Fp-ts: How to write Eq for an union?

[steida]

📖 Documentation

Is this idiomatic fp-ts?

const eqNode: Eq<Text | ElementNotNormalized> = fromEquals((x, y) => {
  if (x.type === 'text') {
    if (y.type === 'element') return false;
    return eqText.equals(x, y);
  } else {
    if (y.type === 'text') return false;
    return eqElement.equals(x, y);
  }
});

[mlegenhausen]

I do it myself the same way, but use a ADT library like morphic-ts or use a switch statements instead of if

const eqNode: Eq<Text | ElementNotNormalized> = fromEquals((x, y) => {
  switch (x.type) {
    case 'element': return y.type === 'element' && eqElement.equals(x, y)
    case 'text': return y.type === 'text' && eqText.equals(x, y)
  }
});

[steida]

@mlegenhausen Thank you. But this approach can explode if we have more than two unioned types, so I am curious if there is any generic way of doing that for n types.

[samhh]

Because you appear to be testing discriminated unions, this is functionally identical to how Eq instances are derived for the likes of Either in fp-ts. There's really no way to do this other than to check against your discriminant as you are doing.

My only suggestion would be to line up the conditions slightly so that it's more readable and scales better:

const eqNode: Eq<Text | ElementNotNormalized> = fromEquals((x, y) => {
  if (x.type === 'text' && y.type === 'text') return eqText.equals(x, y);
  if (x.type === 'element' && y.type === 'element') return eqElement.equals(x, y);
  return false;
});

[steida]

@samhh That makes sense. Thank you!