cheat.sheets/sheets/_scala/Functions

// define function
// GOOD 
def f(x: Int) = { x*x }
// BAD 
// hidden error: without = it’s a Unit-returning procedure; causes havoc
def f(x: Int) { x*x }

// define function
// GOOD
def f(x: Any) = println(x)
// BAD
// syntax error: need types for every arg.
def f(x) = println(x)

// type alias
type R = Double

// call-by-value
def f(x: R)
// call-by-name (lazy parameters)
def f(x: => R)

// anonymous function
(x:R) => x*x    

// anonymous function: underscore is positionally matched arg.
(1 to 5).map(_*2) // vs.
(1 to 5).reduceLeft( _+_ )

//  anonymous function: to use an arg twice, have to name it.
(1 to 5).map( x => x*x )

// anonymous function: bound infix method. Use 2*_ for sanity’s sake instead.
// GOOD 
(1 to 5).map(2*)
// BAD
(1 to 5).map(*2)

// anonymous function: block style returns last expression.
(1 to 5).map { x => val y=x*2; println(y); y }

// anonymous functions: pipeline style. (or parens too).
(1 to 5) filter {_%2 == 0} map {_*2}

// anonymous functions: to pass in multiple blocks, need outer parens.
def compose(g:R=>R, h:R=>R) = (x:R) => g(h(x))
val f = compose({_*2}, {_-1})

// currying, obvious syntax.
val zscore = (mean:R, sd:R) => (x:R) => (x-mean)/sd

// currying, obvious syntax
def zscore(mean:R, sd:R) = (x:R) => (x-mean)/sd

// currying, sugar syntax. but then:
def zscore(mean:R, sd:R)(x:R) = (x-mean)/sd

// need trailing underscore to get the partial, only for the sugar version.
val normer = zscore(7, 0.4) _

// using curried parameters with a partially applied function
// can be called as "add(1)(2)" to return "3"
def add(x: Int) = x + (_: Int)

// generic type.
def mapmake[T](g:T=>T)(seq: List[T]) = seq.map(g)

// infix sugar.
5.+(3); 5 + 3
(1 to 5) map (_*2)

// varargs
def sum(args: Int*) = args.reduceLeft(_+_)

// default parameters
def countTo(i: Int = 5) = 1 to i
countTo(3) = Range(1,2,3)
countTo()  = Range(1,2,3,4,5)

// higher order functions
// as long as a function returns the correct type, it can be used as a parameter in another function
def sum(a: Int, b: Int): Int = a + b
def double(x: Int): Int = x * 2
double(sum(1, 1)) // returns 4