Clojure
“Lisp Reloaded”
Versions of LISP
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Lisp is an old language with many variants
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LISP is an acronym for List Processing language
Lisp is alive and well today
Most modern versions are based on Common Lisp
Scheme is one of the major variants
Racket is the latest version of Scheme
Clojure is the latest in a long line of dialects
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Clojure uses a new approach to concurrency, Software
Transactional Memory (STM)
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Lisp syntax vs. Clojure syntax
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Basic data types I
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Numbers: Clojure has integers, floating point numbers, and ratios
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Strings are enclosed in double quotes, and can contain escaped characters
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Within a string, \n represents a newline, as usual
Character literals are written as \c to indicate the character c
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Integers include decimal integers (255), octals numbers (012), hexadecimal
numbers (0xff) and radix numbers (2r1111), where the radix is an integer
between 2 and 36, inclusive
Floating point numbers include standard notation (3.1416) and scientific
notation(1.35e-12).
Ratios are “fractions” such as 1/3; they are not subject to roundoff error
Since \n represents the character n, \newline is used to represent a newline
Keywords are not reserved words, but are like “atoms” in Prolog or Erlang; a
keyword begins with a colon (:foo) and stands for itself
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Basic data types II
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A list is a sequence of values enclosed in parentheses, for example,
(one 2 "buckle my shoe")
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Elements of the list are separated by whitespace or commas
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A list represents either data or a function call, depending on context
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A vector is a sequence of values enclosed in brackets, for example,
[one 2 "buckle my shoe”]
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A map or hash is a sequence of key/value pairs, enclosed in braces, for
example,
{:ace 1, :deuce 2, "trey" 3}
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Elements of the vector are separated by whitespace or commas
Elements are separated by whitespace or commas
It is helpful to use commas between key/value pairs
A set is a pound sign (#) followed sequence of values enclosed in braces, for
example #{a b c}
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Functions
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The syntax to define a named function is:
(defn function_name docstring? [arguments]
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The docstring after the function name is optional
The value of the function is the value of the last expression evaluated
The syntax of a function call is
(function_name arguments)
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expressions)
Notice that the name of the function being called is the first thing inside the
parentheses
The syntax of an anonymous function, or lambda, is
(fn [arguments] expressions)
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This syntax can be used in place of a function name
Example: ((fn [lst] (first lst)) my-list) is equivalent to
(first my-list)
Alternative syntax: #(expressions), where %1 (or just %), %2, %3, … stand for
parameters
Since functions are first-class values, this syntax is convenient for creating a
function to be used as an argument to another function
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Sequences
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Lists, vectors, maps, and sets are all “sequences,” and many of the same
operations apply
(first seq) returns the first element of seq
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If the seq is empty, the empty list, (), is returned
(rest seq) returns the seq without its first element
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If the seq is empty, the empty list, (), is returned
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(cons value seq) returns a sequence of the same type with the value
inserted as the new first element
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Some sequences may be lazy; the elements of the sequence are not actually
computed until they are used
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This allows Clojure to have infinite sequences, for example, “all positive integers”
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Fast and slow operations
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Sequence is an interface roughly analogous to Java’s Iterator:
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It is a small interface (a couple of methods) that can be applied to a large range of data types
Many (most?) of the common Clojure operations are defined on sequences
For example, map and reduce work across almost all data types.
Clojure.org describes sequences as “logical lists”
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The primary sequence operations are first, rest and cons
Everything else is built around these operations.
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The set of cheap (constant time) operations are the same as for lists:
Add/remove/access the first element or step to the next element
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Operations like nth and last are expensive (O(n) time) since they must
traverse the entire list.
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count -- getting the length of a sequence -- is also expensive.
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Sequence operations
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Sequence operations can be used on almost any compound type
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Collections: Vector, list, map, set, struct
Strings (as char sequence)
Java arrays, collections, sets, maps, iterator, enumerable
IO streams
Producer functions: Many lazy seqs do not get their values from a backing data set,
but from a function that calculates the value when asked for.
Some collections can be used directly as sequences, others need
to be converted
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The rules are somewhat different for different kinds of collections, so the simplest
general strategy is to
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convert to sequence, using seq
do the processing
if you need a particular non-sequence type (like vector or array), convert the return
value back
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Predicates
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A predicate (in any computer language) is a function that
returns either “true” or “false”
In Clojure,
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“false” is one of the atoms false or nil
“true” is the atom true
In addition, anything that isn’t false or nil is considered to be
true
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In particular, the empty list, (), is “true”
Hence, a predicate returns either false, nil, or any other
value to mean “true”
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Predicates often return “true” values other than true, especially if
the returned value might be useful
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Function calls and data
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A function call is written as a list
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Example: (println "Hello" "World")
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The first element is the name of the function
Remaining elements are the arguments
Calls function println with arguments "Hello" and "World"
The function prints Hello World and returns nil
Data is written as atoms or lists
Example: (println "Hello" "World") is a list of three
elements
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Quoting
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Is (f a b) a call to f, or is it just data?
Lists and atoms must be quoted
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x indicates the variable x, while (quote x) is just the atom x
(quote (f a b)) is the list (f a b)
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Exceptions include nil, true, and nil, which do not need to be quoted
quote is not a function, but a special form
The special form gets the unevaluated arguments, and is control of when or
whether to evaluate them
Special forms, unlike functions, are not first-class values
'(f a b) is a shorthand way of doing the same thing
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There is just one single quote at the beginning
It quotes one expression
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Examples
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(defn my-first [x]
(first x))
user=> (my-first '(1 2 3))
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defn my-second [lst]
(first (rest lst)))
user=> (my-second '(a b c))
b
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(defn my-third
"This is a doc comment"
[lst]
(first (rest (rest lst)))
)
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user=> (my-third
"Whatever")
\a
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user=> (doc my-third)
------------------------user/my-third
([lst])
This is a doc comment
nil
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Arithmetic
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+
*
/
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Returns the sum of its arguments; (+) returns 0
Subtracts the rest of the numbers from the first number
Returns the product of its arguments;(*) returns 1.
Divides the rest of the numbers into the first number
If operands are integers, result is a ratio
If only one number, returns its inverse
quot Returns the quot[ient] of integer division of the first number by the rest
of the numbers
rem remainder of dividing the first number by the second
mod Modulus of first and second number; truncates toward negative infinity
inc Returns a number one greater than its argument
dec Returns a number one less than its argument
max Returns the largest of its arguments
min Returns the smallest of its arguments
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Numeric comparisons
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=
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Returns true if all arguments are equal
This is a true equality test, not an identity test
Collections are compared in a type-independent manner; for example,
(= '(1 2 3) [1 2 3]) returns true
== Returns true if numeric arguments all have the same value, otherwise
false
not= Same as (not (= obj1 obj2))
<
Returns true if numeric arguments are in monotonically increasing order
>
Returns true if numeric arguments are in monotonically decreasing order,
otherwise false
<= Returns true if numeric arguments are in monotonically non-decreasing
order, otherwise false
>= Returns true if numeric arguments are in monotonically non-increasing
order, otherwise false
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Conditional execution
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if is a special form that takes exactly three arguments:
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A predicate
An expression to evaluate if the predicate is true
An expression to evaluate if the predicate is false
Syntax: (if
Example:
predicate
true-branch false-branch)
(defn sum [lst]
(if (= lst ())
0
(+ (first lst) (sum (rest lst))) ) )
(defn average [lst]
(/ (sum lst) (count lst)) )
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The problem with if
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if expressions nest rather awkwardly
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(defn collatz [n]
(println n)
(if (= n 1)
1
(if (= (rem n 2) 0)
(collatz (/ n 2))
(collatz (inc (* 3 n))) ) ) )
In most cases, the more general cond is preferred
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cond
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cond implements the
if...then...elseif...then...elseif...then...
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control structure
Like if, cond is a special form, not a function
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That is, the arguments to cond are not evaluated before cond
is called; rather, cond evaluates the arguments as it pleases
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Syntax of the cond
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The syntax of the cond special form is:
(cond condition result condition result ...)
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Example:
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(defn pos-neg-or-zero
"Determines whether n is positive, negative, or zero"
[n]
(cond
(< n 0) "negative"
(> n 0) "positive"
:else "zero" ) )
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The last condition, :else, is true, and reads better than the atom true
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Rules for Recursion
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Handle the base (“simplest”) cases first
Recur only with a “simpler” case
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“Simpler” = more like the base case
Don’t alter global variables
Don’t look down into the recursion
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Example: member
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As an example we define member, to test membership
in a list
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(defn member [a lat]
(cond
(empty? lat) false
(= a (first lat)) true
:else (member a (rest lat)) ) )
user=> (member :b '(:a :b :c))
true
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Guidelines for recursive Clojure functions
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Unless the function is trivial, use a cond
Handle the base case(s) first
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Avoid having more than one base case
The base case is often testing for an empty list
Do something with the first and recur with the rest
Use tail recursion wherever possible
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(To be explained later)
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Example: union
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(defn union [set1 set2]
(cond
(empty? set1) set2
(member (first set1) set2)
(union (rest set1) set2)
:else (cons (first set1)
(union (rest set1) set2)) ) )
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This example uses the previously defined member function
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It appears that, if function A calls function B, Clojure requires function B
to be defined before defining function A
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Of all the languages I have used, only C and its variants make this
requirement
Unless I’m missing something, this is a real step backward for Lisp dialects
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Tests
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The following can be used with any type of argument
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nil?
identical?
Returns true if its argument is nil, false otherwise.
Tests if its two arguments are the same object
The following will throw an exception if given a non-numeric argument
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zero?
pos?
neg?
even?
odd?
Returns true if its argument is zero, else false
Returns true if its argument is greater than zero
Returns true if its argument is less than zero
Returns true if its argument is even
Returns true if its argument is odd
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Type tests
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coll?
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seq?
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vector?
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list?
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map?
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set?
Returns true if x implements IPersistentCollection
Return true if x implements ISeq
Return true if x implements IPersistentVector
Returns true if x implements IPersistentList
Return true if x implements IPersistentMap
Returns true if x implements IPersistentSet
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Content Tests
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(contains? map key)
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Returns true if key is present in the map
(distinct? args)
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Returns true if no two of the arguments are =
(empty? collection)
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Returns true if collection has no items - same as (not (seq coll))
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Clojure programmers use the idiom (seq x) rather than (not (empty? x))
(every? predicate collection)
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Returns true if (predicate x) is true for every x in collection
(not-every? predicate collection)
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Returns false if (predicate x) is true for every x in collection
(not-any? predicate collection)
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Returns false if (predicate x) is true for any x in collection
(some predicate collection)
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Returns the first value in the collection for which the predicate is true
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I/O
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*in*
*out*
*err*
print, pr
printf
println
prn
newline
read-line
slurp
spit
A java.io.Reader object representing standard input
A java.io.Writer object representing standard output
A java.io.Writer object representing standard error
Prints the object(s) to the *out* output stream, separated by spaces
Prints formatted output, according to a format
Same as print followed by newline
Same as pr followed by newline. Observes *flush-on-newline*
Writes a newline to the output stream *out*
Reads the next line from stream *in*
Reads the file into a string and returns it
Opposite of slurp. Opens file with writer, writes content, then closes it
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Dealing with side effects
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Functions compute values; but printing is a side effect
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The value returned by println is nil—not too useful
You can put multiple expressions in a defn
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The result of calling the function is the result of evaluating the last expression
(defn five []
(println "Just five")
5 )
(five)
Just five
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You can also use do; the result is the value of the last expression
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(do (println "one" "two")
(+ 1 2) )
one two
3
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REPL commands
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(load-file filename) loads a file containing Clojure functions
*1 bound in a repl thread to the most recent value printed
*2 bound in a repl thread to the second most recent value printed
*3 bound in a repl thread to the third most recent value printed
*e bound in a repl thread to the most recent exception caught by the repl
*print-dup*
When set to logical true, objects will be printed in a
way that preserves their type when read in later
*print-length* controls how many items of each collection the print
*print-level*
controls how many levels deep the printer will print
*print-meta*
If set to logical true, when printing an object, its
metadata will also be printed in a form that can be read back by the reader
*print-readably* When set to logical false, strings and characters will be
printed with non-alphanumeric characters converted to the appropriate escape
sequences
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Dealing with the REPL
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The last time I used Clojure’s REPL, it seemed to be very buggy
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I got a lot of errors along these lines:
java.lang.Exception: Unable to resolve symbol: union
in this context (NO_SOURCE_FILE:43)
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Here are two workarounds that seem to solve most such problems:
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(It’s a Java exception because Clojure is compiled to the JVM)
Start a new Clojure shell, or
Just copy the function definition and paste it directly into the Clojure shell
That was a year ago; the new version may be better
You might also try out clooj, which is a very simple Clojure IDE
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https://github.com/arthuredelstein/clooj
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The End
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