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[CLJ-200] Extend cond to support inline let, much like for Created: 18/Oct/09  Updated: 01/Jul/16  Resolved: 01/Jul/16

Status: Resolved
Project: Clojure
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Enhancement Priority: Major
Reporter: Anonymous Assignee: Mark Engelberg
Resolution: Declined Votes: 11
Labels: None

Attachments: File clj-200-cond-let-clauses-fixed-test-v2.diff     Text File clj-200-cond-let-clauses-fixed-test-v2-patch.txt    
Patch: Code and Test


I find it occasionally very useful to do a few tests in a cond, then introduce some new symbols (for both clarity and efficiency) that can be referenced in later tests (or matching expressions). This parallels similar functionality inside the for macro, where the :let keyword is matched against a vector of symbol bindings and forms an implicit let around the remainder of the comprehension.

I'll be adding a patch for this shortly.

Comment by Assembla Importer [ 24/Aug/10 1:51 PM ]

Converted from http://www.assembla.com/spaces/clojure/tickets/200

Comment by Assembla Importer [ 24/Aug/10 1:51 PM ]

hlship said: Trickier than I thought because cond is really wired into other fundamentals, like let.

Comment by Assembla Importer [ 24/Aug/10 1:51 PM ]

cgrand said: Howard, what do you think of http://gist.github.com/432712 ?

Comment by Mark Engelberg [ 23/Nov/12 2:33 AM ]

Patch cond-let-clauses.diff on 23/Nov/12 adds inline :let clauses to cond, implementing CLJ-200. The code is based off of code by cgrand, with some tweaks so the implementation only relies on constructs defined earlier in core.clj, since when cond is defined, things aren't yet fully bootstrapped. Also added a test to control.clj.

Comment by Christophe Grand [ 23/Nov/12 3:06 AM ]

Some comments: the docstring is missing, I believe you don't have to modify the original cond (except the docstring maybe), just redefine it later on once most of the language is defined – a bit like what is done for let for example.

There is still the unlikely eventuality that some code uses :let as :else. What about shipping a cond which complains on keywords (in test position) other than :else?

Comment by Mark Engelberg [ 23/Nov/12 3:47 AM ]

cond-let-clauses-with-docstring.diff contains the same patches as cond-let-clauses, but includes the original docstring for cond along with an additional sentence about the :let bindings.

Comment by Mark Engelberg [ 23/Nov/12 3:54 AM ]

Cgrand, I did see your example of redefining cond after most of the language is defined, but since I was able to figure out how to do it in the proper place, that makes the :let bindings available for users of cond downstream and avoids any unforeseen complications that might come from rebinding.

As for your other point, I think it is highly improbable that someone would have used :let in the :else position. However I can imagine someone intentionally using something like :true or :default. I think the idea of warning for other keywords is actually more likely to cause complications than the unlikely problem it is meant to solve.

I did resubmit the patch with the docstring restored. Thanks for pointing out that problem. I'm excited about this patch – I use :let bindings within the cond in my own code all the time. Thanks again for the blog post that started me on that path.

Comment by Christophe Grand [ 23/Nov/12 4:13 AM ]

True, it's :unlikely for :let to happen.
However once :let is officially blessed, it may be better to provision for future other "special" keywords and thus to warn on "unsupported" keywords. Plus it will help out-of-order typists (like myself) to catch earlier a :elt instead of a :let
This is only my point of view. Thanks for trying to get :let in cond supported.

Comment by Andy Fingerhut [ 29/Nov/12 8:46 PM ]

Mark, could you remove the obsolete earlier patch now that you have added the one with the doc string? Instructions for removing patches are under the heading "Removing Patches" on this page: http://dev.clojure.org/display/design/JIRA+workflow

Comment by Mark Engelberg [ 29/Nov/12 10:50 PM ]


Comment by Andy Fingerhut [ 30/Nov/12 1:24 AM ]

I haven't figured out what is going wrong yet. I can apply the patch cond-let-clauses-with-docstring.diff to the latest Clojure master just fine. I can do "ant jar" and it will build a jar. When I do "ant", it fails with the new test for cond with :let, throwing a StackOverflowException. I can enter that same form into the REPL and it evaluates just as the test says it should. I can comment out that new test and all of the rest pass. But the new test doesn't pass when inside of the control.clj file. Anyone know why?

Comment by Christophe Grand [ 30/Nov/12 4:54 AM ]

It's because of the brutal replacement performed by test/are: the placeholders for this are form are x and y but in Mark's test there are used as local names and are tries to substitute them recursively...
If one changes the local names to a and b for example it works.

Comment by Mark Engelberg [ 02/Dec/12 8:20 AM ]

cond-let-clauses-fixed-test.diff on 02/Dec/12 contains the same patch, but with the x,y locals in the test case changed to a,b so that it works properly in the are clause which uses x and y.

Comment by Mark Engelberg [ 02/Dec/12 8:27 AM ]

On Windows, I can't get Clojure's test suite task to work, either via ant or maven, which has made it difficult for me to verify the part of the patch that applies to the test suite works as expected; I had tested it as best I could in the REPL, using a version of Clojure built with the patch applied, but using this process, I missed the subtle interaction between are and the locals in the test case. Sorry about that. If someone can double-check that the test suite task now works with the newest patch, that would be great, and then I'll go ahead and remove the obsoleted patch. Thanks.

Comment by Andy Fingerhut [ 02/Dec/12 6:29 PM ]

clj-200-cond-let-clauses-fixed-test-v2-patch.txt dated Dec 2 2012 is identical to Mark Engelberg's cond-let-clauses-fixed-test.diff of the same date, except it applies cleanly to the latest Clojure master.

I've verified that it compiles and passes all tests with latest Clojure master as of this date.

Mark, I've made sure to keep your name in the patch, since you wrote it. You should be able to remove your two attachments now, so the screener won't be confused which patch should be examined.

Comment by Andy Fingerhut [ 02/Dec/12 6:31 PM ]

Mark, besides general issues with Windows not being used much (or maybe not at all?) by Clojure developers, there is the issue right now filed as CLJ-1076 that not all tests pass when run on Windows due to CR-LF line ending differences that cause several Clojure tests to fail, regardless of whether you use ant or maven to run them.

Comment by Andy Fingerhut [ 22/Oct/13 8:01 PM ]

clj-200-cond-let-clauses-fixed-test-v2.diff is identical to earlier patch clj-200-cond-let-clauses-fixed-test-v2-patch.txt, except it removes unnecessarily trailing whitespace that causes warnings when applying the patch, and with .diff suffix for easier diff-style viewing in some editors.

Comment by Mark Engelberg [ 01/Jul/16 7:09 PM ]

It was just brought to my attention that this issue was declined today.

FWIW, earlier today I wrote up a Rationale about this issue as part of my effort to address it temporarily at the library level. I hope you will take a look:


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