<Playground>
+i .= 1
+squares := {
+  do
+    [i]: i * i
+  while i++ < 10
+}
+</Playground>
+
+The loop can exist anywhere a property is expected.
+It can be freely mixed with other object properties.
+
+<Playground>
+rateLimits := {
+  admin: Infinity,
+  for user of users
+    [user.name]: getRemainingLimit(user)
+}
+</Playground>
+
 ### Infinite Loop
 
 <Playground>

source/parser.hera

@@ -3422,6 +3422,40 @@ ObjectPropertyDelimiter
 # https://262.ecma-international.org/#prod-PropertyDefinition
 # NOTE: Must start on same line
 PropertyDefinition
+  # NOTE: This needs to be above NamedProperty so that
+  # a `do` block doesn't get treated as {do} property
+  _?:ws InsertDotDotDot:dots IterationExpression:exp ->
+    let { statement } = exp
+    // Treat empty-bodied `do` and `loop` (which require no expression)
+    // as keys instead of iterations
+    if (exp.block.implicit &&
+        (statement.type === "DoStatement" || statement.subtype === "loop")) {
+      return $skip
+    }
+
+    // immutably set exp.statement.object = true
+    statement = { ...statement, object: true }
+    exp = {
+      ...exp,
+      statement,
+      children: exp.children.map(($) => $ === exp.statement ? statement : $),
+    }
+
+    const children = [ws, dots, exp]
+    if (statement.reduction) {
+      children.unshift({
+        type: "Error",
+        message: "Reduction loops are forbidden in object literals",
+      })
+    }
+
+    return {
+      type: "SpreadProperty",
+      children,
+      names: exp.names,
+      dots,
+      value: exp,
+    }
   _?:ws NamedProperty:prop ->
     return prepend(ws, prop)
 
@@ -3540,6 +3574,7 @@ PropertyDefinition
   #_?:ws IdentifierReference:id ->
   #  return prepend(ws, id)
 
+
 NamedProperty
   # NOTE: CoverInitializedName early error doesn't seem necessary with this parser
   # NOTE: Using PostfixedExpression to allow If/Switch expressions and postfixes
@@ -6453,6 +6488,10 @@ DotDotDot
   "…" ->
     return { $loc, token: "..." }
 
+InsertDotDotDot
+  "" ->
+    return { $loc, token: "..." }
+
 DoubleColon
   "::" ->
     return { $loc, token: $1 }

source/parser/function.civet

@@ -670,6 +670,8 @@ function iterationDeclaration(statement: IterationStatement | ForStatement)
         when "max" then "-Infinity"
         when "product" then "1"
         else "0"
+  else if statement.object
+    declaration.children.push "={}"
   else
     // Assign [] directly only in const case, so TypeScript can better infer
     if decl is "const"
@@ -684,6 +686,7 @@ function iterationDeclaration(statement: IterationStatement | ForStatement)
       return declaration
     unless block.empty
       assignResults block, (node) =>
+        return [ "Object.assign(", resultsRef, ",", node, ")" ] if statement.object
         return [ resultsRef, ".push(", node, ")" ] unless reduction
         switch reduction.subtype
           when "some"