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Composable directives

Oftentimes, a directive cannot be applied on a field, because it has an input which is different than the field's output. For instance, directive @strUpperCase receives a string as input, so it can't be applied on field User.capabilities, which returns an array of strings.

With composable directives, a directive can augment another directive to modify its behavior or fill a gap. This removes the need to duplicate fields or directives just to change their input or return types, avoiding bloat.

In this query, directive @underEachArrayItem iterates over an array of strings, and applies its nested directive @strUpperCase on each of them, fixing the type mismatch:

query {
  users {