Doing haskell on the side for a while (first attempt before 2011, real use in 2012)
Doing java/typescript at work nowadays
Introduced javaslang in our work environment (6 developers full-time)
Real-world app. in fact, JEE
Javaslang
Java 8+
First release one day after java8
Scala is the reference
Emphasis on the collections
Javaslang has zero external dependencies!
Why javaslang?
It has an interface named λ. I rest my case ∎
FP key features
Immutability
Composability
High-level paradigms
Explicitness
Big picture
By avoiding global state, being explicit about input and output for functions, we can understand
completely a piece of code by looking at it in isolation.
Much better for tests! explicit input, same input→same output
Immutability gives you guarantees that functions won't modify data behind your back
"Hidden" inputs & ouputs: unchecked exceptions, side-effects, global state, class members
Immutable?
No 'void' modifiers. Example of mutable classes: List, Map, old Date, Calendar
List newList = existing.prepend(extraCar);
Note that we now have two lists: existing is still available and unchanged, and newList is added, with the change included.
We say of a 'void' returning function that it's only side-effecting.
Immutability, what for
No more 'getters' which allow to modify your state
Mutability increases complexity by adding the dimension of time to all objects.
Valid or Invalid, but a validation will accumulate the errors instead of short-circuiting
map, mapError, get, getError, sequence
Functional and Reactive domain modeling, Debasish Ghosh, p125
Tuple
Multiple arities are available
mapN, _N
Zipping (REPL)
A map can be viewed as a list of pairs
List.partition & variants
Function.tupled
Expressions vs statements
"if" is a statement, ternaries are an expression. Expressions compose, statements don't.
1. good old
Expressions vs statements
2. java streams
Patterns are extracted and immediately visible, while a for loop could perform anything.
Expressions vs statements
3. javaslang
Expressions vs statements
Functions
In mathematics, a is a relation between a set of inputs and a set of permissible outputs with the property that each input is related to exactly one output.
Wikipedialist.map(f).map(g) is equivalent to list.map(f.andThen(g)) only if both functions are pure (free of side-effects).
Side effects: anything that depends on an external system. Filesystem, global mutable state..
Function composition
You can wrap a function with another one, or compose them.
Composed function are better testable (in tests and in the REPL)
You can wrap a function with another one, or compose them.
Composed function are better testable (in tests and in the REPL)
Don't underestimate Option!
Javaslang embraces Option
Map.get() returns Option
Traversable.headOption
Option is invaluable to us, as a documentation for return types
Don't underestimate Option!
Javaslang embraces Option
Map.get() returns Option
Traversable.headOption
Option is invaluable to us, as a documentation for return types
Option and DTOs: before
Option and DTOs: after
Infinite streams
Pattern matching again
Ready-made predicates like instanceOf, isIn
Or custom check with lambda -- x -> x %2 == 0 || x < 0
Pattern matching deconstruction 1/2
Option
Note that Match is pushing the java8 type inference. Sometimes you must really fight the compiler and the code is sensitive on the type annotations and difficult to write.
Pattern matching deconstruction 2/2
And more...
Lifting
Property testing
Currying
For
Future
Lazy
Memoization
Jackson integration
Universality
These concepts are general, and you will find them in many environments.
Scala, Haskell but also Javascript libraries, F#, elm, lisps...
January 2017
