JavaScript: "What do we show when we don't know the value in the URL?"
Designer: "That won't happen."
JavaScript: "K."

Later a null pointer happens in an 'impossible situation'. Gnashing of teeth & lol'ing at how this could happen.

Elm: "What do we show when we don't know the value?"
Designer: "That won't happen."
Elm: "K, I'll put 'designer choose not to design this screen'".
Designer: "Hah, if you say so."

Later error screen shows in an impossible situation. Visually & immediately clear what failed.

The compiler is forcing me to have uncomfortable error scenario conversations with my Designer because null pointers/Maybes are now his problem too. As a front-end dev, this is a wonderful tool, man.

... and by wonderful, it's clearly one sided here on me, the developer. The poor Designer has to think through more scenarios, recognize the strange error conditions, and make a judgement call on what we should/should not design for to make progress.