Cats Api Documentation

Like ->, but with monadic binding instead of pure application. A mnemonic for the name is a pun on >>=, the monadic bind operator, and clojure’s regular arrow macros.

You can think of it as generalizing the some-> thread macro to all Monads instead of just Maybe.

Alternatively, if you think of the regular thread macro as sugar for let:

(-> :a b (c (other args)) d) => (let [res (b :a) res (c res (other args)) res (d res)] res)

Then ->= is sugar for cats.core/mlet:

(->= m-a b (c (other args)) d) (mlet [res m-a res (c res (other args)) res (d res)] (return res))

Note that extra args in this context are assumed pure, and will be evaluated along with the function itself; this also matches the behavior of some-> wrt extra args.

Threading through pure functions is somewhat awkward, but can be done:

(->= m-a monadic-fn (-> pure-fn other-pure-fn m/return) other-monadic-fn)

Like ->>, but with monadic binding instead of pure application. See cats.labs.sugar/->= for more in-depth discussion.

Apply a pure function to applicative arguments, e.g.

(ap + (just 1) (just 2) (just 3)) ;; => #<Just [6]> (ap str [“hi” “lo”] [“bye” “woah” “hey”]) ;; => [“hibye” “hiwoah” “hihey” “lobye” “lowoah” “lohey”]

ap is essentially sugar for (apply fapply (pure f) args), but for the common case where you have a pure, uncurried, possibly variadic function.

ap actually desugars in alet form:

(macroexpand-1 `(ap + (just 1) (just2))) ;; => (alet [a1 (just 1) a2 (just 2)] (+ a1 a2))

That way, variadic functions Just Work, without needing to specify an arity separately.

If you’re familiar with Haskell, this is closest to writing “in Applicative style”: you can straightforwardly convert pure function application to effectful application by with some light syntax (<$> and <*> in case of Haskell, and ap here).

See the original Applicative paper for more inspiration: http://staff.city.ac.uk/~ross/papers/Applicative.pdf

Thread like ->, within an applicative idiom.

Compare:

(macroexpand-1 `(-> a b c (d e f))) => (d (c (b a) e f)

with:

(macroexpand-1 `(ap-> a b c (d e f)) => (ap d (ap c (ap b a) e f))

Thread like ->>, within an applicative idiom. See cats.labs.sugar/ap-> for more in-depth discussion.

Like as->, but with monadic binding instead of pure application. See cats.labs.sugar/->= for more in-depth discussion.

Thread like as->, within an applicative idiom. See cats.labs.sugar/ap-> for more in-depth discussion.