Dumb question but I recently was looking at Elm and ReasonML (by Facebook) and don't really know the difference... they seem to try to achieve the same thing no?
Their goals might align quite a lot (functional programming for the browser; compiling to JS), but Elm seems to me more "pure" than ReasonML, and places extra emphasis on correctness ("no runtime errors").
For example, ReasonML allows you to put JS code inside its source [1]. Which might be great if you want that escape hatch, but might mean that runtime errors are common when working with ReasonML. Elm trades a bit of convenience for the enforced reliability.
Different trade-offs were made.