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Monads
@matyo91
29-09-2021
In PHP
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Why?
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29-09-2021
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A new way to program with
effects
“
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29-09-2021
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“Once you understand Monads
You lose the ability to explain
them to other people”
Crockford’s Paradox
“
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29-09-2021
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What are Monads?
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29-09-2021
As functional programming
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Data Expr = Val Int | Div Expr Expr
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29-09-2021
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Math world
1
6 / 2
6 / ( 3 / 1 )
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29-09-2021
Haskell world
Val 1
Div (Val 6) (Val 2)
Div (Val 6) (Div (Val 3) (Val 1))
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eval :: Expr -> Int
eval (Val n) = n
eval (Div (Expr x) (Expr y)) = eval(x) / eval(y)
@matyo91
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29-09-2021
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safediv :: Int -> Int -> MayBe Int
safediv (Int n) (Int m) = if m == 0 then Nothing else Just(n / m)
@matyo91
@matyo91
29-09-2021
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eval :: Expr -> Maybe
eval (Val n) = Just(n)
eval (Div (Expr x) (Expr y)) = case eval(x) at
Nothing -> Nothing
Just n -> case eval(y) of
Nothing -> Nothing
Just m -> safediv (n m)
@matyo91
@matyo91
29-09-2021
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case [m] as Maybe of
Nothing -> Nothing
Just x -> [f] as function x
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29-09-2021
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m >>= f case m of
Nothing -> Nothing
Just x -> f x
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29-09-2021
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eval :: Expr -> Maybe Int
eval (Val n) = return n
eval (Div (Expr x) (Expr y)) = eval(x) >>= (ƛn ->
eval(y) >>= (ƛm ->
safediv n m))
@matyo91
@matyo91
29-09-2021
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eval :: Expr -> Maybe Int
eval (Val n) = return n
eval (Div (Expr x) (Expr y)) = do n <- end x
m <- end y
safediv n m
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@matyo91
29-09-2021
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return :: a -> Maybe a
>>= :: Maybe a -> (a -> Maybe b) -> Maybe b
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29-09-2021
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Pure Function
29-09-2021 @matyo91
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Referential
transparency and
side effects
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29-09-2021
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“Go learn Haskell”
“Go learn Category Theory”
“A monad is just a monoid in category of endofunctors”
(M t) -> (t -> Mu) -> (M u)
(>>=) :: (Monad m) => m a -> (a -> m b) -> m b
What is a Monad?
@matyo91
@matyo91
29-09-2021
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I don’t fully understand them
I will share what I’ve learned so far
You fill in the blanks
Monads are a loophole in the “Functional contract”
A way to write pure functions with referential transparency
The loophole
@matyo91
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29-09-2021
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Fundamentals
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29-09-2021
And theory
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function unit($value)
function bind($monad, function($value) { … })
$value passed to function in bind <=> $value passed to unit
All return a new monad
Function that returns new thing <=> constructor
All monads are constructors
Not all constructors are monads
Unit & bind
@matyo91
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29-09-2021
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bind(unit($value), $fn) <=> $fn($value)
bind($monad, unit) <=> $monad
bind(bind($monad, $fnA), $fnB)
<=>
bind($monad, function($value) {
return bind($fnA($value), $fnB);
})
Axioms
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29-09-2021
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OOP
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29-09-2021
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Monad::unit($value)->bind($fn) <=> $fn($value)
$monad->bind(Monad::unit) <=> $monad
$monad->bind($fnA)->bind($fnB)
<=>
$monad->bind(function($value) {
return $fnA($value)->bind($fnB);
})
Axioms of class
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29-09-2021
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Implementation
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29-09-2021
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Applications
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And examples
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Identity
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Maybe
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Maybe applied to GrandParentName
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Maybe applied to GrandParentName
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Maybe applied to GrandParentName
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Everywhere
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Everywhere
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29-09-2021
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ircmaxell/monad-php : Monad, Identity, Maybe, Chain, Deferred,
ListMonad, Promise
schmittjoh/php-option : Option, LazyOption, Some, None
GrahamCampbell/Result-Type : Result, Success, Error
whsv26/functional : Option (do notation), Some, None, Either
darkwood-fr/railway-fbp : Rail
Github
@matyo91
@matyo91
29-09-2021
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Monads are category of design pattern
We have been using them for years
Most common forms : simple implementation
Strong mathematical knowledge are not required
Go play !
Conclusion
@matyo91
@matyo91
29-09-2021
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Hello!
I Am Mathieu Ledru
You can contact me at @matyo91
29-09-2021 @matyo91
Thanks!
Any questions?