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Functional Programming in
PHP
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Lochemem
Bruno
Michael
● PHP enthusiast from Kampala, Uganda
● Functional Programming aficionado
● Writes PHP, JavaScript, and some C++
● Recently graduated college
● Maintains PHP userland packages and
extensions
● Authored a book
● Loves hoops, movies, and video games
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Functional Programming, huh?
● A declarative programming paradigm
● Mandates that pure functions be written
● Saliently features the use of expressions
● Emphasizes the importance of composition
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This is not an FP
vs OOP debate!
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What are the benefits?
● Functional Programming helps address complexity
○ Complexity often deters project advancement
○ Functions exist only as callable units and do not affect histories
● FP techniques truncate the cognitive load of developing software
○ Short-term memory is a finite resource
○ FP conditions one to parameterize constructs
○ The paradigm sturdies one’s intellection when writing programs
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4-7
Quanta in working memory models
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All of this in PHP?
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Expressions
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First-class
functions
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Do not forget
everything you know
about programming!
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Functions
everywhere!
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User-defined
functions
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Anonymous
functions
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Class methods
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Keep the functions
pure, please!
Side effects
Side causes
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Not so pure!
● Mutable global variable
(side-cause)
● Value mutated after function is
called (side-effect)
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An improvement?
● Use a closure
● Mutate only within the function
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Parameterize the
things!
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Another fix?
● Parameterize the accumulator
● Return one integer for every
$x-$acc parameter
combination
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Every argument
input should yield
the same output.
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Referential Transparency
● Is a quality of pure functions
● Dictates that pure functions calls
should be substitutable with their
outputs
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What about
classes?
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OOP has a
Forgotten
History!
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What we know
● Classes are Composite Data Types
● Encapsulation
● Property visibility
● Inheritance (is-a relationships)
● Polymorphism
● Setters and Getters
What we may not know
● Alan Kay’s biological analogy
● Immutability in objects
● A message-passing API
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Not pure!
● Setter mutates internal state
whenever invoked
● Each setter call increases
temporal coupling
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The cardinal rule
of purity still
applies!
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Either
● Remove the setter
● Single $empName value for
each object
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Or
● Make the setter a pure
function
● Create new copy of
Employee
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Pure Functions
● Do not create side-effects
● Yield the same output given the same input
● Are referentially transparent
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Compose the
functions!
f(x) = x + 1
g(x) = 2x
f ∘ g = f(g(x))
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“If you’re chaining, you’re composing”
- From Eric Elliott in Composing Software: An Introduction
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Traditionally
● Invoke functions as function
arguments
● Cumbersome to deal with
long chains
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Approaching
helper library
territory!
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bingo-functional
● Suite of FP libraries for PHP and JavaScript
● REPL for FP experimentation
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Point-free?
● No explicit parameter
mentions
● Seemingly more compact
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const for
functions?
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Some FP sauce to add to your program
● Partial application
● Currying
● Map, Filter, Fold/Reduce
● Pattern Matching
● Persistent Data Structures
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You might have
used some of
these techniques
before.
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Currying
● Decompose a function into a
series of sub-functions
● Each sub-function takes one
argument
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Partial Application
● Similar to currying
● Also useful for deferred function
calls
● Each sub-function has variable
arity
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Map, Filter, Fold
● Higher-order functions for array
transformations
● Highly composable
● Single iteration for every
transformation
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Pattern Matching
● Native support in pure FP
languages
● Useful whenever data has
discernible patterns
● Especially good for flow-control
● Composable, as a function
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Immutable Lists
● Persistent data structure
● Each transformation outputs a
new copy
● Single iteration for each
transformation
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Real world IO is
messy!
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Messy, huh? Why?
● Many operations are inherently impure
● Exceptions - used commonly - are also inherently impure
● IO can get complicated
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Impure IO?
● Filesystem interaction
● Writing to and reading from standard output and standard input
● Opening windows
● Database interactions
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Exceptions
● Inherently impure
● Not good for function chains
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How you can cope with this
● Use functors and monads
● Parameterize (might lead to callback hell)
● Favor composition with forward function chaining
● Use persistent data structures
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Programming
with effects...
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Functor
● Something you can map over
● Typically a type class
● A type of category
● Supports forward chaining
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Monad
● A special type of functor
● Convenient because of inherent
ad-hoc value
● Especially good for flow-control
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Real world monads
● Either and Maybe union types
● IO monad
● State monad
● Reader monad
● Writer monad
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Maybe Monad
● Composed of Just and Nothing
types
● Can evaluate to one of either
sub-type at runtime
● Useful for error handling
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Either Monad
● Composed of Left and Right
types
● Allows for more succinct error
handling
● Great for the Railway pattern
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IO Monad
● Great for I/O-related tasks
● Encapsulated within the monad
is an executable function
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State Monad
● Stores an initial state
● Creates copies of said state;
retains both
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Reader Monad
● Useful for sharing data amongst
functions
● Akin to Dependency Injection (DI)
in OO
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Writer Monad
● Great for logging data
● Can, in some contexts, be used
for event sourcing
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How do I know I’m doing it right?
The functions you write should have the following qualities:
● Purity
● Referential Transparency
● Composability
● Conciseness
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The future looks bright
for FP in PHP!
● New language features
● New RFCs
● New libraries
● More explanatory material
● All-around language consistency
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My Book
Functional Programming in PHP
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Thank you!