either_monads.php

<?php

use DaveRoss\FunctionalProgrammingUtils\Left as Left;
use DaveRoss\FunctionalProgrammingUtils\Right as Right;

/**
 * @param string $json A JSON string
 * @return Either
 */
function parse_json_returning_either( $json ) {
  $value = json_decode( $json );
  if( is_null( $value ) ) {
    return Left::of( $value );
  }
  else {
    return Right::of( $value );
  }
}

/**
 * Extract the 'value' member from the parsed JSON
 */
function extract_value( $obj ) {
  return $obj->value;
};

$good = parse_json_returning_either( '{ "label": "This is valid JSON", "value": 5 }' );
$bad  = parse_json_returning_either( '{ "label": This is invalid JSON, "value": 5 }' );

// It doesn't matter if these are Left or Right monads, we still map() our functions
$good_value = $good->map( 'extract_value' );
$bad_value = $bad->map( 'extract_value' );

if( $good_value instanceof Left ) {
  // This doesn't get echoed because $good_value is a Right monad
  echo "An error occurred parsing the good JSON";
}

if( $bad_value instanceof Left ) {
  // This echoes because $bad_value is a Left monad
  echo "An error occurred parsing the bad JSON";
}