Hello, interpolation!

You may have noticed in Command-line Clojure that we glued the string together in pieces

(println (str "Hello, " name "!"))

What are we, Neanderthals? There is a string format in Clojure, but that’s not much better

(println (format "Hello, %s!" name))

Doesn’t Clojure have interpolation? Apparently not, but Chas Emerick wrote a macro for it some years ago. According to that, we should be able to use

(println (<< "Hello, ~{name}!"))

instead. The “<<” is a macro that enables variable interpolation in the string with “~{}”. It does a whole lot more too. Nice!

So how do we use it? It’s not in the core of Clojure yet, but it has been moved into something called the core.incubator. It says there we just need to

Add a dependency on core.incubator to your Leinigen project.clj:

[org.clojure/core.incubator “0.1.4”]

So if we create a new project

$ lein new hello-world
$ cd hello-world

and then add that dependency

$ cat project.clj 
(defproject hello-world "0.1.0-SNAPSHOT"
  :description "Hello interpolation"
  :dependencies [[org.clojure/clojure "1.8.0"]
                 [org.clojure/core.incubator "0.1.4"]])

we should be able to use the macro.

$ cat src/hello_world/core.clj 
(ns hello-world.core)

(defn hello
  ([]
   (hello "World"))
  ([name]
   (<< "Hello, ~{name}!")))

But that doesn’t work. We still need to add the namespace. And not clojure.core.incubator as we might expect, but clojure.core.strint!

(ns hello-world.core
  (:use [clojure.core.strint]))

(defn hello
  ([]
   (hello "World"))
  ([name]
   (<< "Hello, ~{name}!")))

Ah, interpolation! Sweet, sweet, interpolation!

$ rlwrap lein repl
user=> (use 'hello-world.core)
user=> (hello)
"Hello, World!"
user=> (hello "Hank")
"Hello, Hank!"

I do hope they add the “<<” macro to the core soon!

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Hello, interpolation!

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