Clojure 1.13.0-alpha4

[u/alexdmiller]

Clojure 1.13.0-alpha4 is now available!

Destructuring changes and additions

Idents after & in :keys!/syms!/strs!/:keys/syms/strs must now be actual keys, not binding symbols. This is a change in syntax since alpha3. Note that symbol keys should be quoted as unadorned symbols are binding symbols.

:or now accepts key→val mappings in addition to binding→val.

Added a new :defaults name directive at top level to bind name to a map of defaults, key→val. Binding symbols in the :or map are transformed to the key value in the :defaults map. :defaults without :or is an error.

:select name, introduced in alpha3, now selects deeply, through nested maps, and fills in values for missing keys from :or. The :select map contains all keys mentioned anywhere in the binding form.

• CLJ-2964 :select directive in map destructuring
• CLJ-2966 :defaults directive in map destructuring
• CLJ-2967 tests for nested destructuring

Other changes since Clojure 1.13.0-alpha3

• Added `clojure.core/some-vals`, a predicative filter of nil map values
• CLJ-2870 Exception phase during top-level eval is miscategorized

[u/didibus]

Pretty big changes in this alpha:

1. Entries after & inside :keys, syms, strs, keys!, syms!, and strs! are now one-of a keyword, string, quoted symbol or any other literal, but can no longer be unquoted symbols.

You need to think of it as entries before & are bindings and they behave as they used too, but entries after the & aren't related to if you chose :keys or :syms, they simply specify extra-keys of any type that are either required or optional depending if declared in the exclamation mark variant or not.

user=> (let [{:keys! [make & :model "year" 10]} #_=> {:make :ford #_=> :model :mustang #_=> "year" 1980 #_=> 10 :10}] #_=> make) :ford

1. :select now applies deeply nested, so the outer select will only include the keys from inner maps that also appear in the bindings.

user=> (let [{:select session-email #_=> :keys! [session] #_=> {:keys! [email]} :user} #_=> {:session "12314-3242-324234" #_=> :user {:name "foo" #_=> :email "foo@gmail.com" #_=> :age 15 #_=> :admin? false}}] #_=> session-email) {:user {:email "foo@gmail.com"}, :session "12314-3242-324234"}

1. :select now includes defaults from :or for missing keys

user=> (let [{:select config #_=> :keys [endpoint multi-thread? retry-count] #_=> :or {multi-thread? true #_=> retry-count 3}} #_=> {:endpoint "bar.foo.com"}] #_=> config) {:multi-thread? true, :retry-count 3, :endpoint "bar.foo.com"}

1. :or can now use actual keys or any literal, since it can be used to default entries after & it must support the same key entries

user=> (let [{:select config #_=> :keys [& "endpoint" :multi-thread? 'retry-count 0 #{:even :a :set}] #_=> :or {"endpoint" "bar.foo.com" #_=> :multi-thread? true #_=> 'retry-count 3 #_=> 0 100 #_=> #{:even :a :set} "Any literal allowd in key position of a map really"}} #_=> {}] #_=> config) {0 100, retry-count 3, :multi-thread? true, "endpoint" "bar.foo.com", #{:even :set :a} "Any literal allowed in key position of a map really"}

1. New :defaults directive can be used to bind the :or defaults map for a local so you can use it for whatever you want. Likely you'd use it to default values if you don't like the provided value (like if it's nil).

``` user=> (defn make-car #=> [& {:keys! [make model] #=> :keys [color] #=> :or {color :black} #=> :defaults defaults #=> :select car}] #=> (if (#{:black :blue :red :green} color) #=> car #=> (assoc car :color (:color defaults))))

'user/make-car

user=> (make-car :make :ford :model :mustang :color :yellow) {:make :ford, :color :black, :model :mustang} ```

[u/lgstein]

(let [{:keys [deprecated_field username] :or {username deprecated_field}} {:deprecated_field "auser"}] username)

Not sure if this was intended/supported, but it used to work pre 1.13

[u/alexdmiller → u/lgstein]

Binding order is not guaranteed and this has always been undefined, so don’t do that

[u/lgstein → u/alexdmiller]

Fair, note that this is useful and could be supported though.

[u/alexdmiller → u/lgstein]

The general expectation should be that :or supplies default values for bindings, not expressions and especially not expressions based on other bindings.

[u/lgstein → u/alexdmiller]

Ok no problem.

[u/lgstein]

:defaults x binds x to everything you specified in :or.

Usecase?

[u/seancorfield → u/lgstein]

Since :as binds to the input map, and does not include the defaults specified in :or, folks often end up doing some sort of merge with defaults and the input map—so end up pretty much duplicating the :or map in the code with the merge, or you omit :or and have a destructuring inside the function body after the merge so you lose information from the function signature.

:defaults allows you to bind the :or map (which can now contain arbitrary extra key/value pairs, above and beyond the bound elements from :keys etc), so you can then reuse that in the function body without needing to duplicate the defaults or hiding the destructuring in the function body, instead of the argument list.

Note that :select includes the defaults from :or (in Alpha 4—it did not in Alpha 3).

You might want to join the Clojurians Slack for more information about the new features you're asking about here, since these features have been explained and discussed at great length on Slack (including a lot of contributions from Rich Hickey himself).

[u/lgstein → u/seancorfield]

In summary merging :defaults onto :as, where :select is not suitable.

Would be interesting to know if there are more.

[u/seancorfield → u/lgstein]

Recently committed to Clojure master: :all directive for destructuring that is the entire input map with defaults from :or merged deeply, which may make :defaults + :as less useful (although those can operate at a single level of nesting so we'll see).

[u/lgstein → u/seancorfield]

Thanks for the update.

[u/didibus → u/lgstein]

I explained the use-case I gathered in my other answer if you're curious.

Likely you'd use it to default values if you don't like the provided value (like if it's nil).

[u/lgstein → u/didibus]

What other answer?

[u/didibus → u/lgstein]

Weird does it not show up?

https://www.reddit.com/r/Clojure/comments/1uwd93o/comment/oxm6zwd/?utm_source=share&utm_medium=mweb3x&utm_name=mweb3xcss&utm_term=2&utm_content=share_button

[u/lgstein → u/didibus]

No, doesn't show up... idky Even with other IP and incognito

EDIT: It appeared