fsharpVisitor.js

import { types as t } from "@babel/core";

const fsharpVisitor = {
  BinaryExpression(path) {
    const { scope } = path;
    const { node } = path;
    const { operator, left } = node;
    let { right } = node;
    if (operator !== "|>") return;

    let optimizeArrow =
      t.isArrowFunctionExpression(right) &&
      t.isExpression(right.body) &&
      !right.async &&
      !right.generator;
    let param;

    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 applied =
      right.type === "AwaitExpression"
        ? t.awaitExpression(t.cloneNode(placeholder))
        : t.callExpression(right, [t.cloneNode(placeholder)]);
    const call = optimizeArrow ? right.body : applied;
    path.replaceWith(
      t.sequenceExpression([
        t.assignmentExpression("=", t.cloneNode(placeholder), left),
        call,
      ]),
    );
  },
};

export default fsharpVisitor;