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[CLJ-1269] RFC: Anonymous functions interaction with -> and ->> threading macros Created: 28/Sep/13  Updated: 09/Dec/13  Resolved: 09/Dec/13

Status: Closed
Project: Clojure
Component/s: None
Affects Version/s: Release 1.6
Fix Version/s: None

Type: Enhancement Priority: Minor
Reporter: Michael O. Church Assignee: Unassigned
Resolution: Declined Votes: 0
Labels: None


This is more of a starting point for discussion than a feature request. It'd be easy to write and submit the patch, but I want to ask if it's a good idea, since it would alter the semantics of two core macros, '> and '>>; and if it is a good idea, what considerations need to be balanced.

For threading macros, I'd like to special-case forms that begin with 'fn and 'fn*. It's often useful (but maybe a bad idea; that's why I'd like to start the discussion) to use the threading macros in conjunction with anonymous functions in addition to forms, like so (contrived example):

(defn sigmoid [x] (-> x 
                      (+ 1) 
                      #(/ 1 %)))

This won't compile; #(/ 1 %) expands to the form

(fn* [p1__1389#] (/ 1 p1__1389#))

modulo gensym, of course. The threading macro, not treating fn* and fn specially, alters that to:

(fn* (clojure.core/-> (clojure.core/-> (clojure.core/-> x -) Math/exp) (+ 1)) [p1__1417#] (/ 1 p1__1417#))

which is a fn* with a non-symbol (illegal label) before its binding vector, raising an error.

Is this worth "fixing", or are the benefits to small to justify the added complexity of a special case in the ->, ->> threading macros?

Comment by Michael O. Church [ 29/Sep/13 8:52 AM ]

Thanks Alex!

Comment by Gary Fredericks [ 09/Dec/13 7:53 AM ]

Just one of the special cases you'd want to consider is when fn does not mean clojure.core/fn, which is certainly realistic.

Comment by Michael O. Church [ 09/Dec/13 10:03 AM ]

Yes, I realize that.

Looking at the ticket again, I retract my support of this change.

Here's why: (1) that "fix" would alter the semantics of the threading macros, which would break existing code, and (2) there's a really easy way to get anonymous functions into threading macros: just surround the

expression with an extra set of parentheses.

I retract my request for this feature. It's easy enough to do this: instead of,

(-> x f #(g h %))

it's this:

(-> x f (#(g h %)))

Saving 2 characters, like so, is not worth a breaking change IMO.

Comment by Alex Miller [ 09/Dec/13 10:15 AM ]

Retracted by submitter

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