Currently we don't show warnings in the playground: https://melange.re/unstable/playground/?language=Reason&code=dHlwZSBmb28gPQogIHwgT25lCiAgfCBUd287CgpsZXQgYmFyID0gdiA9PgogIHN3aXRjaCAodikgewogIHwgT25lKF8pID0%2BIDAuMAogIH07Cg%3D%3D&live=off

I think we should. cc @anmonteiro in case there are some ideas on how to this better, maybe we can make all warnings as errors at the compiler level?