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 []const u8

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

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

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