@spec check_for_duplicates(list(), list(), config()) :: :ok

Checks for duplicate field names in a field selection list.

Normalizes field names using the input formatter before checking for duplicates. Throws {:duplicate_field, field_name, path} if duplicates are found.

This runs before any field-existence check, so it sees every name a client sent, valid or not. A name that matches no existing atom stays a string here and is reported as such; the caller rejects it as an unknown field moments later.

Parameters

• fields - List of field selections
• path - Current path for error reporting
• config - Configuration map with :input_field_formatter

@spec field_exists?(keyword(), term()) :: boolean()

Checks whether field_specs declares field_name.

Accepts a string name, which Keyword.has_key?/2 rejects outright. A client name that resolved to no existing atom stays a string, and no field spec can be keyed by one, so the answer is always false — the caller then reports it as an unknown field.

@spec validate_field_exists!(atom() | String.t(), keyword(), list(), String.t()) ::
  :ok

Validates that a field exists in the given field specs.

Throws {:unknown_field, field_name, error_type, path} if not found.

Parameters

• field_name - The field name to check
• field_specs - Keyword list of field specifications
• path - Current path for error reporting
• error_type - Error type string for error messages

@spec validate_non_empty(term(), term(), list(), atom()) :: :ok

Validates that nested fields are non-empty for fields that require selection.

Throws appropriate errors if validation fails.

Parameters

• nested_fields - The nested field selections
• field_name - The field name for error messages
• path - Current path for error reporting
• error_type - The type of error to throw (default: :relationship)