REPL 中的程式設計：導覽命名空間

$ clj
Clojure 1.10.0
user=> *ns*
#object[clojure.lang.Namespace 0x7d1cfb8b "user"]
user=> (ns myapp.foo.bar) ;; creating and switching to the myapp.foo.bar namespace - `ns` will be explained later in this guide.
nil
myapp.foo.bar=> (require '[clojure.set :as cset :refer [union]]) ;; this will only affect the current namespace
nil
myapp.foo.bar=> (cset/union #{1 2} #{2 3})
#{1 3 2}
myapp.foo.bar=> (union #{1 2} #{2 3})
#{1 3 2}
myapp.foo.bar=> (cset/intersection #{1 2} #{2 3})
#{2}
myapp.foo.bar=> (in-ns 'user) ;; now switching back to the `user` namespace - `in-ns` will be explained later in this guide.
#object[clojure.lang.Namespace 0x7d1cfb8b "user"]
user=> (union #{1 2} #{2 3})  ;; won't work, because `union` has not been :refer'ed in the `user` namespace
Syntax error compiling at (REPL:1:1).
Unable to resolve symbol: union in this context
user=> (cset/intersection #{1 2} #{2 3}) ;; won't work, because the `cset` alias has not been defined in the current namespace.
Syntax error compiling at (REPL:1:1).
No such namespace: cset
user=>

$ clj
Clojure 1.10.0
user=> (ns myapp.foo.bar) ;; creating and switching to the `myapp.foo.bar` namespace - NOTE `ns` will be explained later in this guide
nil
myapp.foo.bar=> (def my-favorite-number 42) ;; defining a Var named `my-favorite-number`
#'myapp.foo.bar/my-favorite-number
myapp.foo.bar=> my-favorite-number
42
myapp.foo.bar=> (ns myapp.baz) ;; creating and switching to another namespace `myapp.baz`
nil
myapp.baz=> myapp.foo.bar/my-favorite-number ;; referring to `my-favorite-number`
42
myapp.baz=> (require '[myapp.foo.bar :as foobar]) ;; we can also use an alias to make it shorter
nil
myapp.baz=> foobar/my-favorite-number
42

$ clj
Clojure 1.10.0
user=> (ns myapp.foo-bar)
nil
myapp.foo-bar=> *ns*
#object[clojure.lang.Namespace 0xacdb094 "myapp.foo-bar"]
myapp.foo-bar=> (def x 42)
#'myapp.foo-bar/x

$ clj
Clojure 1.10.0
user=> (ns myapp.ns1) ;; creating a new namespace and defining a Var `x` and an alias `str/`:
nil
myapp.ns1=> (def x 42)
#'myapp.ns1/x
myapp.ns1=> x
42
myapp.ns1=> (require '[clojure.string :as str])
nil
myapp.ns1=> (str/upper-case "hello")
"HELLO"
myapp.ns1=> (ns myapp.ns2) ;; now switching to another namespace:
nil
myapp.ns2=> x ;; won't work, because x has not been defined in namespace `myapp.ns2`
Syntax error compiling at (REPL:0:0).
Unable to resolve symbol: x in this context
myapp.ns2=> (str/upper-case "hello") ;; won't work, because alias `str` has not been defined in namespace `myapp.ns2`
Syntax error compiling at (REPL:1:1).
No such namespace: str

$ clj
Clojure 1.10.0
user=> (ns myapp.some-ns) ;;;; creating the namespace `myapp.some-ns`
nil
myapp.some-ns=> *ns* ;; where are we?
#object[clojure.lang.Namespace 0xacdb094 "myapp.some-ns"]
myapp.some-ns=> (def x 42) ;; defining `x`
#'myapp.some-ns/x
myapp.some-ns=> (in-ns 'user) ;;;; switching back to `user`
#object[clojure.lang.Namespace 0x4b45dcb8 "user"]
user=> *ns* ;; where are we?
#object[clojure.lang.Namespace 0x4b45dcb8 "user"]
user=> (in-ns 'myapp.some-ns) ;;;; ...switching back again to `myapp.some-ns`
#object[clojure.lang.Namespace 0xacdb094 "myapp.some-ns"]
myapp.some-ns=> *ns* ;; where are we?
#object[clojure.lang.Namespace 0xacdb094 "myapp.some-ns"]
myapp.some-ns=> x ;; `x` is still here!
42

$ clj
Clojure 1.10.0
user=> (in-ns 'myapp.never-created)
#object[clojure.lang.Namespace 0x22356acd "myapp.never-created"]
myapp.never-created=> (defn say-hello [x] (println "Hello, " x "!"))
Syntax error compiling at (REPL:1:1).
Unable to resolve symbol: defn in this context

myapp.never-created=> (clojure.core/refer-clojure)
nil
myapp.never-created=> (defn say-hello [x] (println "Hello, " x "!"))
#'myapp.never-created/say-hello
myapp.never-created=> (say-hello "Jane")
Hello,  Jane !
nil