Documentation

Returns Just if the precondition is fulfilled.

This is an infix version of fmap for writing select style expressions using test functions, that produce Maybes.

The precedence is chosen to be higher than '(:)', in order to allow:

alternatives default $
   checkForA ?-> (\a -> f a) :
   checkForB ?-> (\b -> g b) :
   []

The operation is left associative in order to allow to write

checkForA ?-> f ?-> g

which is equivalent to

checkForA ?-> g . f

due to the functor law.