Macrobrew: Clojure macros distilled

Macrobrew: Clojure macros
distilled
Abhinav Omprakash
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Macros look similar to functions
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Arguments passed to the macro
are not evaluated
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(defn func [x]
(println "inside the body")
x)
=>(func (println "hey!"))
hey!
inside the body
nil
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(defmacro mac [x]
(println "inside the body")
x)
=>(mac (println "hey!"))
inside the body
hey!
nil
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Data returned by the macro is
evaluated
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(defn foo []
(list 1 2 3))
=>(foo)
(1 2 3)
=>(eval (foo))
; Execution error (ClassCastException); class
;java.lang.Long cannot be cast to class
clojure.lang.IFn
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(defmacro foo []
(list 1 2 3))
=>(foo)
; Execution error (ClassCastException
; class java.lang.Long cannot be cast to class
clojure.lang.Ifn
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=>(macroexpand '(foo))
(1 2 3)
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Double Evaluation
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What does a lisper do?
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(func arg1 arg2)
‘(1 2 3)
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user=> (count '(func arg1 arg2))
3
user=> (count '(1 2 3))
3
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In lisp, we create lists.
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The origin of the mighty macro
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Symbols & Values
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(def one 7)
user=> one
7
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user=> 'one
one
user=> `one
user/one
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(def one 7)
user=> (eval 'one)
7
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Symbols can be assigned other
symbols
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(def two ‘one)
user=> two
one
user=> (eval (two))
7
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Are symbols values?
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Pre-defined vs user-defined
symbols
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420
[1 2 3]
"Not-a-string"
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(defn add [& nums]
(apply + nums))
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(defn add-2 [num1 num2]
(add num1 num2))
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Revisiting double evaluation
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symbols Value?
(write time)
Value?
(Compile
time)
Value?
(run time)
Predefined
symbols
NO Yes Yes
User defined
symbols
NO No yes
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In functions,
we manipulate values.
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In macros,
we manipulate symbols and values.
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Let’s write a macro
(finally!)
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(1 + 2) => (+ 1 2)
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(lt op rt) => (op lt rt)
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(defmacro infix
[[lt op rt]]
`(~op ~lt ~rt))
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=>(infix (1 + 2))
3
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A 3-step process
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1. Think about the data the macro will get.
(1 + 2)
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2. Think about the data the macro will return.
(+ 1 2)
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3. Think about how to transform that data.
(1 + 2) => (+ 1 2)
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user=>(print-els [1 2 3])
1
2
3
[1 2 3]
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(defmacro print-els [coll]
`(do ~(map println coll)
~coll))
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`(do ~(map println coll)
~coll))
user=>(print-els [1 2 3])
1
2
3
; Syntax error (IllegalArgumentException)
; Can't call nil, form: (nil nil nil)
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(macroexpand-1 '(print-els [1 2 3]))
1
2
3
(do (nil nil nil) [1 2 3])
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=>(do (map println [1 2 3]))
1
2
3
(nil nil nil)
=>(do nil nil nil [1 2 3])
[1 2 3]
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=>(macroexpand-1 '(print-els [1 2 3]))
;expected
(do nil nil nil [1 2 3])
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The unquote splice aka ~@ operator
`(do ~@(map println coll)
~coll))
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user=>(macroexpand-1 '(print-els [1 2 3]))
1
2
3
(do nil nil nil [1 2 3])
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Symbol capture
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(defmacro return-some-list [x]
(list 'let ['one 1]
['one x]))
user=>(return-some-list 5)
[1 5]
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(def one "one")
user=>(return-some-list one)
[1 1]
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user=>(macroexpand-1 '(return-some-list one))
(let [one 1] [one one])
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The autogensym aka #
=>`one#
one__9411__auto__
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`(let [one 1]
[one ~x]))
`(let [one# 1]
[one# ~x]))
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(def one "one")
user=>(return-some-list one)
[1 "one"]
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Common gotchas
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1. Not resolving arguments to a
value inside the macro
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(defmacro infix [[lt op rt]]
‘(op lt rt))
=>(macroexpand-1 '(infix (1 + 2)))
(op lt rt)
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2. The macro is NOT the first
element in the list
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(map infix [(1 + 2) (3 + 4)])
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3. Passing a symbol instead of a
value
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(defmacro mul-2 [xs]
`(* 2 ~@xs))
=>(mul-2 [1 2])
4
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(defmacro mul-2 [xs]
`(* 2 ~@xs))
(defn multiply-by-2 [nums]
(mul-2 nums))
; Syntax error
; Don't know how to create ISeq from:
clojure.lang.Symbol
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Practical Advice
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Always use a syntax quote
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“Don’t write a macro”
-Not me
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Actions have consequences
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Macros are infectious
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(defmacro add [& args]
`(+ ~@args))
user=>(add 1 2 3)
6
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=>(map #(apply + %) [[1 2 3] [2 4]])
(6 6)
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(defmacro add-vecs [vecs]
(loop [f (first vecs)
r (rest vecs)
res `(list)]
(if (seq r)
(recur (first r) (rest r) (concat
res `((add ~@f))))
(concat res `((add ~@f))))))
user=> (add-vecs [[1 2] [3 4]])
(3 7) @the_lazy_folder

Macros are not reusable
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Macros are not composable
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Composition is king.
(even the OO folks know that)
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When should you write a macro?
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When a function just won’t do
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When you need to prevent
evaluation of arguments
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Incanter’s infix notation
User=> ($= 7 + 8 - 2)
13
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When you want to extend
another macro
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Syntactic sugar
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Speed
(no, not that kind)
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I promised a wedding photo
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Macrobrew: Clojure macros distilled

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