7 ineffective coding habits many F# programmers don't have

7 ineffective
coding
habits
MANY
F#
programmers
DON’T
have

habit
ˈhabɪt/
A settled or regular tendency
or practice, especially one
that is hard to give up.

“I’m not a great programmer;
I’m just a good programmer
with great habits.”
- Kent Beck

Noisy Code
Visual Dishonesty
Lego Naming
Underabstraction
Unencapsulated State
Getters and Setters
Uncohesive Tests

@theburningmonk
does the language I use
make a difference?

“Programming languages
have a devious influence:
they shape our thinking
habits.”
- Edsger W. Dijkstra

@theburningmonk
every LOC is a cost

@theburningmonk
more code
more chance for bugs

@theburningmonk
more code
more engineers

@theburningmonk
You should do whatever possible to increase the
productivity of individual programmers in terms of
the expressive power of the code they write. Less
code to do the same thing (and possibly better).
Less programmers to hire. Less organizational
communication costs.

does the language I use
make a difference?

@theburningmonk
source http://bit.ly/1oBHHh1

@theburningmonk
no { }
no nulls
fewer syntactic noise

@theburningmonk
fewer code
fewer noise

@theburningmonk
fewer noise
higher SNR

@theburningmonk
fewer code
more productivity

- Dan North
“Lead time to someone saying
thank you is the only reputation
metric that matters.”

“…a clean design is one that
supports visual thinking so
people can meet their
informational needs with a
minimum of conscious effort.”
- Daniel Higginbotham (www.visualmess.com)

@theburningmonk
public void MyCleverMethod(
int firstArg,
string secondArg)
signifies hierarchy

“You convey information by the way you
arrange a design’s elements in relation to
each other. This information is understood
immediately, if not consciously, by the
people viewing your designs.”

“This is great if the visual relationships are
obvious and accurate, but if they’re not,
your audience is going to get confused.
They’ll have to examine your work carefully,
going back and forth between the different
parts to make sure they understand.”

@theburningmonk
Whilst talking with an ex-colleague, a question came up on how to implement the Stable Marriage
problem using a message passing approach. Naturally, I wanted to answer that question with Erlang!
Let’s first dissect the problem and decide what processes we need and how they need to interact with
one another.
The stable marriage problem is commonly stated as:
Given n men and n women, where each person has ranked all members of the opposite sex with a
unique number between 1 and n in order of preference, marry the men and women together such that
there are no two people of opposite sex who would both rather have each other than their current
partners. If there are no such people, all the marriages are “stable”. (It is assumed that the participants
are binary gendered and that marriages are not same-sex).
From the problem description, we can see that we need:
* a module for man
* a module for woman
* a module for orchestrating the experiment
In terms of interaction between the different modules, I imagined something along the lines of…
how we read ENGLISH
see also http://bit.ly/1KN8cd0

@theburningmonk
Whilst talking with an ex-colleague, a question came up on how to implement the Stable Marriage
problem using a message passing approach. Naturally, I wanted to answer that question with Erlang!
Let’s first dissect the problem and decide what processes we need and how they need to interact with
one another.
Given n men and n women, where each person has ranked all members of the opposite sex with a
unique number between 1 and n in order of preference, marry the men and women together such that
there are no two people of opposite sex who would both rather have each other than their current
partners. If there are no such people, all the marriages are “stable”. (It is assumed that the participants
* a module for man
In terms of interaction between the different modules, I imagined something along the lines of…
2.top-to-bottom
1.left-to-right
how we read ENGLISH

@theburningmonk
how we read CODE
public void DoSomething(int x, int y)
{
Foo(y,
Bar(x,
Zoo(Monkey())));
}

@theburningmonk
how we read CODE
{
Foo(y,
Bar(x,
Zoo(Monkey())));
}
2.bottom-to-top
1.right-to-left

@theburningmonk
Whilst talking with an ex-colleague, a question came up on
how to implement the Stable Marriage problem using a
message passing approach. Naturally, I wanted to answer
that question with Erlang!
Let’s first dissect the problem and decide what processes we
need and how they need to interact with one another.
Given n men and n women, where each person has ranked
all members of the opposite sex with a unique number
between 1 and n in order of preference, marry the men and
women together such that there are no two people of
opposite sex who would both rather have each other than
their current partners. If there are no such people, all the
marriages are “stable”. (It is assumed that the participants
* a module for man
In terms of interaction between the different modules, I
imagined something along the lines of…
2.top-to-bottom
1.left-to-right
how we read ENGLISH
{
Foo(y,
Bar(x,
Zoo(Monkey())));
}
2.top-to-bottom
1.right-to-left
how we read CODE

@theburningmonk
|>

@theburningmonk
how we read CODE
let drawCircle x y radius =
circle radius
|> ﬁlled (rgb 150 170 150)
|> alpha 0.5
|> move (x, y)

@theburningmonk
how we read CODE
let drawCircle x y radius =
circle radius
|> ﬁlled (rgb 150 170 150)
|> alpha 0.5
|> move (x, y)
2.top-to-bottom
1.left-to-right

@theburningmonk
public ResultType MyCleverMethod(
int firstArg,
string secondArg,
string thirdArg) {
var localVar =
AnotherCleverMethod(firstArg, secondArg);
if (localVar.IsSomething(
thirdArg, MY_CONSTANT)) {
DoSomething(localVar);
}
return localVar.GetSomething();
}

@theburningmonk
XXXXXX XXXXXXXXXX XXXXXXXXXXXXXX
XXX XXXXXXXX
XXXXXX XXXXXXXXX
XXXXXX XXXXXXXX
XXX XXXXXXXX
XXXXXXXXXXXXXXXXXXX XXXXXXXX XXXXXXXXX
XX XXXXXXXX XXXXXXXXXXX
XXXXXXXX XXXXXXXXXX
XXXXXXXXXXX XXXXXXXX
XXXXXX XXXXXXXX XXXXXXXXXXXX

“This is great if the visual relationships are
obvious and accurate, but if they’re not,
your audience is going to get confused.
They’ll have to examine your work carefully,
going back and forth between the different
parts to make sure they understand.”

@theburningmonk
public ResultType MyCleverMethod(
int firstArg,
string secondArg,
string thirdArg)
{
var localVar =
AnotherCleverMethod(firstArg, secondArg);
if (localVar.IsSomething(
thirdArg, MY_CONSTANT))
{
DoSomething(localVar);
}
return localVar.GetSomething();
}

- Douglas Crockford
“It turns out that style
matters in programming for
the same reason that it
matters in writing.
It makes for better reading.”

@theburningmonk
two competing rules for
structuring code in
C-style languages

@theburningmonk
Compiler
{ }
Human
{ } + whitespace

@theburningmonk
Compiler
whitespace
Human
whitespace

@theburningmonk
xxx
{
}
xxx {
}
no
braces no
problem

@theburningmonk
There should be one - and preferably only
one - obvious way to do it.
- the Zen of Python

@theburningmonk
let myCleverFunction x y z =
let localVar = anotherCleverFunction x y
if localVar.IsSomething(z, MY_CONSTANT) then
doSomething localVar
localVar.GetSomething()

@theburningmonk
XXX XXXXXXXXXXXXXXXX X X X
XXX XXXXXXXX XXXXXXXXXXXXXXXXXXXX X X
XX XXXXXXXX XXXXXXXXXXX X XXXXXXXXXX XXXX
XXXXXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXXXXXX

@theburningmonk
one way to describe
hierarchy

@theburningmonk
naming is HARD

- Phil Karlton
“There are only two hard things
in Computer Science: cache
invalidation and naming things.”

- Mike Mahemoff
“Names are the one and only
tool you have to explain what a
variable does in every place it
appears, without having to
scatter comments everywhere.”

@theburningmonk
Lego Naming
Gluing common words together in an
attempt to create meaning.

@theburningmonk
Strategy
Process
Create
Add
Controller
Factory
Proxy
Object
Exception
Enable
Do
Disable
Service
Remove
Check
Get
Set
Update
Validate

@theburningmonk
see http://methodnamer.com

@theburningmonk
this is not naming

@theburningmonk
this is not naming
this is labelling

@theburningmonk
anonymous functions
aka lambdas

@theburningmonk
fewer things to name

@theburningmonk
words
|> Array.map (fun x -> x.Count)
|> Array.reduce (+)

@theburningmonk
smaller scope
shorter names

@theburningmonk
http://bit.ly/1ZpAByu
When x, y, and z are
great variable names

@theburningmonk
"The length of a name should be related to the
length of the scope. You can use very short
variable names for tiny scopes, but for big
scopes you should use longer names.
Variable names like i and j are just fine if their
scope is five lines long."
- Robert C. Martin

@theburningmonk
object expressions

@theburningmonk
enterpriseCrew.OrderBy(
(fun c -> c.Current),
{ new IComparer<Occupation> with
member this.Compare(x, y) =
x.Position.CompareTo(y.Position) })

@theburningmonk
tuples + pattern matching

@theburningmonk
fewer abstractions

@theburningmonk
words
|> Seq.groupBy id
|> Seq.map (fun (word, gr) ->
word, Seq.length gr)
|> Seq.iter (fun (word, len) ->
printfn “%s - %s” word len)

@theburningmonk
Lego Naming can also be
the symptom of a failure to
identify the right level of
abstractions.

@theburningmonk
the RIGHT level of
abstraction might be
smaller than “object”

@theburningmonk
public interface ConditionChecker
{
bool CheckCondition();
}

@theburningmonk
public interface Condition
{
bool IsTrue();
}

@theburningmonk
type Condition = unit -> bool

source https://vimeo.com/113588389

@theburningmonk
ClassNotFoundException
IllegalArgumentException
IndexOutOfBoundsException
NoSuchMethodException
UnsupportedOperationException

@theburningmonk
ClassNotFound
IllegalArgument
IndexOutOfBounds
NoSuchMethod
UnsupportedOperation

@theburningmonk
ArithmeticException
ArrayStoreException
ClassCastException
InstantiationException
NullPointerException
SecurityException

@theburningmonk
IntegerDivisionByZero
IllegalArrayElementType
CastToNonSubclass
ClassCannotBeInstantiated
NullDereferenced
SecurityViolation

@theburningmonk
lightweight exception syntax

@theburningmonk
open System
open System.IO
exception InsufficientBytes

@theburningmonk
open System
open System.IO
exception InsufficientBytes
what could this type represent?

@theburningmonk
F# < > silver bullet

@theburningmonk
anonymous functions

@theburningmonk
tuple + pattern matching

@theburningmonk
no abstraction is too small

@theburningmonk
public Result DoSomething(
int a,
string b,
string c,
string d,
DateTime e,
DateTime f,
string g,
MyEnum h)

“If you have a procedure
with ten parameters, you
probably missed some.”
- Alan Perlis

source https://vimeo.com/97507575

@theburningmonk
lightweight syntax for
types and hierarchies

@theburningmonk
type Employee =
{
FirstName : string
Surname : string
Salary : int<Pound>
}

@theburningmonk
type Employee =
{
FirstName : string
Surname : string
Salary : int<Pound>
}
immutable by default

@theburningmonk
let promote emp raise =
{
emp with Salary <- emp.Salary + raise
}

@theburningmonk
mutable state
complects
value and time

@theburningmonk
type Employee =
{
FirstName : string
Surname : string
Salary : int<Pound>
}
unit-of-measure

@theburningmonk
[<Measure>]
type Pound
e.g. 42<Pound>
153<Pound>

10<Meter> / 2<Second> = 5<Meter/Second>
10<Meter> * 2<Second> = 20<Meter Second>
10<Meter> + 10<Meter> = 20<Meter>
10<Meter> * 10 = 100<Meter>
10<Meter> * 10<Meter> = 100<Meter2>
10<Meter> + 2<Second> // error
10<Meter> + 2 // error

@theburningmonk
discriminated
unions

@theburningmonk
type PaymentMethod =
| Cash
| Cheque of ChequeNumber
| Card of CardType * CardNumber

@theburningmonk
public class RecentlyUsedList
{
private List<string> items = new List<string>();
public List<string> Items
{
get { return items; }
}
…
}

@theburningmonk
type RecentlyUsedList (?items) =
let items = defaultArg items [ ]
member this.Items = Array.ofList items
member this.Count = List.length items
member this.Add newItem =
newItem::(items |> List.filter ((<>) newItem))
|> RecentlyUsedList

@theburningmonk
Affordance
an affordance is a quality of an object, or
an environment, which allows an individual
to perform an action. For example, a knob
affords twisting, and perhaps pushing,
whilst a cord affords pulling.

source https://www.youtube.com/watch?v=aAb7hSCtvGw

@theburningmonk
your abstractions should
afford right behaviour,
whilst make it impossible
to do the wrong thing

@theburningmonk
“Make illegal states unrepresentable”
- Yaron Minsky

@theburningmonk
type PaymentMethod =
| Cash
| Cheque of ChequeNumber
| Card of CardType * CardNumber
finite, closed set of valid states ONLY

@theburningmonk
match paymentMethod with
| Cash -> …
| Cheque chequeNum -> …
| Card (cardType, cardNum) -> …

@theburningmonk
make illegal state
unrepresentable

“When it’s not necessary to change,
it’s necessary to not change.”
- Lucius Cary

“Now we have shortcuts to do the
wrong thing.
We used to have type lots to do the
wrong thing, not anymore.”
- Kevlin Henney

@theburningmonk
immutability by default

@theburningmonk
type Person =
{
Name : string
Age : int
}

@theburningmonk
type Person =
{
mutable Name : string
mutable Age : int
}

@theburningmonk
MethodA
MethodB
When_…Then_… ()
When_…Then_… ()
When_…Then_… ()
When_…Then_… ()
When_…Then_… ()
When_…Then_… ()

@theburningmonk
MethodA
MethodB
MethodC
FeatureA
FeatureB

@theburningmonk
complexities & potential
bugs in the way methods
work together

@theburningmonk
…especially when states
are concerned

@theburningmonk
Test Driven Development

“For tests to drive development they
must do more than just test that code
performs its required functionality: they
must clearly express that required
functionality to the reader. That is, they
must be clear specification of the
required functionality.”
- Nat Pryce & Steve Freeman

@theburningmonk
how many tests?

@theburningmonk
every test has a cost

@theburningmonk
did we cover all the
edge cases?

@theburningmonk
Property-Based Testing
(with FsCheck)

@theburningmonk
List.rev
reverse + reverse = original
length of list is invariant
append + reverse = reverse + prepend

@theburningmonk
List.rev
property : reverse + reverse = original
let ``reverse + reverse = original`` rev aList =
aList |> rev |> rev = aList
Check.Quick (``reverse + reverse = original`` List.rev)
// Ok, passed 100 tests.

@theburningmonk
List.rev
property : length of list is invariant
let ``length of list is invariant`` rev aList =
List.length (rev aList) = List.length aList
Check.Quick (``length of list is invariant`` List.rev)

@theburningmonk
List.rev
property : append + reverse = reverse + prepend
let ``append + reverse = reverse + prepend`` rev x aList =
(aList @ [x]) |> rev = x::(aList |> rev)
Check.Quick (``append + reverse = reverse + prepend``
List.rev)

@theburningmonk
Check.Verbose (``append + reverse = reverse + prepend``
List.rev)
//
0: ‘005' []
1: false ["N "]
2: “" [false; '{']
3: ‘017' [true; true; 'W']
4: “" [""; false]
5: “yg]" [“HnOq6"; null; false; false; '#']
6: true [“"]
…
11: <null> ['014'; '0'; “nRH”; "<#oe"; true; false; ‘O']
…

@theburningmonk
Check.Quick (``append + reverse = reverse + prepend`` id)
// Falsifiable, after 2 tests (4 shrinks) (StdGen
(1855582125,296080469)):
Original:
‘013' ["}k"; ""; “"]
Shrunk:
true [false]

@theburningmonk
let computers do the
grunt work

source : http://bit.ly/1kEpEso

@theburningmonk
Types vs Tests

@theburningmonk
unit-testing
distr. systems
system-testing

@theburningmonk
Jepsenproperty-based
unit-testing system-testing
distr. systems

@theburningmonk
Jepsenproperty-based
unit-testing
types as proof TLA+
distr. systems
system-testing

“Practice does not make perfect.
Only perfect practice makes perfect.”
- Vince Lombardi

“Perfection is not attainable. But if we
chase perfection, we can catch excellence.”
- Vince Lombardi

“One of the most disastrous
thing we can learn is the ﬁrst
programming language, even
if it's a good programming
language.”
- Alan Kay

@theburningmonk
what about ineffective
coding habits SOME F#/FP
programmers DO have?

@theburningmonk
people are too puritanical about purity

…premature optimization
is the root of all evil. Yet
we should not pass up
our opportunities in that
critical 3%
- Donald Knuth

@theburningmonk
F# Map vs .Net array vs Dictionary

@theburningmonk
Explicit is better than implicit.
- the Zen of Python

@theburningmonk
Simple is better than Complex.
Complex is better than Complicated.
- the Zen of Python

@theburningmonk
Special cases aren't special enough to
break the rules.
- the Zen of Python

@theburningmonk
Special cases aren't special enough to
break the rules.
Although practicality beats purity.
- the Zen of Python

@theburningmonk
If the implementation is hard to explain,
it's a bad idea.
- the Zen of Python

@theburningmonk
@theburningmonk
theburningmonk.com
github.com/theburningmonk

@theburningmonk
is hiring :-)
http://tech.just-eat.com/jobs

7 ineffective coding habits many F# programmers don't have

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