Heikki Hulkko hhulkko

A Quick and Dirty Lens primer

Why does Lens exist? Well, Haskell records suck, for a number of reasons. I will enumerate them using this sample record.

data User = User { login    :: Text
                 , password :: ByteString
                 , email    :: Text
                 , created  :: UTCTime
 }

Refactoring Booleans to Functions

Boolean parameters are a plague, responsible for non-composable, monolithic functions that never quite do enough, and countless programming bugs:

"Oops, meant to pass that as the 2nd boolean flag, not the 1st!"
"Oops, accidentally inverted the meaning of that boolean in the implementation!"
"Oops, the function isn't flexible enough for my use case, let's add a 6th boolean flag!"
"Oops, got the meaning of that boolean flag wrong, time to dig into the source code!"

All boolean parameters should be refactored into functions that effect the change otherwise encoded in the parameter.

	# best practice: linux
	nano ~/.pgpass
	:5432::username:password
	chmod 0600 ~/.pgpass

	# best practice: windows
	edit %APPDATA%\postgresql\pgpass.conf
	:5432::username:password

	# linux

	import java.util.function.BiFunction;
	import java.util.function.Function;
	import java.util.function.Supplier;

	/*
	* Java 8 fantasy land spec:
	*
	* A monad is a parameterized data type `M` with the following constraints
	* - An instance `M<A>` must have a method: `public <B> M<B> bind(Function<A, M<B>> f)`
	* - `M` must have a static method: `public static <A> M<A> point(A value)`

	// Usage:
	// p(instance)('privateMethod)(arg1, arg2, arg3)

	class PrivateMethodCaller(x: AnyRef, methodName: String) {
	def apply(_args: Any*): Any = {
	val args = _args.map(_.asInstanceOf[AnyRef])
	def _parents: Stream[Class[_]] = Stream(x.getClass) #::: _parents.map(_.getSuperclass)
	val parents = _parents.takeWhile(_ != null).toList
	val methods = parents.flatMap(_.getDeclaredMethods)
	val method = methods.find(_.getName == methodName).getOrElse(throw new IllegalArgumentException("Method " + methodName + " not found"))