The big thing with Gren is that it's easy to anticipate what your code will do, even when calling functions you don't know the implementation of. It's hard to do this when function calls can mutate state, throw unchecked exceptions or perform arbitrary and unmanaged side effects.

Gren tries to do without these (mutation, unchecked exceptions, unmanaged side-effects) without requiring too much effort on the developer's part. In return, you can easier reason about the behaviour of your program.

Elm proved, at least to me, that this was good way to write programs. Gren simply tries to take it a step further by including first class support for backend applications and command line tools through NodeJS.