From 3eceb2be15bd665ce5f58721b299b88f55e6c0ee Mon Sep 17 00:00:00 2001
From: Thiago Arrais <thiago.arrais@gmail.com>
Date: Sat, 25 May 2019 07:48:30 -0300
Subject: [PATCH] Transform for F# Pipeline (#9984)

* Transform for F#-style await

Inludes support for optimizing single-parameter arrow functions

* Wait until optimization before pushing placeholder into scope
---
 .../src/buildOptimizedSequenceExpression.js   | 54 +++++++++++++++++++
 .../src/fsharpVisitor.js                      | 25 +++++++++
 .../src/index.js                              |  2 +
 .../src/minimalVisitor.js                     | 48 ++++-------------
 .../fsharp/arrow-functions-parenless/exec.js  | 27 ++++++++++
 .../fixtures/fsharp/arrow-functions/exec.js   | 14 +++++
 .../fixtures/fsharp/arrow-functions/input.js  | 14 +++++
 .../fixtures/fsharp/arrow-functions/output.js | 15 ++++++
 .../test/fixtures/fsharp/await/exec.js        | 13 +++++
 .../test/fixtures/fsharp/await/input.js       |  6 +++
 .../test/fixtures/fsharp/await/options.json   |  9 ++++
 .../test/fixtures/fsharp/await/output.js      |  5 ++
 .../test/fixtures/fsharp/basic/exec.js        |  3 ++
 .../test/fixtures/fsharp/basic/input.js       |  3 ++
 .../test/fixtures/fsharp/basic/output.js      |  5 ++
 .../fixtures/fsharp/indirect-eval/exec.js     |  7 +++
 .../fixtures/fsharp/indirect-eval/input.js    |  7 +++
 .../fixtures/fsharp/indirect-eval/output.js   |  8 +++
 .../fsharp/multiple-argument-use/input.js     |  5 ++
 .../fsharp/multiple-argument-use/output.js    |  5 ++
 .../fsharp/optimize-0-param-arrow/exec.js     |  8 +++
 .../fsharp/optimize-0-param-arrow/input.js    |  8 +++
 .../fsharp/optimize-0-param-arrow/output.js   |  5 ++
 .../test/fixtures/fsharp/options.json         |  8 +++
 .../minimal/arrow-functions-parenless/exec.js | 20 +++++++
 .../arrow-functions-parenless/input.js        | 20 +++++++
 .../arrow-functions-parenless/output.js       | 23 ++++++++
 .../minimal/arrow-functions/output.js         |  8 +--
 .../minimal/multiple-argument-use/output.js   |  4 +-
 .../src/index.js                              |  2 +-
 30 files changed, 335 insertions(+), 46 deletions(-)
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/src/buildOptimizedSequenceExpression.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/src/fsharpVisitor.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions-parenless/exec.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/exec.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/input.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/output.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/exec.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/input.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/options.json
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/output.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/exec.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/input.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/output.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/exec.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/input.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/output.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/multiple-argument-use/input.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/multiple-argument-use/output.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/exec.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/input.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/output.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/options.json
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/exec.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/input.js
 create mode 100644 packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/output.js

diff --git a/packages/babel-plugin-proposal-pipeline-operator/src/buildOptimizedSequenceExpression.js b/packages/babel-plugin-proposal-pipeline-operator/src/buildOptimizedSequenceExpression.js
new file mode 100644
index 000000000000..2d0d3f23ea8f
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/src/buildOptimizedSequenceExpression.js
@@ -0,0 +1,54 @@
+import { types as t } from "@babel/core";
+
+// tries to optimize sequence expressions in the format
+//   (a = b, ((c) => d + e)(a))
+// to
+//   (a = b, a + e)
+const buildOptimizedSequenceExpression = ({ assign, call, path }) => {
+  const { left: placeholderNode, right: pipelineLeft } = assign;
+  const { callee: calledExpression } = call;
+
+  let optimizeArrow =
+    t.isArrowFunctionExpression(calledExpression) &&
+    t.isExpression(calledExpression.body) &&
+    !calledExpression.async &&
+    !calledExpression.generator;
+  let param;
+
+  if (optimizeArrow) {
+    const { params } = calledExpression;
+    if (params.length === 1 && t.isIdentifier(params[0])) {
+      param = params[0];
+    } else if (params.length > 0) {
+      optimizeArrow = false;
+    }
+  } else if (t.isIdentifier(calledExpression, { name: "eval" })) {
+    const evalSequence = t.sequenceExpression([
+      t.numericLiteral(0),
+      calledExpression,
+    ]);
+
+    call.callee = evalSequence;
+
+    path.scope.push({ id: placeholderNode });
+
+    return t.sequenceExpression([assign, call]);
+  }
+
+  if (optimizeArrow && !param) {
+    // Arrow function with 0 arguments
+    return t.sequenceExpression([pipelineLeft, calledExpression.body]);
+  }
+
+  path.scope.push({ id: placeholderNode });
+
+  if (param) {
+    path.get("right").scope.rename(param.name, placeholderNode.name);
+
+    return t.sequenceExpression([assign, calledExpression.body]);
+  }
+
+  return t.sequenceExpression([assign, call]);
+};
+
+export default buildOptimizedSequenceExpression;
diff --git a/packages/babel-plugin-proposal-pipeline-operator/src/fsharpVisitor.js b/packages/babel-plugin-proposal-pipeline-operator/src/fsharpVisitor.js
new file mode 100644
index 000000000000..f32f8e33fd99
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/src/fsharpVisitor.js
@@ -0,0 +1,25 @@
+import { types as t } from "@babel/core";
+import buildOptimizedSequenceExpression from "./buildOptimizedSequenceExpression";
+
+const fsharpVisitor = {
+  BinaryExpression(path) {
+    const { scope, node } = path;
+    const { operator, left, right } = node;
+    if (operator !== "|>") return;
+
+    const placeholder = scope.generateUidIdentifierBasedOnNode(left);
+
+    const call =
+      right.type === "AwaitExpression"
+        ? t.awaitExpression(t.cloneNode(placeholder))
+        : t.callExpression(right, [t.cloneNode(placeholder)]);
+    const sequence = buildOptimizedSequenceExpression({
+      assign: t.assignmentExpression("=", t.cloneNode(placeholder), left),
+      call,
+      path,
+    });
+    path.replaceWith(sequence);
+  },
+};
+
+export default fsharpVisitor;
diff --git a/packages/babel-plugin-proposal-pipeline-operator/src/index.js b/packages/babel-plugin-proposal-pipeline-operator/src/index.js
index 488cd02da9e0..241a99b87828 100644
--- a/packages/babel-plugin-proposal-pipeline-operator/src/index.js
+++ b/packages/babel-plugin-proposal-pipeline-operator/src/index.js
@@ -2,10 +2,12 @@ import { declare } from "@babel/helper-plugin-utils";
 import syntaxPipelineOperator from "@babel/plugin-syntax-pipeline-operator";
 import minimalVisitor from "./minimalVisitor";
 import smartVisitor from "./smartVisitor";
+import fsharpVisitor from "./fsharpVisitor";
 
 const visitorsPerProposal = {
   minimal: minimalVisitor,
   smart: smartVisitor,
+  fsharp: fsharpVisitor,
 };
 
 export default declare((api, options) => {
diff --git a/packages/babel-plugin-proposal-pipeline-operator/src/minimalVisitor.js b/packages/babel-plugin-proposal-pipeline-operator/src/minimalVisitor.js
index 960082a09591..5975949779bb 100644
--- a/packages/babel-plugin-proposal-pipeline-operator/src/minimalVisitor.js
+++ b/packages/babel-plugin-proposal-pipeline-operator/src/minimalVisitor.js
@@ -1,51 +1,21 @@
 import { types as t } from "@babel/core";
+import buildOptimizedSequenceExpression from "./buildOptimizedSequenceExpression";
 
 const minimalVisitor = {
   BinaryExpression(path) {
-    const { scope } = path;
-    const { node } = path;
-    const { operator, left } = node;
-    let { right } = node;
+    const { scope, node } = path;
+    const { operator, left, right } = node;
     if (operator !== "|>") return;
 
-    let optimizeArrow =
-      t.isArrowFunctionExpression(right) &&
-      t.isExpression(right.body) &&
-      !right.async &&
-      !right.generator;
-    let param;
+    const placeholder = scope.generateUidIdentifierBasedOnNode(left);
 
-    if (optimizeArrow) {
-      const { params } = right;
-      if (params.length === 1 && t.isIdentifier(params[0])) {
-        param = params[0];
-      } else if (params.length > 0) {
-        optimizeArrow = false;
-      }
-    } else if (t.isIdentifier(right, { name: "eval" })) {
-      right = t.sequenceExpression([t.numericLiteral(0), right]);
-    }
-
-    if (optimizeArrow && !param) {
-      // Arrow function with 0 arguments
-      path.replaceWith(t.sequenceExpression([left, right.body]));
-      return;
-    }
-
-    const placeholder = scope.generateUidIdentifierBasedOnNode(param || left);
-    scope.push({ id: placeholder });
-    if (param) {
-      path.get("right").scope.rename(param.name, placeholder.name);
-    }
-
-    const call = optimizeArrow
-      ? right.body
-      : t.callExpression(right, [t.cloneNode(placeholder)]);
+    const call = t.callExpression(right, [t.cloneNode(placeholder)]);
     path.replaceWith(
-      t.sequenceExpression([
-        t.assignmentExpression("=", t.cloneNode(placeholder), left),
+      buildOptimizedSequenceExpression({
+        assign: t.assignmentExpression("=", t.cloneNode(placeholder), left),
         call,
-      ]),
+        path,
+      }),
     );
   },
 };
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions-parenless/exec.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions-parenless/exec.js
new file mode 100644
index 000000000000..03ffe5328721
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions-parenless/exec.js
@@ -0,0 +1,27 @@
+const y = 2;
+
+const f = (x) => (x |> (y) => y + 1)
+  |> (z) => z * y
+
+const _f = (x) => x
+  |> (y) => y + 1
+  |> (z) => z * y
+
+const g = (x) => x
+  |> (y) => (y + 1 |> (z) => z * y)
+
+const _g = (x) => x
+  |> (y => (y + 1 |> (z) => z * y))
+
+const __g = (x) => x
+  |> (
+    y => {
+      return (y + 1 |> (z) => z * y);
+    }
+  )
+
+expect(  f(1)).toBe(4);
+expect( _f(1)).toBe(4);
+expect(  g(1)).toBe(2);
+expect( _g(1)).toBe(2);
+expect(__g(1)).toBe(2);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/exec.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/exec.js
new file mode 100644
index 000000000000..2411e82738ee
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/exec.js
@@ -0,0 +1,14 @@
+var result = [5,10]
+  |> (_ => _.map(x => x * 2))
+  |> (_ => _.reduce( (a,b) => a + b ))
+  |> (sum => sum + 1)
+
+expect(result).toBe(31);
+
+
+var inc = (x) => x + 1;
+var double = (x) => x * 2;
+
+var result2 = [4, 9].map( x => x |> inc |> double )
+
+expect(result2).toEqual([10, 20]);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/input.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/input.js
new file mode 100644
index 000000000000..2411e82738ee
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/input.js
@@ -0,0 +1,14 @@
+var result = [5,10]
+  |> (_ => _.map(x => x * 2))
+  |> (_ => _.reduce( (a,b) => a + b ))
+  |> (sum => sum + 1)
+
+expect(result).toBe(31);
+
+
+var inc = (x) => x + 1;
+var double = (x) => x * 2;
+
+var result2 = [4, 9].map( x => x |> inc |> double )
+
+expect(result2).toEqual([10, 20]);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/output.js
new file mode 100644
index 000000000000..cd2b80132c22
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/arrow-functions/output.js
@@ -0,0 +1,15 @@
+var _ref, _ref2, _ref3;
+
+var result = (_ref = (_ref2 = (_ref3 = [5, 10], _ref3.map(x => x * 2)), _ref2.reduce((a, b) => a + b)), _ref + 1);
+expect(result).toBe(31);
+
+var inc = x => x + 1;
+
+var double = x => x * 2;
+
+var result2 = [4, 9].map(x => {
+  var _ref4, _x;
+
+  return _ref4 = (_x = x, inc(_x)), double(_ref4);
+});
+expect(result2).toEqual([10, 20]);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/exec.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/exec.js
new file mode 100644
index 000000000000..38503c1d0d23
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/exec.js
@@ -0,0 +1,13 @@
+const triple = (x) => x * 3;
+
+async function myFunction(n) {
+  return n
+    |> Math.abs
+    |> Promise.resolve
+    |> await
+    |> triple;
+}
+
+return myFunction(-7).then(result => {
+  expect(result).toBe(21);
+});
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/input.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/input.js
new file mode 100644
index 000000000000..ad69aa432bbf
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/input.js
@@ -0,0 +1,6 @@
+async function myFunction(n) {
+  return n
+    |> Math.abs
+    |> Promise.resolve
+    |> await;
+}
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/options.json b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/options.json
new file mode 100644
index 000000000000..c5a39e971027
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/options.json
@@ -0,0 +1,9 @@
+{
+  "plugins": [
+    ["proposal-pipeline-operator", { "proposal": "fsharp" }]
+  ],
+  "parserOpts": {
+    "allowReturnOutsideFunction": true
+  },
+  "minNodeVersion": "8.0.0"
+}
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/output.js
new file mode 100644
index 000000000000..bf7848696774
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/await/output.js
@@ -0,0 +1,5 @@
+async function myFunction(n) {
+  var _ref, _ref2, _n;
+
+  return _ref = (_ref2 = (_n = n, Math.abs(_n)), Promise.resolve(_ref2)), await _ref;
+}
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/exec.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/exec.js
new file mode 100644
index 000000000000..a11e8073bb3f
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/exec.js
@@ -0,0 +1,3 @@
+var inc = (x) => x + 1
+
+expect(10 |> inc).toBe(11);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/input.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/input.js
new file mode 100644
index 000000000000..a11e8073bb3f
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/input.js
@@ -0,0 +1,3 @@
+var inc = (x) => x + 1
+
+expect(10 |> inc).toBe(11);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/output.js
new file mode 100644
index 000000000000..6ac05ce0fbc9
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/basic/output.js
@@ -0,0 +1,5 @@
+var _;
+
+var inc = x => x + 1;
+
+expect((_ = 10, inc(_))).toBe(11);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/exec.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/exec.js
new file mode 100644
index 000000000000..30efd15ea92a
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/exec.js
@@ -0,0 +1,7 @@
+(function() {
+  'use strict';
+  var result = '(function() { return this; })()'
+    |> eval;
+
+  expect(result).not.toBeUndefined();
+})();
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/input.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/input.js
new file mode 100644
index 000000000000..30efd15ea92a
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/input.js
@@ -0,0 +1,7 @@
+(function() {
+  'use strict';
+  var result = '(function() { return this; })()'
+    |> eval;
+
+  expect(result).not.toBeUndefined();
+})();
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/output.js
new file mode 100644
index 000000000000..3c53e6d4f3bf
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/indirect-eval/output.js
@@ -0,0 +1,8 @@
+(function () {
+  'use strict';
+
+  var _functionReturn;
+
+  var result = (_functionReturn = '(function() { return this; })()', (0, eval)(_functionReturn));
+  expect(result).not.toBeUndefined();
+})();
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/multiple-argument-use/input.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/multiple-argument-use/input.js
new file mode 100644
index 000000000000..f29037ed294e
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/multiple-argument-use/input.js
@@ -0,0 +1,5 @@
+var array = [10,20,30];
+
+var last = array |> (a => a[a.length-1]);
+
+expect(last).toBe(30);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/multiple-argument-use/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/multiple-argument-use/output.js
new file mode 100644
index 000000000000..121f15e8a584
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/multiple-argument-use/output.js
@@ -0,0 +1,5 @@
+var _array;
+
+var array = [10, 20, 30];
+var last = (_array = array, _array[_array.length - 1]);
+expect(last).toBe(30);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/exec.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/exec.js
new file mode 100644
index 000000000000..d8ab9b2ebcf2
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/exec.js
@@ -0,0 +1,8 @@
+var a = 1,
+    b = 2,
+    c = 3;
+var result = a
+  |> (() => b)
+  |> (() => c);
+
+expect(result).toBe(c);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/input.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/input.js
new file mode 100644
index 000000000000..d8ab9b2ebcf2
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/input.js
@@ -0,0 +1,8 @@
+var a = 1,
+    b = 2,
+    c = 3;
+var result = a
+  |> (() => b)
+  |> (() => c);
+
+expect(result).toBe(c);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/output.js
new file mode 100644
index 000000000000..4fcc70527ca8
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/optimize-0-param-arrow/output.js
@@ -0,0 +1,5 @@
+var a = 1,
+    b = 2,
+    c = 3;
+var result = ((a, b), c);
+expect(result).toBe(c);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/options.json b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/options.json
new file mode 100644
index 000000000000..4e41244cc302
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/fsharp/options.json
@@ -0,0 +1,8 @@
+{
+  "plugins": [
+    ["proposal-pipeline-operator", { "proposal": "fsharp" }]
+  ],
+  "parserOpts": {
+    "allowReturnOutsideFunction": true
+  }
+}
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/exec.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/exec.js
new file mode 100644
index 000000000000..88762f8e568f
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/exec.js
@@ -0,0 +1,20 @@
+const y = 2;
+const f = (x) => x
+  |> (y => y + 1)
+  |> (z => z * y)
+
+const g = (x) => x
+  |> (y =>
+    y + 1
+    |> (z => z * y)
+  )
+
+const h = (x) => x
+  |> (y => (
+    y + 1
+    |> (z => z * y)
+  ))
+
+expect(f(1)).toBe(4);
+expect(g(1)).toBe(2);
+expect(h(1)).toBe(2);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/input.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/input.js
new file mode 100644
index 000000000000..88762f8e568f
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/input.js
@@ -0,0 +1,20 @@
+const y = 2;
+const f = (x) => x
+  |> (y => y + 1)
+  |> (z => z * y)
+
+const g = (x) => x
+  |> (y =>
+    y + 1
+    |> (z => z * y)
+  )
+
+const h = (x) => x
+  |> (y => (
+    y + 1
+    |> (z => z * y)
+  ))
+
+expect(f(1)).toBe(4);
+expect(g(1)).toBe(2);
+expect(h(1)).toBe(2);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/output.js
new file mode 100644
index 000000000000..926086963984
--- /dev/null
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions-parenless/output.js
@@ -0,0 +1,23 @@
+const y = 2;
+
+const f = x => {
+  var _ref, _x;
+
+  return _ref = (_x = x, _x + 1), _ref * y;
+};
+
+const g = x => {
+  var _x2, _ref2;
+
+  return _x2 = x, (_ref2 = _x2 + 1, _ref2 * _x2);
+};
+
+const h = x => {
+  var _x3, _ref3;
+
+  return _x3 = x, (_ref3 = _x3 + 1, _ref3 * _x3);
+};
+
+expect(f(1)).toBe(4);
+expect(g(1)).toBe(2);
+expect(h(1)).toBe(2);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions/output.js
index f9e5075a4800..cd2b80132c22 100644
--- a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions/output.js
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/arrow-functions/output.js
@@ -1,6 +1,6 @@
-var _sum, _ref, _ref2;
+var _ref, _ref2, _ref3;
 
-var result = (_sum = (_ref = (_ref2 = [5, 10], _ref2.map(x => x * 2)), _ref.reduce((a, b) => a + b)), _sum + 1);
+var result = (_ref = (_ref2 = (_ref3 = [5, 10], _ref3.map(x => x * 2)), _ref2.reduce((a, b) => a + b)), _ref + 1);
 expect(result).toBe(31);
 
 var inc = x => x + 1;
@@ -8,8 +8,8 @@ var inc = x => x + 1;
 var double = x => x * 2;
 
 var result2 = [4, 9].map(x => {
-  var _ref3, _x;
+  var _ref4, _x;
 
-  return _ref3 = (_x = x, inc(_x)), double(_ref3);
+  return _ref4 = (_x = x, inc(_x)), double(_ref4);
 });
 expect(result2).toEqual([10, 20]);
diff --git a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/multiple-argument-use/output.js b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/multiple-argument-use/output.js
index b26deb11cd97..121f15e8a584 100644
--- a/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/multiple-argument-use/output.js
+++ b/packages/babel-plugin-proposal-pipeline-operator/test/fixtures/minimal/multiple-argument-use/output.js
@@ -1,5 +1,5 @@
-var _a;
+var _array;
 
 var array = [10, 20, 30];
-var last = (_a = array, _a[_a.length - 1]);
+var last = (_array = array, _array[_array.length - 1]);
 expect(last).toBe(30);
diff --git a/packages/babel-plugin-syntax-pipeline-operator/src/index.js b/packages/babel-plugin-syntax-pipeline-operator/src/index.js
index 4cb84850ebac..df904e77644c 100644
--- a/packages/babel-plugin-syntax-pipeline-operator/src/index.js
+++ b/packages/babel-plugin-syntax-pipeline-operator/src/index.js
@@ -1,6 +1,6 @@
 import { declare } from "@babel/helper-plugin-utils";
 
-export const proposals = ["minimal", "smart"];
+export const proposals = ["minimal", "smart", "fsharp"];
 
 export default declare((api, { proposal }) => {
   api.assertVersion(7);