Functional Programming Patterns (NDC London 2014)

Functional Design Patterns
(NDC London 2014)
@ScottWlaschin
fsharpforfunandprofit.com

@ScottWlaschin
X

@ScottWlaschin

Making fun of Enterprise OO
is like shooting fish in a barrel
I but...

I but...
Making fun of Enterprise OO
is like shooting IMarineVertebrates
in an AbstractBarrelProxyFactory
Inspired by @sjkilleen

This is what happens when you dabble too
much in functional programming

https://www.flickr.com/photos/37511372@N08/5488671752/
Haskell programmers
F#/OCaml programmers
Visual Basic programmers
Lisp
programmers

• Single Responsibility Principle
• Open/Closed principle
• Dependency Inversion
Principle
• Interface Segregation
Principle
• Factory pattern
• Strategy pattern
• Decorator pattern
• Visitor pattern
OO pattern/principle

Principle
Principle
• Factory pattern
• Visitor pattern
OO pattern/principle Borg response

Principle
Principle
• Factory pattern
• Visitor pattern
• Functions
• Functions
• Functions, also
• Functions
• You will be assimilated!
• Functions again
• Functions
• Resistance is futile!
OO pattern/principle FP equivalent
Seriously, FP patterns are different

Functional patterns
• Apomorphisms
• Dynamorphisms
• Chronomorphisms
• Zygohistomorphic prepromorphisms

Functional patterns
• Core Principles of FP design
– Functions, types, composition
• Functions as parameters
– Functions as interfaces
– Partial application & dependency injection
– Continuations, chaining & the pyramid of doom
• Monads
– Error handling, Async
• Maps
– Dealing with wrapped data
– Functors
• Monoids
– Aggregating data and operations

This talk
A whirlwind tour of many sights
Don't worry if you don't understand everything

(SOME) CORE PRINCIPLES OF
FUNCTIONAL PROGRAMMING
Important to understand!

Core principles of FP
Function
Types are not classes
Functions are things
Composition everywhere

Core principle:
Functions are things
Function

Functions as things
The Tunnel of
Transformation
Function
apple -> banana
A function is a standalone thing,
not attached to a class

Functions as things
let z = 1
int-> int->int
1
let add x y = x + y

Functions as inputs and outputs
let useFn f = (f 1) + 2
let add x = (fun y -> x + y)
int->(int->int)
int
int
let transformInt f x = (f x) + 1
int int
int->int
Function as output
Int ->int
Function as input
Function as parameterint->int
(int->int)->int
int->int

Core principle:
Composition everywhere

Function composition
Function 1
apple -> banana
Function 2
banana -> cherry

>>
Function 1
apple -> banana
Function 2
banana -> cherry

New Function
apple -> cherry
Can't tell it was built from
smaller functions!

Design paradigm:
Functions all the way down

“Functions in the small,
objects in the large”

Low-level operation
ToUpper
stringstring

Low-level operation
Service
AddressValidator
A “Service” is just like a microservice
but without the "micro" in front
Validation
Result
Address
Low-level operation Low-level operation

Service
Use-case
UpdateProfileData
ChangeProfile
Result
ChangeProfile
Request
Service Service

Use-case
Web application
Http
Response
Http
Request
Use-case Use-case

Core principle:

Set of
valid inputs
Set of
valid outputs
So what is a type?

Types separate data from behavior
Lists Lists
List.map
List.collect
List.filter
List.etc

Types can be composed too
“algebraic types"

Product types
× = Alice, Jan 12th
Bob, Feb 2nd
Carol, Mar 3rd
Set of people Set of dates
type Birthday = Person * Date

Sum types
Set of Cash values
Set of Cheque values
Set of CreditCard
values
+
+
type PaymentMethod =
| Cash
| Cheque of ChequeNumber
| Card of CardType * CardNumber

Design principle:
Strive for totality

twelveDividedBy(x)
input x maps to 12/x
…
3
2
1
0
…
Domain (int) Codomain (int)
…
4
6
12
…
Totality
int TwelveDividedBy(int input)
{
switch (input)
{
case 3: return 4;
case 2: return 6;
case 1: return 12;
case 0: return ??;
}
}

twelveDividedBy(x)
…
3
2
1
0
…
…
4
6
12
…
Totality
{
switch (input)
{
case 3: return 4;
case 2: return 6;
case 1: return 12;
case 0: return ??;
}
} What happens here?

twelveDividedBy(x)
…
3
2
1
0
…
…
4
6
12
…
Totality
{
switch (input)
{
case 3: return 4;
case 2: return 6;
case 1: return 12;
case 0:
throw InvalidArgException;
}
}
int -> int
This type signature is a lie!
You tell me you can
handle 0, and then you
complain about it?

twelveDividedBy(x)
…
3
2
1
-1
…
NonZeroInteger int
…
4
6
12
…
Totality
{
switch (input)
{
case 3: return 4;
case 2: return 6;
case 1: return 12;
case -1: return -12;
}
}
Constrain the input
0 doesn’t have to
be handled
NonZeroInteger -> int
0 is missing
Types are documentation

twelveDividedBy(x)
…
3
2
1
0
-1
…
int Option<Int>
…
Some 4
Some 6
Some 12
None
…
Totality
{
switch (input)
{
case 3: return Some 4;
case 0: return None;
}
}
Extend the output
0 is valid input
int -> int option
Types are documentation

Design principle:
Use static types for domain
modelling and documentation
Static types only!
Sorry Clojure and JS
developers 

Big topic! Not enough time today !
More on DDD and designing with types at
fsharpforfunandprofit.com/ddd

Guideline:
Parameterize all the things

let printList() =
for i in [1..10] do
printfn "the number is %i" i

It's second nature to parameterize the data input:
let printList aList =
for i in aList do
printfn "the number is %i" i

let printList anAction aList =
for i in aList do
anAction i
FPers would parameterize the action as well:
We've decoupled the
behavior from the data.
Any list, any action!

public static int Product(int n)
{
int product = 1;
for (int i = 1; i <= n; i++)
{
product *= i;
}
return product;
}
public static int Sum(int n)
{
int sum = 0;
for (int i = 1; i <= n; i++)
{
sum += i;
}
return sum;
}

let product n =
let initialValue = 1
let action productSoFar x = productSoFar * x
[1..n] |> List.fold action initialValue
let sum n =
let initialValue = 0
let action sumSoFar x = sumSoFar+x
[1..n] |> List.fold action initialValue
Lots of collection functions like this:
"fold", "map", "reduce", "collect", etc.

Tip:
Function types are "interfaces"

Function types are interfaces
interface IBunchOfStuff
{
int DoSomething(int x);
string DoSomethingElse(int x);
void DoAThirdThing(string x);
}
Let's take the
Single Responsibility Principle and the
Interface Segregation Principle
to the extreme...
Every interface should have
only one method!

Function types are interfaces
interface IBunchOfStuff
{
int DoSomething(int x);
}
An interface with one method is a just a function type
type IBunchOfStuff: int -> int
Any function with that type is compatible with it
let add2 x = x + 2 // int -> int
let times3 x = x * 3 // int -> int

Strategy pattern
class MyClass
{
public MyClass(IBunchOfStuff strategy) {..}
int DoSomethingWithStuff(int x)
{
return _strategy.DoSomething(x)
}
}
Object-oriented strategy pattern:

Strategy pattern
int int
int->int
int->int
let DoSomethingWithStuff strategy x =
strategy x
Functional strategy pattern:
IBunchOfStuff as parameter

Decorator pattern using function parameter:
let isEven x = (x % 2 = 0) // int -> bool
isEvenint bool
loggerint bool
let logger f input =
let output = f input
// then log input and output
// return output

Decorator pattern using function parameter:
isEvenint bool
loggerint bool
let isEvenWithLogging = logger isEven
// int -> bool
Substitutable for original isEven

Decorator pattern using function composition:
isEvenint bool
isEvenint boollogint int logbool bool
let isEvenWithLogging = log >> isEven >> log
// int -> bool
Substitutable for original isEven
Composition!

Bad news:
Composition patterns
only work for functions that
have one parameter! 

Good news!
Every function is a
one parameter function 

Writing functions in different ways
let add x y = x + y
let add = (fun x y -> x + y)
let add x = (fun y -> x + y)
int-> int->int
int-> int->int
int-> (int->int)
Normal (Two parameters)

let three = 1 + 2
let three = (+) 1 2
let three = ((+) 1) 2
"+" is a two parameter function

let three = 1 + 2
let three = (+) 1 2
let three = ((+) 1) 2
let add1 = (+) 1
let three = add1 2
No, "+" is a one param function!

let name = "Scott"
printfn "Hello, my name is %s" name
let name = "Scott"
(printfn "Hello, my name is %s") name
let name = "Scott"
let hello = (printfn "Hello, my name is %s")
hello name
Can reuse "hello" in many places now!

Pattern:
Use partial application when
working with lists

let names = ["Alice"; "Bob"; "Scott"]
names |> List.iter hello
let hello = printfn "Hello, my name is %s"
let add1 = (+) 1
let equals2 = (=) 2
[1..100]
|> List.map add1
|> List.filter equals2

Pattern:
Use partial application to do
dependency injection

type GetCustomer = CustomerId -> Customer
let getCustomerFromDatabase connection
(customerId:CustomerId) =
// from connection
// select customer
// where customerId = customerId
type of getCustomerFromDatabase =
DbConnection -> CustomerId -> Customer
let getCustomer1 = getCustomerFromDatabase myConnection
// getCustomer1 : CustomerId -> Customer
Persistence ignorant
This function requires a
connection 
The partially applied function does
NOT require a connection – it's baked in! 

type GetCustomer = CustomerId -> Customer
let getCustomerFromMemory dict (customerId:CustomerId) =
dict.Get(customerId)
type of getCustomerFromMemory =
Dictionary<Id,Customer> -> CustomerId -> Customer
let getCustomer2 = getCustomerFromMemory dict
// getCustomer2 : CustomerId -> Customer
This function requires a
dictionary 
The partially applied function does
NOT require a dictionary – it's baked in! 

Pattern:
The Hollywood principle*:
continuations

Continuations
int Divide(int top, int bottom)
{
if (bottom == 0)
{
throw new InvalidOperationException("div by 0");
}
else
{
return top/bottom;
}
}
Method has decided to throw
an exception
(who put it in charge?)

Continuations
void Divide(int top, int bottom,
Action ifZero, Action<int> ifSuccess)
{
if (bottom == 0)
{
ifZero();
}
else
{
ifSuccess( top/bottom );
}
}
Let the caller decide what
happens
what happens next?

Continuations
let divide ifZero ifSuccess top bottom =
if (bottom=0)
then ifZero()
else ifSuccess (top/bottom)
F# version
Four parameters is a lot
though!
Wouldn't it be nice if we could somehow
"bake in" the two behaviour functions....

Continuations
if (bottom=0)
then ifZero()
let ifZero1 () = printfn "bad"
let ifSuccess1 x = printfn "good %i" x
let divide1 = divide ifZero1 ifSuccess1
//test
let good1 = divide1 6 3
let bad1 = divide1 6 0
setup the functions to:
print a message
Partially apply the
continuations
Use it like a normal function –
only two parameters

if (bottom=0)
then ifZero()
Continuations
let ifZero2() = None
let ifSuccess2 x = Some x
//test
return an Option
only two parameters
Partially apply the
continuations

if (bottom=0)
then ifZero()
Continuations
let ifZero3() = failwith "div by 0"
let ifSuccess3 x = x
//test
throw an exception
only two parameters
Partially apply the
continuations

Pattern:
Chaining callbacks with
continuations

Pyramid of doom: null testing example
Let example input =
let x = doSomething input
if x <> null then
let y = doSomethingElse x
if y <> null then
let z = doAThirdThing y
if z <> null then
let result = z
result
else
null
else
null
else
null
I know you could do early
returns, but bear with me...

Pyramid of doom: async example
let taskExample input =
let taskX = startTask input
taskX.WhenFinished (fun x ->
let taskY = startAnotherTask x
taskY.WhenFinished (fun y ->
let taskZ = startThirdTask y
taskZ.WhenFinished (fun z ->
z // final result

Pyramid of doom: null example
let example input =
if x <> null then
let y = doSomethingElse x
if y <> null then
let z = doAThirdThing y
if z <> null then
let result = z
result
else
null
else
null
else
null
Nulls are a code smell:
replace with Option!

Pyramid of doom: option example
let example input =
if x.IsSome then
let y = doSomethingElse (x.Value)
if y.IsSome then
let z = doAThirdThing (y.Value)
if z.IsSome then
let result = z.Value
Some result
else
None
else
None
else
None
Much more elegant, yes?
No! This is fugly!
But there is a pattern we can exploit...

let example input =
if x.IsSome then
if y.IsSome then
if z.IsSome then
// do something with z.Value
// in this block
else
None
else
None
else
None

let example input =
if x.IsSome then
if y.IsSome then
// do something with y.Value
// in this block
else
None
else
None

let example input =
if x.IsSome then
// do something with x.Value
// in this block
else
None
Can you see the pattern?

if opt.IsSome then
//do something with opt.Value
else
None

let ifSomeDo f opt =
if opt.IsSome then
f opt.Value
else
None

let example input =
doSomething input
|> ifSomeDo doSomethingElse
|> ifSomeDo doAThirdThing
|> ifSomeDo (fun z -> Some z)
let ifSomeDo f opt =
if opt.IsSome then
f opt.Value
else
None

A switch analogy
Some
None
Input ->

Connecting switches
on Some
Bypass on None

Composing switches
>> >>
Composing one-track functions is fine...

Composing switches
>> >>
... and composing two-track functions is fine...

Composing switches
 
... but composing switches is not allowed!

How to combine the
mismatched functions?

“Bind” is the answer!
Bind all the things!

Composing switches
Two-track input Two-track input
One-track input Two-track input



Building an adapter block
Two-track input
Slot for switch function
Two-track output

Two-track input Two-track output

let bind nextFunction optionInput =
match optionInput with
| Some s -> nextFunction s
| None -> None
Two-track input Two-track output

Pattern:
Use bind to chain options

Pyramid of doom: using bind
let bind f opt =
match opt with
| Some v -> f v
| None -> None
let example input =
if x.IsSome then
if y.IsSome then
if z.IsSome then
let result = z.Value
Some result
else
None
else
None
else
None

let example input =
doSomething input
|> bind doSomethingElse
|> bind doAThirdThing
|> bind (fun z -> Some z)
Pyramid of doom: using bind
Let bind f opt =
match opt with
| Some v -> f v
| None -> None
No pyramids!
Code is linear and clear.
This pattern is called “monadic bind”

Pattern:
Use bind to chain tasks

Connecting tasks
When task
completesWait Wait

Pyramid of doom: using bind for tasks
let taskBind f task =
task.WhenFinished (fun taskResult ->
f taskResult)
let taskX = startTask input
taskX.WhenFinished (fun x ->
let taskY = startAnotherTask x
taskY.WhenFinished (fun y ->
let taskZ = startThirdTask y
taskZ.WhenFinished (fun z ->
z // final result
a.k.a “promise” “future”

Pyramid of doom: using bind for tasks
let taskBind f task =
task.WhenFinished (fun taskResult ->
f taskResult)
startTask input
|> taskBind startAnotherTask
|> taskBind startThirdTask
|> taskBind (fun z -> z)
This pattern is also a “monadic bind”

Pattern:
Use bind to chain error handlers

Use case without error handling
string UpdateCustomerWithErrorHandling()
{
var request = receiveRequest();
validateRequest(request);
canonicalizeEmail(request);
db.updateDbFromRequest(request);
smtpServer.sendEmail(request.Email)
return "OK";
}

Use case with error handling
{
var isValidated = validateRequest(request);
if (!isValidated) {
return "Request is not valid"
}
db.updateDbFromRequest(request);
return "OK";
}

{
if (!isValidated) {
}
var result = db.updateDbFromRequest(request);
if (!result) {
return "Customer record not found"
}
return "OK";
}

{
if (!isValidated) {
}
try {
if (!result) {
}
} catch {
return "DB error: Customer record not updated"
}
return "OK";
}

{
if (!isValidated) {
}
try {
if (!result) {
}
} catch {
return "DB error: Customer record not updated"
}
if (!smtpServer.sendEmail(request.Email)) {
log.Error "Customer email not sent"
}
return "OK";
}

A structure for managing errors
Request SuccessValidate
Failure
let validateInput input =
if input.name = "" then
Failure "Name must not be blank"
else if input.email = "" then
Failure "Email must not be blank"
else
Success input // happy path

Connecting switches
Validate UpdateDb SendEmail

World of normal values
int string bool
World of options
int option string option bool option

World of options
int string bool


World of options

int string bool

let add42ToOption opt =
if opt.IsSome then
let newVal = add42 opt.Value
Some newVal
else
None
How not to code with options
Let’s say you have an int wrapped in an Option, and
you want to add 42 to it:
let add42 x = x + 42


World of options
add42


World of options
add42

Lifting
World of options
T -> U
option<T> -> option<U>
Option.map

World of options
add42
add42ToOptionSome 1 |>
1 |> // 43
// Some 43

The right way to code with options
1 |> add42 // 43
let add42ToOption = Option.map add42
Some 1 |> add42ToOption // Some 43


The right way to code with options
1 |> add42 // 43
Some 1 |> Option.map add42 // Some 43


Lifting to lists
World of lists
T -> U
List<T> -> List<U>
List.map

The right way to code with wrapped types
[1;2;3] |> List.map add42
// [43;44;45]

Lifting to async
World of async
T -> U
async<T> -> async<U>
Async.map

Guideline:
Most wrapped generic types
have a “map”. Use it!

Guideline:
If you create your own generic type,
create a “map” for it.

Thinking like a mathematician
1 + 2 = 3
1 + (2 + 3) = (1 + 2) + 3
1 + 0 = 1
0 + 1 = 1

1 + 2 = 3
Some things
A way of combining
them

2 x 3 = 6
Some things
A way of combining
them

"a" + "b" = "ab"
Some things
A way of combining
them

concat([a],[b]) = [a; b]
Some things
A way of combining
them

1 + 2
1 + 2 + 3
1 + 2 + 3 + 4
Is an integer
Is an integer
A pairwise operation has
become an operation that
works on lists!

1 + (2 + 3) = (1 + 2) + 3
Order of combining doesn’t matter
1 + 2 + 3 + 4
(1 + 2) + (3 + 4)
((1 + 2) + 3) + 4
All the same

1 - (2 - 3) = (1 - 2) - 3
Order of combining does matter

1 + 0 = 1
0 + 1 = 1
A special kind of thing that when
you combine it with something, just
gives you back the original
something

42 * 1 = 42
1 * 42 = 42
A special kind of thing that when
you combine it with something, just
gives you back the original
something

"" + "hello" = "hello"
"hello" + "" = "hello"
“Zero” for strings

The generalization
• You start with a bunch of things, and some way of
combining them two at a time.
• Rule 1 (Closure):The result of combining two things is
always another one of the things.
• Rule 2 (Associativity):When combining more than
two things, which pairwise combination you do first
doesn't matter.
• Rule 3 (Identity element):There is a special thing
called "zero" such that when you combine any thing
with "zero" you get the original thing back. A monoid!

• Rule 1 (Closure):The result of combining two
things is always another one of the things.
• Benefit: converts pairwise operations into
operations that work on lists.
1 + 2 + 3 + 4
[ 1; 2; 3; 4 ] |> List.reduce (+)

1 * 2 * 3 * 4
[ 1; 2; 3; 4 ] |> List.reduce (*)

"a" + "b" + "c" + "d"
[ "a"; "b"; "c"; "d" ] |> List.reduce (+)

• Rule 2 (Associativity):When combining more
than two things, which pairwise combination
you do first doesn't matter.
• Benefit: Divide and conquer, parallelization, and
incremental accumulation.
1 + 2 + 3 + 4

(1 + 2) (3 + 4)
3 + 7

(1 + 2 + 3)

(1 + 2 + 3) + 4

(6) + 4

Issues with reduce
• How can I use reduce on an empty list?
• In a divide and conquer algorithm, what should I
do if one of the "divide" steps has nothing in it?
• When using an incremental algorithm, what
value should I start with when I have no data?

• Rule 3 (Identity element):There is a special
thing called "zero" such that when you combine
any thing with "zero" you get the original thing
back.
• Benefit: Initial value for empty or missing data

Pattern:
Simplifying aggregation code with monoids

type OrderLine = {Qty:int; Total:float}
let orderLines = [
{Qty=2; Total=19.98}
{Qty=1; Total= 1.99}
{Qty=3; Total= 3.99} ]
let addPair line1 line2 =
let newQty = line1.Qty + line2.Qty
let newTotal = line1.Total + line2.Total
{Qty=newQty; Total=newTotal}
orderLines |> List.reduce addPair
// {Qty=6; Total= 25.96}
Any combination
of monoids is
also a monoid
Write a pairwise combiner
Profit!

Pattern:
Convert non-monoids to monoids

Customer
+
Customer
+
Customer
Customer Stats
+
Customer Stats
+
Customer Stats
Reduce
Map
Not a monoid A monoid
Customer Stats
(Total)

Hadoop make me a sandwich
https://twitter.com/daviottenheimer
/status/532661754820829185

Guideline:
Convert expensive monoids
to cheap monoids

Log file (Mon)
+
Log File (Tue)
+
Log File (Wed)
=
Really big file
Summary (Mon)
+
Summary (Tue)
+
Summary (Wed)
Map
Strings are monoids A monoid
Much more efficient for
incremental updates
“Monoid homomorphism”

Pattern:
Seeing monoids everywhere

Monoids in the real world
Metrics guideline:
Use counters rather than rates
Alternative metrics guideline:
Make sure your metrics are monoids
• incremental updates
• can handle missing data

Is function composition a monoid?
>>
Function 1
apple -> banana
Function 2
banana -> cherry
New Function
apple -> cherry
Not the same
thing.
Not a monoid 

>>
Function 1
apple -> apple
Same thing
Function 2
apple -> apple
Function 3
apple -> apple
A monoid! 

“Functions with same type of input and output”
Functions where the input and output are the same type are
monoids! What shall we call these kinds of functions?

“Functions with same type of input and output”
“Endomorphisms”
Functions where the input and output are the same type are
monoids! What shall we call these kinds of functions?
All endomorphisms are monoids

let plus1 x = x + 1 // int->int
let times2 x = x * 2 // int->int
let subtract42 x = x – 42 // int->int
Reduce
plus1ThenTimes2ThenSubtract42 // int->int
Endomorphisms
Another endomorphism!

Event sourcing
Any function containing an endomorphism can
be converted into a monoid!
Event application function:
Is an endomorphism
Event -> State -> State

apply event1 // State -> State
Reduce
applyAllEventsAtOnce // State -> State
Endomorphism
Another endomorphism!
Partial application of event
• incremental updates
• can handle missing events

Series combination
=+
Result is same
kind of thing
(Closure)

Series combination
+
Order not
important
(Associative) Monoid!
+( )
++( )

Parallel combination
=+
Same thing
(Closure)
Order not
important
(Associative)
Monoid!

Monad laws
• The Monad laws are just the monoid
definitions in diguise
– Closure, Associativity, Identity
• What happens if you break the monad laws?
– You go to jail
– You lose monoid benefits such as aggregation

A monad is just a monoid in
the category of endofunctors!

Functional Programming Patterns (NDC London 2014)

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