From 00b4837f845602e02b7ca1e61c7181249ba3edb7 Mon Sep 17 00:00:00 2001
From: Matt Kantor <the.matt.kantor@gmail.com>
Date: Mon, 27 Jan 2025 17:04:58 -0500
Subject: [PATCH] Fix unparsing immediately-invoked function expressions

They were missing parentheses around the function expression.
---
 src/language/compiling/unparsing.test.ts | 38 ++++++++++++++++++++
 src/language/unparsing/plz-utilities.ts  | 44 +++++++++++++++---------
 2 files changed, 66 insertions(+), 16 deletions(-)

diff --git a/src/language/compiling/unparsing.test.ts b/src/language/compiling/unparsing.test.ts
index b5513e5..59792ba 100644
--- a/src/language/compiling/unparsing.test.ts
+++ b/src/language/compiling/unparsing.test.ts
@@ -46,6 +46,18 @@ testCases(
     },
     either.makeRight('{ identity: a => :a, test: :identity("it works!") }'),
   ],
+  [
+    {
+      0: '@apply',
+      function: {
+        0: '@function',
+        parameter: 'a',
+        body: { 0: '@lookup', 1: 'a' },
+      },
+      argument: 'it works!',
+    },
+    either.makeRight('(a => :a)("it works!")'),
+  ],
 ])
 
 testCases(
@@ -82,6 +94,18 @@ testCases(
       '{\n  identity: a => :a\n  test: :identity("it works!")\n}',
     ),
   ],
+  [
+    {
+      0: '@apply',
+      function: {
+        0: '@function',
+        parameter: 'a',
+        body: { 0: '@lookup', 1: 'a' },
+      },
+      argument: 'it works!',
+    },
+    either.makeRight('(a => :a)("it works!")'),
+  ],
 ])
 
 testCases(
@@ -120,4 +144,18 @@ testCases(
       '{\n  "identity": {\n    "0": "@function",\n    "parameter": "a",\n    "body": {\n      "0": "@lookup",\n      "1": "a"\n    }\n  },\n  "test": {\n    "0": "@apply",\n    "function": {\n      "0": "@lookup",\n      "1": "identity"\n    },\n    "argument": "it works!"\n  }\n}',
     ),
   ],
+  [
+    {
+      0: '@apply',
+      function: {
+        0: '@function',
+        parameter: 'a',
+        body: { 0: '@lookup', 1: 'a' },
+      },
+      argument: 'it works!',
+    },
+    either.makeRight(
+      '{\n  "0": "@apply",\n  "function": {\n    "0": "@function",\n    "parameter": "a",\n    "body": {\n      "0": "@lookup",\n      "1": "a"\n    }\n  },\n  "argument": "it works!"\n}',
+    ),
+  ],
 ])
diff --git a/src/language/unparsing/plz-utilities.ts b/src/language/unparsing/plz-utilities.ts
index 8c1038c..f641ff5 100644
--- a/src/language/unparsing/plz-utilities.ts
+++ b/src/language/unparsing/plz-utilities.ts
@@ -128,25 +128,37 @@ const escapeStringContents = (value: string) =>
 const unparseSugaredApply = (
   expression: ApplyExpression,
   unparseAtomOrMolecule: UnparseAtomOrMolecule,
-) =>
-  either.flatMap(serializeIfNeeded(expression.function), serializedFunction =>
+) => {
+  const functionUnparseResult = either.map(
     either.flatMap(
-      unparseAtomOrMolecule(serializedFunction),
-      functionToApplyAsString =>
-        either.flatMap(
-          serializeIfNeeded(expression.argument),
-          serializedArgument =>
-            either.map(
-              unparseAtomOrMolecule(serializedArgument),
-              argumentAsString =>
-                functionToApplyAsString
-                  .concat(openParenthesis)
-                  .concat(argumentAsString)
-                  .concat(closeParenthesis),
-            ),
-        ),
+      serializeIfNeeded(expression.function),
+      unparseAtomOrMolecule,
     ),
+    unparsedFunction =>
+      either.isRight(readFunctionExpression(expression.function))
+        ? // Immediately-applied function expressions need parentheses.
+          openParenthesis.concat(unparsedFunction).concat(closeParenthesis)
+        : unparsedFunction,
+  )
+  if (either.isLeft(functionUnparseResult)) {
+    return functionUnparseResult
+  }
+
+  const argumentUnparseResult = either.flatMap(
+    serializeIfNeeded(expression.argument),
+    unparseAtomOrMolecule,
   )
+  if (either.isLeft(argumentUnparseResult)) {
+    return argumentUnparseResult
+  }
+
+  return either.makeRight(
+    functionUnparseResult.value
+      .concat(openParenthesis)
+      .concat(argumentUnparseResult.value)
+      .concat(closeParenthesis),
+  )
+}
 
 const unparseSugaredFunction = (
   expression: FunctionExpression,