[a0]

Being a Vim/Merlin user I normally just type `<Leader>t` to see the type of the value under the cursor (this maps to `:MerlinTypeOf` if I recall correctly).

What I do use type annotations for is debugging. If the compiler finds a type error, in some situations it helps adding type annotations to find where the actual error is.

[johnisgood]

I do use that feature, too, for functions, in Emacs. So in the mode line I see for example: Array.length: 'a array -> int. I find it quite useful.