Simple Cell recomputation

Cell.idr

data Recomputation a  = Same a
                      | Different a

data Cell : Type where
  MkCell : Eq b => (computation : (a -> b)) -> (value : Recomputation (a, b)) -> Cell

recompute : Cell -> Cell
recompute (MkCell computation (Same x)) = MkCell computation (Same x)
recompute (MkCell computation (Different (a, b))) =
  let
    result = computation a
  in
    case result == b of
      True => MkCell computation (Same (a, b))
      False => MkCell computation (Different (a, result))

testCell : Cell
testCell = MkCell negate (Different (1, 2))

nextCell : Cell
nextCell = recompute testCell
-- Which results in `MkCell negate (Different (1, -1))`

Obviously the ideal would be that Same and Different are guaranteed by the type to be valid, so that you can’t construct a Same cell if (computation input) != output.

My Java is rusty:

interface Computable {
  public Object compute (Object argument);
}

abstract class Cell {
  private Computable computation;
  private Object input;
  private Object output;

  public Computable getComputation () {
    return computation;
  }

  public Object getInput () {
    return input;
  }

  public Object getOutput () {
    return output;
  }
}

class SameCell extends Cell {
  public SameCell (Object input, Object output, Computable computation) {
    this.input = input;
    this.output = output;
    this.computation = computation;
  }

  public Cell compute () {
    return this;
  }
}

class DifferentCell extends Cell {
  public DifferentCell (Object input, Object output, Computable computation) {
    this.input = input;
    this.output = output;
    this.computation = computation;
  }

  public Cell compute () {
    Object result = this.computation.compute(this.input);

    if (result == this.output) {
      return new SameCell(this.input, this.output, this.computation);
    } else {
      return new DifferentCell(this.input, result, this.computation);
    }
  }
}

Modern PHP (super rusty too):

<?
$recompute = function ($cell) {
  if ($cell['different'] == true) {
    $result = $cell['computation']($cell['input']);

    if ($result == $output) {
      return array(
        'different' => false,
        'computation' => $cell['computation'],
        'input' => $cell['input'],
        'output' => $cell['output']
      );
    } else {
      return array(
        'different' => true,
        'computation' => $cell['computation'],
        'input' => $cell['input'],
        'output' => $result
      );
    }
  } else {
    return $cell;
  }
};

$testCell = array(
  'different' => true,
  'computation' => function ($int) { return -$int; },
  'input' => 1,
  'output' => 2
);
$nextCell = $recompute($testCell);
$finalCell = $recompute($nextCell);

var_dump($testCell);
var_dump($nextCell);
var_dump($finalCell);
?>