(* Programming Languages, Dan Grossman, CSE341 *)
(* Lecture 7: First-Class Functions *)
(* functions can go anywhere other values go *)
fun double x = 2*x
fun incr x = x+1
val a_tuple = (double, incr, double(incr 7))
val eighteen = (#1 a_tuple) 9
(* it should *pain* us to write the next three functions separately,
but we do not have to *)
fun increment_n_times_lame (n,x) = (* silly example, this is addition (n+x) *)
if n=0
then x
else 1 + increment_n_times_lame(n-1,x)
fun double_n_times_lame (n,x) =
if n=0
then x
else 2 * double_n_times_lame(n-1,x)
fun nth_tail_lame (n,xs) =
if n=0
then xs
else tl (nth_tail_lame(n-1,xs))
(* this is much better: as always, abstract the common pieces into a function
n_times(f,n,x) returns f(f(...(f(x)))) where there are n calls to f
note: if we gave x type int, then we could not use this for lists
*)
fun n_times (f,n,x) =
if n=0
then x
else f (n_times(f,n-1,x))
fun increment x = x+1
val x1 = n_times(double,4,7)
val x2 = n_times(increment,4,7)
val x3 = n_times(tl,2,[4,8,12,16])
(* and we can define functions that use n_times *)
fun addition (n,x) = n_times(increment,n,x) (* assumes n >=0 *)
fun double_n_times (n,x) = n_times(double,n,x)
fun nth_tail (n,x) = n_times(tl,n,x)
(* we can then use n_times for more things we did not plan on originally *)
fun triple x = 3*x
fun triple_n_times (n,x) = n_times(triple,n,x)
(* higher-order functions are often polymorphic based on "whatever
type of function is passed" but not always: *)
fun times_until_zero (f,x) =
(* note: a better implementation would be tail-recursive *)
if x=0 then 0 else 1 + times_until_zero(f, f x)
(* conversely, we have seen polymorphic functions that are not higher-order *)
fun len xs =
case xs of
[] => 0
| x::xs' => 1 + len xs'
(* motivating and introducing anonymous functions *)
fun triple_n_times2 (n,x) =
let fun triple x = 3*x in n_times(triple,n,x) end
(* actually since used only once, we could define it
right where we need it *)
fun triple_n_times3 (n,x) =
n_times((let fun triple y = 3*y in triple end), n, x)
(* This does not work: a function /binding/ is not an /expression/ *)
(* fun triple_n_times3 (n,x) = n_times((fun triple y = 3*y), n, x) *)
(* This /anonymous function/ expression works and is the best style: *)
(* Notice the function has no name *)
fun triple_n_times4 (n,x) = n_times((fn y => 3*y), n, x)
(* because triple_n_times4 does not call itself, we could use a val-binding
to define it, but the fun binding above is better style *)
val triple_n_times5 = fn (n,x) => n_times((fn y => 3*y), n, x)
(* unnecessary function wrapping *)
(* bad style: the if e then true else false of functions *)
fun nth_tail_poor (n,xs) = n_times((fn y => tl y), n, xs)
(* good style: *)
fun nth_tail_good (n,x) = n_times(tl, n, x)
(* bad style *)
fun rev_poor xs = List.rev xs
val rev_poor = fn xs => List.rev xs
(* good style *)
val rev_good = List.rev
(* here is a very, very useful and common example *)
fun map (f,xs) =
case xs of
[] => []
| x::xs' => (f x)::(map(f,xs'))
val x4 = map ((fn x => x+1), [4,8,12,16])
val x5 = map (hd, [[1,2],[3,4],[5,6,7]])
(* another very, very useful and common example *)
fun filter (f,xs) =
case xs of
[] => []
| x::xs' => if f x
then x::(filter (f,xs'))
else filter (f,xs')
fun is_even v =
(v mod 2 = 0)
fun all_even xs =
filter(is_even,xs)
fun all_even_snd xs =
filter((fn (_,v) => is_even v), xs)
(* Returning a function *)
fun double_or_triple f =
if f 7
then fn x => 2*x
else fn x => 3*x
val dbl = double_or_triple (fn x => x-3 = 4)
val nine = (double_or_triple (fn x => x = 42)) 3
(* Higher-order functions over our own datatype bindings *)
datatype exp = Constant of int
| Negate of exp
| Add of exp * exp
| Multiply of exp * exp
fun true_of_all_constants(f,e) =
case e of
Constant i => f i
| Negate e1 => true_of_all_constants(f,e1)
| Add(e1,e2) => true_of_all_constants(f,e1)
andalso true_of_all_constants(f,e2)
| Multiply(e1,e2) => true_of_all_constants(f,e1)
andalso true_of_all_constants(f,e2)
fun all_even_exp e = true_of_all_constants((fn x => x mod 2 = 0),e)