-> operator that breaks evaluation on encountering a nil/false return

Question

I want to thread an input through a series of functions, just what the -> operator does. However if any of the functions returns nil / false then I would like to break the evaluation and return back an error message. How do I do that, is there some operator / macro that provides this functionality ?

Answer 1

Try this one: -?> From documentation:

(-?> "foo" .toUpperCase (.substring 1)) returns "OO"
(-?> nil .toUpperCase (.substring 1)) returns nil

If you will use -> macro for second example, you will definitely get NullPointerException.

Answer 2

There is also the maybe-m monad in clojure.algo.monads. Being part of the monads framework it is more heavyweight than the -?> macro, so it makes sense to use maybe-m if you are using monads anyway or if your computation graph is more complicated than a simple chain of functions.

Unlike the threading macros, thedomonad composition can handle multiple argument functions that take arguments from multiple previous steps of computation:

(domonad maybe-m
    [a 1
     b nil
     c (* a b)]
    c)

In this example,(* a b) won't get evaluated, since b is nil. The whole expression will return nil instead of throwin an exception from trying to multiply by nil.