GC 163011 Functional Programming Lecture 9 Higher Order

GC 16/3011 Functional Programming Lecture 9 Higher Order Functions 3/4/2021 1

Contents • Higher Order Functions • Definition • Example: composition • Combinators • Definition • Example: S, K, I • Capturing common forms of recursion on lists • Examples: map and filter 3/4/2021 2

Higher Order Functions • Definition: • A Higher Order Function is a function that either (i) takes a function as an argument, or (ii) returns a function as a result, or (iii) both. f g x = (g x) h x = (+ x) j f p g = (+ (f p)), if p = (+ (g p)), otherwise 3/4/2021 3

Higher Order Functions: types f : : (* -> **) -> ** f g x = (g x) h : : num -> (num -> num) h x = (+ x) j : : (bool-> num) -> bool -> (bool -> num) -> (num -> num) j f p g = (+ (f p)), if p = (+ (g p)), otherwise 3/4/2021 4

Function composition compose : : (**->***) ->(*->**)->*->*** compose f g x = f (g x) • Can partially apply “compose”: fred = (compose (+ 1) abs) main = fred (-3) § Built-in operator “. ” fred = ((+1). abs) 3/4/2021 5

Function composition (2) • twice x = x * 2 • many x = twice (twice x))) • mymany : : num -> num • mymany = (twice. twice) 3/4/2021 6

Combinators • Definition • A combinator is a function that contains no free variables • Examples: fst (x, y) = x id x = x cancel x y = x swap f x y = f y x distribute f g x = f x (g x) || “I” || “K” || “C” || “S” 3/4/2021 7

Combinators (2) • Basis for implementation • S & K computationally complete • All required data available via arguments (passed on the stack) so no need to search for distant bindings 3/4/2021 8

Example HOF • “myiterate” repeatedly applies its second parameter to its final parameter; the final parameter is an accumulator for the result. The function loops n times, where n is the first parameter: myiterate : : num -> (* -> *) -> * myiterate 0 f state = state myiterate n f state = myiterate (n-1) f (f state) 3/4/2021 9

Recursion on lists: capturing common forms • Mapping a function across the values of a list: notlist [] = [] notlist (x: xs) = ((~ x) : (notlist xs)) inclist [] = [] inclist (x: xs) = ((x+1) : (inclist xs)) abslist [] = [] abslist (x: xs) = ((abs x) : (abslist xs)) 3/4/2021 11

Recursion on lists: map • Built-in function “map” makes life easier: notlist any = map (~) any inclist any = map (+1) any abslist any = map abs any • Or, simply: notlist = map (~) inclist = map (+1) abslist = map abs 3/4/2021 12

Definition of map f [] = [] map f (x: xs) = ((f x): (map f xs)) • So, what’s the type of map? • Write it here: 3/4/2021 13

Recursion on lists: capturing common forms • Filtering some elements out of a list: firsts [] firsts (x: xs) = = = not 34 [] = not 34 (x: xs) = = [] (x : (firsts xs)), if (x>=70) firsts xs, otherwise [] (x : (not 34 xs)), if (x~=34) not 34 xs, otherwise 3/4/2021 14

Recursion on lists: filter • Built-in function “filter” makes life easier: firsts any = filter (>=70) any not 34 any = filter (~=34) any • Or, simply: firsts = filter (>=70) not 34 = filter (~=34) 3/4/2021 15

Definition of filter f [] = [] filter f (x: xs) = (x: (filter f xs)), if (f x) = filter f xs, otherwise • So, what’s the type of filter? • Write it here: 3/4/2021 16

Challenge • • Can you write a function that takes a list of numbers (containing only the values 1, 2 and 0, where at least one 0 must occur) and returns a three-tuple containing: • The number of 1 s before the first 0 • The number of 2 s before the first 0 • The length of the longest sequence of 1 s before the first 0 If you find that easy, try designing the program a different way so that it makes use of higher order functions (e. g. filter, dropwhile, takewhile) 3/4/2021 17

Summary • Higher Order Functions • Definition • Example: composition • Combinators • Definition • Example: S, K, I • Use of example HOF • Capturing common forms of recursion on lists • Examples: map and filter 3/4/2021 18