For Elm developers
It is no secret Act is hugely inspired by Elm. Therefore, Act will [I hope] feel sort of natural to anyone with some Elm experience.
The main way Act departs from Elm, and also its main advantage, is that it is written in JavaScript (actually, es2015/es6). Also its main disadvantage is that it is written in JavaScript :cry:.
Being written in JavaScript allows Act to play nicer with other JavaScript libraries and it also appeals to a bigger number of developers than any Haskell inspired language. Also, given its simplicity Act will very likely generate much smaller builds than most frameworks out there.
Being written in JavaScript makes almost impossible for Act – and any JavaScript framework out there – to throw elegant error messages in compile time, and the lack of type system forces us to use strings for message types (like in Redux) and this makes composition of components a little too shabby. Also, even though I stated Act plays nicer with other JavaScript libraries, when Elm implements any of these libraries it will invariably add a layer of safety to them, and that's another unfortunate miss.
Here's a selection of things in which Act departs from Elm: