Argument applyPlan

You've received this error because the applyPlan method on one of your arguments (see the error message to identify which one) has returned a value. Doing so was meaningful during part of Grafast's beta phase, but it was determined that this led to bad patterns and thus it was removed.

It's likely that you just returned a value by accident and did not mean for it to be meaningful. If so, simply remove the return.

However, if you are migrating from an early version of Grafast and this is meaningful then instead of returning something from your existing code:

// Code that produces the error:
function applyPlan($parent, $field, fieldArg, info) {
  // [...]
  return $target;
}

you likely want to directly apply the fieldArg to it:

function applyPlan($parent, $field, fieldArg, info) {
  // [...]
  fieldArg.apply($target);
}

This uses applyInput() under the hood to "apply" input values to a parent step, and thus requires that $target implements the apply() method. The code is roughly equivalent to:

function applyPlan($parent, $field, fieldArg, info) {
  // [...]
  // fieldArg.apply($target) is roughly equivalent to:
  const argType = info.arg.type;
  const $value = fieldArg.getRaw();
  const $callback = applyInput(argType, $value);
  $target.apply($callback);
}

Note that applying inputs like this happens at runtime (rather than plantime) since the inputs could be variables and we don't want to fork the plans based on runtime values; thus the mechanics have changed a bit - please see the applyInput() documentation for more details.