Enterprise Clojure Training

3. Functions

functions

The chief function of the body is to carry the brain around.

— Thomas A. Edison

Defining functions (again)

    (defn square [x]
      (* x x))
    ;=> #my.namespace/square

Functions always return a value; the result of their last expression
defn creates a var

Applying functions

    (square 2)
    ;=> 4

Prefix notation

    (+ (square 2) (square 3))
    ;=> 13

Mathematical operators are functions.

Arguments are evaluated from left to right before the function is called

Unnamed functions

    (fn [a]
      (inc a))
    ;=> #object[Function]

    #(inc %)
    ;=> #object[Function]

Unnamed; anonymous; Lambda (λ) expression

Invoking anonymous functions

    ((fn [a] (inc a)) 1)
    ;=> 2

    (#(inc %) 1)
    ;=> 2

Just like a named function; first item in a list is applied

Closure

Function that captures values from the environment

    (let [who "world"]
      (defn greet []
        (str "Hello " who)))
    (greet)
    ;=> "Hello world"

Passing a function to another function

    (defn do-triple [f]
      (f)
      (f)
      (f))
    (do-triple (fn []
                 (print "hot ")))
    ;;; hot hot hot
    ;=> nil

Functions are values
Can be passed to other functions
Functions apply an input function are considered "higher order functions"

Mapping a function over a sequence

map the function, not hash-map the data structure

Function that applies a function to every element in a sequence

    (map inc [1 2 3])
    ;=> (2 3 4)

    (map (fn [x] (* x x)) [1 2 3 4])
    ;=> (1 4 9 16)

Map a previously defined function

    (defn greet-them [person]
      (str "Hello " person))
    (map greet-them ["Alice" "Bob" "Carol"])
    ;=> ("Hello Alice" "Hello Bob" "Hello Carol")

The greet-them function is easier to test on its own

Map an anonymous function

    (let [x 5]
      (map #(+ x %) [1 2 3]))
    ;=> (6 7 8)

Closure as argument to higher order function
Terse

This is a key source of Clojure’s expressiveness

Named anonymous functions?!?!?!

(fn add-one [x]
  (inc x))
add-one
;;; Unable to resolve symbol: add-one

The name add-one is only available inside the function

Documents purpose
Name appears in stacktraces (searchable clue)
The function can call itself

Named functions

(defn f [x]
  (inc x))

Is shorthand for

(def f
  (fn [x]
    (inc x)))

Variadic arguments

    (defn f [& args]
      args)
    (f 1 2 3)
    ;=> (1 2 3)

    (+ 1 2 3 4 5)
    ;=> 15

Variadic disadvantages

Not always convenient

(bake-cakes cake1 cake2 cake3)
(let [cakes (db/find-cakes "delicious")]
  (apply bake-cakes cakes))

Prefer functions that do one thing

Combine with sequence operations

(doseq [cake (db/find-cakes "delicious")]
  (bake-cake cake))

Variadic pitfall

Bypasses arity checking

Common bad pattern:

    (defn f [x & [y]]
      (if y
        (+ x y)
        (inc x)))
    (f 1 2 3 4 "not a number")
    ;=> 3

Prefer explicit argument lists

    (defn f
      ([x] (inc x))
      ([x y] (+ x y)))
    (f 1)
    ;=> 2

    (f 1 2)
    ;=> 3

    (f 1 2 3)
    ;;; Exception: Wrong number of args

Keyword arguments

    (defn start [& {:keys [port protocol]
                    :or {port 8080,
                         protocol "https"}}]
      protocol)
    (start)
    ;=> "https"

    (start :protocol "ftp")
    ;=> "ftp"

Avoid keyword arguments

    (def config {:protocol "ftp"})
    (start config)

    (apply start (apply concat config))
    ;=> "ftp"

Inconvenient and confusing

Take a map instead

   (defn start2 [{:keys [port protocol]}]
      protocol)
    (start2 {:protocol "gopher"})
    ;=> "gopher"

    (start2 config)
    ;=> "ftp"

Pre and post conditions

    (defn f [x]
      {:pre [(pos? x)]
       :post [(neg? %) (int? %)]}
      (- x))
    (f 1)
    ;=> -1

Pre and post conditions assert

    (f -1)
    ;;; AssertionError Assert failed: (pos? x)

    (f 1.5)
    ;;; AssertionError Assert failed: (int? %)

Pre/Post drawbacks:

Syntax is easy to get wrong, no assertion made
Assertions can be disabled
Less control over error reporting and handling
Rarely used

More common

Check for a condition and throw an exception

(defn f [x]
  (when-not (pos? x)
    (throw (ex-info "bad input" {:x x}))
  (let [result (- x)]
    (if (and (neg? result) (int? result))
      result
      (throw (ex-info "bad result" {:x x})))

Or use spec

Function literals

#(inc %)

#(+ %1 %2)

#(apply + %&)

Terse, powerful expressions

Compare

(map #(* % %) [1 2 3 4])

(map (fn square [x]
       (* x x))
     [1 2 3 4])

Prefer (fn) form

Named parameter
named purpose
stack traces

Exercises

See manual end of section 3

Answers

    (defn square [x]
      (* x x))
    (square 55)
    ;=> 3025

Answers

    (defn square-of-square [x]
      (if (< x 100)
        (square (square x))
        (throw (ex-info "Input too large" {:x x}))))
    (square-of-square 2)
    ;=> 16

    (square-of-square 123)
    ;;; ExceptionInfo Input too large

Answers

    (defn fib-step [a b]
      [b (+ a b)])
    (fib-step 1 1)
    ;=> [1 2]

    (fib-step 1 2)
    ;=> [2 3]

    (fib-step 2 3)
    ;=> [3 5]

Challenge 1: Corgi Cover eligibility

Insuricorp is about to launch a marketing campaign for a new “corgi cover” policy. Only certain people are eligible to register for “corgi cover”. To be eligible they must own a corgi, live in either Illinois (IL), Washington (WA), New York (NY), or Colorado (CO). You are tasked with building a system to validate applications for the policy.

Part 1:

Write a function that will take as input a state and corgi-count, and will return a boolean indicating the person’s eligibility for the “corgi cover” policy.

Test data

Name	State	Corgi count	Existing policy count
Chloe	IL	1	0
Ethan	IL	4	2
Annabelle	WY	19	0
Logan	WA	2	1

Part 2:

A focus group of corgi owners has revealed that “corgi cover” needs to be offered at 3 different tiers: “corgi cover silver”, “corgi cover gold”, and “corgi cover platinum”. Platinum is available when covering 7 or more corgis OR covering at least 3 corgis and also having one other policy with Insuricorp. Gold is available when covering at least 3 corgis. Silver is the original “corgi cover” policy. Create a new function that takes an additional argument policy-count and returns a keyword indicating their eligibility.

See cond

Part 3:

The “corgi cover” applications Insuricorp collect contain more information than necessary to determine eligibility. Create a new function that takes as input a single map data structure as input instead of multiple inputs. It should pick out the values that it needs from the input map. Create some test data and feed it to your function. The data should look something like:

{:name "Chloe", :state "IL", :corgi-count 1, :policy-count 0}

Part 4:

Insuricorp just merged with Megacorp. Platinum level corgi cover is now offered to people with an existing Megacorp policy as well. Because the company is still restructuring, the policy-count input still only contains Insuricorp data. But a new input has been made available to you which is a map of people to policies.

{"Chloe" ["secure goldfish"]
 "Ethan" ["cool cats cover" "megasafe"]}

Create a new function that takes as inputs two maps: the application, and the existing policies. It should apply the same logic, but make use of the Megacorp data.

End Functions

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