Clojure

Quoting from a post I wrote on the clojure-dev ML:

• I really don't like the idea of special-casing `_` here, users are already confused about idioms like `[.. & _]` thinking that `_` is some special token/variable. Making it an actual special token in some occasion wouldn't help.
• I also don't like how we're making the single `:` auto-qualify keywords when used within the context of a qualifying-map. Auto-qualifying keywords has always been the job of the double `::`, changing this would introduce (IMO) needless cognitive overhead.
• The current impl treats `#:foo{'bar 1}` and `'#:foo{bar 1}` differently. I can see why is that, but the difference might be highly unintuitive to some.
• The current proposal makes it feel like quote is auto-qualifying symbols , when that has always been the job of syntax-quote. I know that's not correct, but that's how it's perceived.

Here's an alternative syntax proposal that handles all those issues:

• No #::, only #:foo or #::foo
• No auto-resolution of symbols when the namespaced-map is quoted, only when syntax-quoted
• No special-casing of `_`
• No auto-resolution of single-colon keywords

Here's how the examples in the ticket description would look:

#:person{::first "Han", ::last "Solo", ::ship #:ship{::name "Millenium Falcon", ::model "YT-1300f light freighter"}}
;=> {:person/first "Han" :person/last "Solo" :person/ship {:ship/name "Millenium Falcon" :ship/model "YT-1300f light freighter"}}

#:foo{::kw 1, :n/kw 2, :bare 3, 0 4}
;=> {:foo/kw 1, :n/kw 2, :bare 3, 0 4}

{::kw 1, :n/kw 2, bare 3, 0 4}
;=> {:user/kw 1, :n/kw 2, :bare 3, 0 4}

Note in the previous example how we don't need `#::` at all – `::` already does that job for us

(require '[clojure.string :as s])
#::s{::kw 1, :n/kw 2, bare 3, 0 4}
;=> {:clojure.string/kw 1, :n/kw 2, :bare 3, 0 4}

`{a 1, n/b 2, ~'c 3}
;=> {user/a 1, n/b 2, c 3}

Again, no need for `#::` here, we can just rely on the existing auto-qualifying behaviour of `.

`#:foo{a 1, n/b 2}
;=> {foo/a 1, n/b 2}

I think this would be more consistent with the existing behaviour – it's basically just making `#:foo` or `#::foo` mean: in the top-level keys of the following map expression, resolve keywords/symbols as if ns was bound to `foo`, rather than introducing new resolution rules and special tokens.

I realize that this proposal wouldn't work with EDNReader as-is, given its lack of support for `::` and "`". I don't have a solution to that other than "let's just bite the bullet and implement them there too", but maybe that's not acceptable.

Nicola, thanks for the proposal, we talked through it. We share your dislike for :_/kw syntax and you should consider that a placeholder for this behavior for the moment - it may be removed or replaced before we get to a published release.

For the rest of it:

• requiring syntax quote is a non-starter
• supporting a mixture of default ns and the current ns is important and this is not possible with your proposal. Like #:foo{:bar 1 ::baz 2}.
• there is a lot of value to changing the scope of a map without modifying the contents, which is an advantage of the syntax in the ticket

Why restrict this feature to a single namespace? (this doesn't preclude a shorthand for the single mapping) I'd like to locally define aliases (and default ns).

We already have namespace level aliases. You can use :: in the map to leverage those aliases (independently from the default ns):

(ns app 
  (:require [my.domain :as d]
            [your.domain :as y]))

#::{:svc 1, ::d/name 2, ::y/name 3}

;;=> {:app/svc 1, :my.domain/name 2, :your.domain/y 3}

Alex, if existing namespace level aliases are enough when there's more than one namespace used in the key set I fail to understand the real value of this proposal.

Okay I'm lying a little: there are no aliases in edn, so this would bring aliases to edn (and allows printers to factor/alias namespaces out). And for Clojure code you can't define an alias to a non-existing namespace – and I believe that this implementation wouldn't check namespace existence when resolving the default ns #:person{:name}.

Still my points hold for edn (and that's where the value of this proposal seems to be): why not allows local aliases too?

#:person #:employee/e {:name "John Smith", :e/eid "012345"}
;=> {:person/name "John Smith", :employee/eid "012345"}

I have another couple of questions:

• should it apply to other datatypes?
• should it be transitive?

New patch rev supports spaces between the namespace part #:foo and the map in both LispReader and EdnReader.

Any news on the flag to prevent this behavior (mentioned in the screener notes)?

I ask because (pr-str) in Clojure 1.9.0-alpha12 currently produces EDN that can't be consumed by (cljs.reader/read-string) in ClojureScript 1.9.229 (the ClojureScript consumption side is tracked in CLJS-1706).

It would be nice to have a way to disable this behavior until ClojureScript reaches parity, and a lot of fairly widely-used open-source libraries use plain (pr-str) to generate EDN output - for example, ring-middleware-format.

Yes, that was implemented in a separate ticket (CLJ-1993) in Clojure 1.9.0-alpha11. There is now a print-namespace-maps dynvar. It defaults to false, but is set to true in the standard REPL.

So at the REPL you can (set! print-namespace-maps false) to turn it off.

If you're doing stuff in your program outside a REPL, it will be off by default so you probably don't need to do anything as of alpha11.

that's *print-namespace-maps* above, sorry for the formatting

Any plans to extend this to vectors? Would be useful in a number of contexts:

(s/keys :req #:foo[:a :b])
(select-keys m #:foo[:a :b])
(get-in m #:foo[:a :b])

We've considered extending it to vectors and that might be a future addition. One thing with that is it drives the question of whether the namespace syntax should be applicable to all collections and the syntax clashes with the set literal syntax is pretty ugly: #:foo#{:a :b}.

How do I bind *print-namespace-maps* in REPL?

with-bindings hardcodes it to true. Looks like an ugly exception, as all the other bindings pick the root value there.

If you're in a clojure.main repl, as you note it's already bound, so you can just use `set!`:

user=> (set! *print-namespace-maps* false)
false
user=> {:a/b 1}
{:a/b 1}

This won't work in nrepl-based repls that don't set it though, so you can also use `binding`:

user=> (binding [*print-namespace-maps* false] (println {:a/b 1}))
{:a/b 1}

Thanks for the clarification. It's NREPL issue then. I can happily set! other globals there but not this one.

Is namespace that starts with number not allowed in namespaced maps reader macros in clojure 1.9?

user=> {:3/a 1}
#:3{:a 1}

user=> #:3{:a 1}
RuntimeException Namespaced map must specify a valid namespace: 3 clojure.lang.Util.runtimeException (Util.java:221)

I'm using clojure 1.9.0.

Although the same reader macro #:3{:a 1} is accepted in clojurescript/1.9.946

Clearly from your example where Clojure 1.9.0 throws an exception when attempting to read the input #:3{:a 1}, it does not accept a namespace that starts with a number.

Perhaps your question is: "Is this a bug in Clojure 1.9.0, or intended behavior?"

Chenyu Li, you may want to consider creating a new JIRA issue with that question, since this ticket is already closed as resolved, and such a question in a comment here is more likely to go unnoticed.

Thanks Andy. Created an issue here per your suggestion: https://dev.clojure.org/jira/browse/CLJ-2315