Bug: KCL formatter removes 'fn' from closures: (#4718)

# Problem

Before this PR, our formatter reformats
```
squares_out = reduce(arr, 0, fn (i, squares)  {
  return 1
})
```
to 
```
squares_out = reduce(arr, 0, (i, squares) {
  return 1
})
```
i.e. it removes the `fn` keyword from the closure. This keyword is required, so, our formatter turned working code into invalid code.

# Cause

When this closure parameter is formatted, the ExprContext is ::Decl, so `Expr::recast` skips adding the `fn` keyword. The reason it's ::Decl is because the `squares_out = ` declaration sets it, and no subsequent call sets the context to something else.

# Solution

When recasting a call expression, set the context for every argument to `ExprContext::Other`.

docs/kcl/map.md

@@ -45,7 +45,7 @@ circles = map([1..3], drawCircle)
 ```js
 r = 10 // radius
 // Call `map`, using an anonymous function instead of a named one.
-circles = map([1..3], (id) {
+circles = map([1..3], fn(id) {
   return startSketchOn("XY")
     |> circle({ center = [id * 2 * r, 0], radius = r }, %)
 })

docs/kcl/reduce.md

@@ -61,7 +61,7 @@ assertEqual(sum([1, 2, 3]), 6, 0.00001, "1 + 2 + 3 summed is 6")
 // an anonymous `add` function as its parameter, instead of declaring a
 // named function outside.
 arr = [1, 2, 3]
-sum = reduce(arr, 0, (i, result_so_far) {
+sum = reduce(arr, 0, fn(i, result_so_far) {
   return i + result_so_far
 })
 
@@ -84,7 +84,7 @@ fn decagon(radius) {
   // Use a `reduce` to draw the remaining decagon sides.
   // For each number in the array 1..10, run the given function,
   // which takes a partially-sketched decagon and adds one more edge to it.
-  fullDecagon = reduce([1..10], startOfDecagonSketch, (i, partialDecagon) {
+  fullDecagon = reduce([1..10], startOfDecagonSketch, fn(i, partialDecagon) {
     // Draw one edge of the decagon.
     x = cos(stepAngle * i) * radius
     y = sin(stepAngle * i) * radius

docs/kcl/std.json

@@ -100436,7 +100436,7 @@
     "deprecated": false,
     "examples": [
       "r = 10 // radius\nfn drawCircle(id) {\n  return startSketchOn(\"XY\")\n    |> circle({ center = [id * 2 * r, 0], radius = r }, %)\n}\n\n// Call `drawCircle`, passing in each element of the array.\n// The outputs from each `drawCircle` form a new array,\n// which is the return value from `map`.\ncircles = map([1..3], drawCircle)",
-      "r = 10 // radius\n// Call `map`, using an anonymous function instead of a named one.\ncircles = map([1..3], (id) {\n  return startSketchOn(\"XY\")\n    |> circle({ center = [id * 2 * r, 0], radius = r }, %)\n})"
+      "r = 10 // radius\n// Call `map`, using an anonymous function instead of a named one.\ncircles = map([1..3], fn(id) {\n  return startSketchOn(\"XY\")\n    |> circle({ center = [id * 2 * r, 0], radius = r }, %)\n})"
     ]
   },
   {
@@ -146129,8 +146129,8 @@
     "deprecated": false,
     "examples": [
       "// This function adds two numbers.\nfn add(a, b) {\n  return a + b\n}\n\n// This function adds an array of numbers.\n// It uses the `reduce` function, to call the `add` function on every\n// element of the `arr` parameter. The starting value is 0.\nfn sum(arr) {\n  return reduce(arr, 0, add)\n}\n\n/* The above is basically like this pseudo-code:\nfn sum(arr):\n    let sumSoFar = 0\n    for i in arr:\n        sumSoFar = add(sumSoFar, i)\n    return sumSoFar */\n\n\n// We use `assertEqual` to check that our `sum` function gives the\n// expected result. It's good to check your work!\nassertEqual(sum([1, 2, 3]), 6, 0.00001, \"1 + 2 + 3 summed is 6\")",
-      "// This example works just like the previous example above, but it uses\n// an anonymous `add` function as its parameter, instead of declaring a\n// named function outside.\narr = [1, 2, 3]\nsum = reduce(arr, 0, (i, result_so_far) {\n  return i + result_so_far\n})\n\n// We use `assertEqual` to check that our `sum` function gives the\n// expected result. It's good to check your work!\nassertEqual(sum, 6, 0.00001, \"1 + 2 + 3 summed is 6\")",
+      "// This example works just like the previous example above, but it uses\n// an anonymous `add` function as its parameter, instead of declaring a\n// named function outside.\narr = [1, 2, 3]\nsum = reduce(arr, 0, fn(i, result_so_far) {\n  return i + result_so_far\n})\n\n// We use `assertEqual` to check that our `sum` function gives the\n// expected result. It's good to check your work!\nassertEqual(sum, 6, 0.00001, \"1 + 2 + 3 summed is 6\")",
-      "// Declare a function that sketches a decagon.\nfn decagon(radius) {\n  // Each side of the decagon is turned this many degrees from the previous angle.\n  stepAngle = 1 / 10 * tau()\n\n  // Start the decagon sketch at this point.\n  startOfDecagonSketch = startSketchAt([cos(0) * radius, sin(0) * radius])\n\n  // Use a `reduce` to draw the remaining decagon sides.\n  // For each number in the array 1..10, run the given function,\n  // which takes a partially-sketched decagon and adds one more edge to it.\n  fullDecagon = reduce([1..10], startOfDecagonSketch, (i, partialDecagon) {\n    // Draw one edge of the decagon.\n    x = cos(stepAngle * i) * radius\n    y = sin(stepAngle * i) * radius\n    return lineTo([x, y], partialDecagon)\n  })\n\n  return fullDecagon\n}\n\n/* The `decagon` above is basically like this pseudo-code:\nfn decagon(radius):\n    let stepAngle = (1/10) * tau()\n    let startOfDecagonSketch = startSketchAt([(cos(0)*radius), (sin(0) * radius)])\n\n    // Here's the reduce part.\n    let partialDecagon = startOfDecagonSketch\n    for i in [1..10]:\n        let x = cos(stepAngle * i) * radius\n        let y = sin(stepAngle * i) * radius\n        partialDecagon = lineTo([x, y], partialDecagon)\n    fullDecagon = partialDecagon // it's now full\n    return fullDecagon */\n\n\n// Use the `decagon` function declared above, to sketch a decagon with radius 5.\ndecagon(5.0)\n  |> close(%)"
+      "// Declare a function that sketches a decagon.\nfn decagon(radius) {\n  // Each side of the decagon is turned this many degrees from the previous angle.\n  stepAngle = 1 / 10 * tau()\n\n  // Start the decagon sketch at this point.\n  startOfDecagonSketch = startSketchAt([cos(0) * radius, sin(0) * radius])\n\n  // Use a `reduce` to draw the remaining decagon sides.\n  // For each number in the array 1..10, run the given function,\n  // which takes a partially-sketched decagon and adds one more edge to it.\n  fullDecagon = reduce([1..10], startOfDecagonSketch, fn(i, partialDecagon) {\n    // Draw one edge of the decagon.\n    x = cos(stepAngle * i) * radius\n    y = sin(stepAngle * i) * radius\n    return lineTo([x, y], partialDecagon)\n  })\n\n  return fullDecagon\n}\n\n/* The `decagon` above is basically like this pseudo-code:\nfn decagon(radius):\n    let stepAngle = (1/10) * tau()\n    let startOfDecagonSketch = startSketchAt([(cos(0)*radius), (sin(0) * radius)])\n\n    // Here's the reduce part.\n    let partialDecagon = startOfDecagonSketch\n    for i in [1..10]:\n        let x = cos(stepAngle * i) * radius\n        let y = sin(stepAngle * i) * radius\n        partialDecagon = lineTo([x, y], partialDecagon)\n    fullDecagon = partialDecagon // it's now full\n    return fullDecagon */\n\n\n// Use the `decagon` function declared above, to sketch a decagon with radius 5.\ndecagon(5.0)\n  |> close(%)"
     ]
   },
   {

src/wasm-lib/kcl/src/unparser.rs

@@ -166,7 +166,14 @@ pub(crate) enum ExprContext {
 }
 
 impl Expr {
-    pub(crate) fn recast(&self, options: &FormatOptions, indentation_level: usize, ctxt: ExprContext) -> String {
+    pub(crate) fn recast(&self, options: &FormatOptions, indentation_level: usize, mut ctxt: ExprContext) -> String {
+        let is_decl = matches!(ctxt, ExprContext::Decl);
+        if is_decl {
+            // Just because this expression is being bound to a variable, doesn't mean that every child
+            // expression is being bound. So, reset the expression context if necessary.
+            // This will still preserve the "::Pipe" context though.
+            ctxt = ExprContext::Other;
+        }
         match &self {
             Expr::BinaryExpression(bin_exp) => bin_exp.recast(options),
             Expr::ArrayExpression(array_exp) => array_exp.recast(options, indentation_level, ctxt),
@@ -175,11 +182,7 @@ impl Expr {
             Expr::MemberExpression(mem_exp) => mem_exp.recast(),
             Expr::Literal(literal) => literal.recast(),
             Expr::FunctionExpression(func_exp) => {
-                let mut result = if ctxt == ExprContext::Decl {
+                let mut result = if is_decl { String::new() } else { "fn".to_owned() };
-                    String::new()
-                } else {
-                    "fn".to_owned()
-                };
                 result += &func_exp.recast(options, indentation_level);
                 result
             }
@@ -2170,6 +2173,28 @@ sketch002 = startSketchOn({
         assert_eq!(actual, expected);
     }
 
+    #[test]
+    fn unparse_fn_unnamed() {
+        let input = r#"squares_out = reduce(arr, 0, fn(i, squares) {
+  return 1
+})
+"#;
+        let ast = crate::parsing::top_level_parse(input).unwrap();
+        let actual = ast.recast(&FormatOptions::new(), 0);
+        assert_eq!(actual, input);
+    }
+
+    #[test]
+    fn unparse_fn_named() {
+        let input = r#"fn f(x) {
+  return 1
+}
+"#;
+        let ast = crate::parsing::top_level_parse(input).unwrap();
+        let actual = ast.recast(&FormatOptions::new(), 0);
+        assert_eq!(actual, input);
+    }
+
     #[test]
     fn recast_objects_with_comments() {
         use winnow::Parser;