013 reject function call in branch constructor

// Test: Parser should reject function calls in branch constructors
//
// Branch constructors must contain only pure values: literals, bindings,
// field access, and expressions. Function calls introduce side effects
// and are not allowed. Use event chaining instead:
//
//   WRONG:  | ok x |> result { value: computeIt(x) }
//   RIGHT:  | ok x |> compute(input: x)
//                     | value v |> result { value: v }

~import "$std/io"

const std = @import("std");

fn helper(x: []const u8) []const u8 { return x; }

~event greet { name: []const u8 }
| greeting { msg: []const u8 }

~proc greet {
    const buf = std.fmt.comptimePrint("Hello {s}", .{name});
    return .{ .greeting = .{ .msg = buf } };
}

~event transform { input: []const u8 }
| result { value: []const u8 }

// INVALID: function call helper() inside branch constructor
~greet(name: "world")
| greeting g |> result { value: helper(g.msg) }