Mixing passé simple and passé composé ?

Question

The sentence tout le monde éclata de rire quand ils ont vu uses first the passé simple and then the passé composé. Why is the same form of the passé not used for both ? I read in the Lawless French article on the passé simple that it's not used in conjunction with the passé composé (https://www.lawlessfrench.com/grammar/passe-simple/).

Accepted Answer

Mixing passé composé and passé simple in one sentence is very uncommon. Here, I understand it to underline that the cry-out of the people was a singular, sudden event while they were watching. It would be considered literary style writing, where rules can be bent for effect.