1. CSE-321 Programming Languages Introduction to Functional Programming
박성우
POSTECH

2. Programming Paradigms
Structured programming
C, Pascal, …
Object-oriented programming
C++, Java, …
Logic programming
Prolog, …
Functional programming
SML, Haskell, Objective Caml, Lisp, Scheme, F#, …

3. Functional Language SML
Standard ML
ML = Meta-Language
An example of programming language whose design is based on the programming language theory
type theory
Programming language for your assignments

4. Designer of SML

5. Familiar?

6. WTH?

7. Outline Expressions and values Variables Functions Types Recursion
Datatypes
Pattern matching
Higher-order functions
Exceptions
Modules

8. Imperative Language C A program consists of commands.
command = “do something”
Nothing wrong:
if (x == 1) then
x = x + 1;
else
x = x - 1;
Nothing wrong either:

9. Functional Language SML
A program consists of expressions.
expression = “obtain a value”
Nothing wrong:
if (x = 1) then
x + 1
else
x - 1
But this does not make sense:
what is the value if x <> 1?

10. Expression Evaluation An expression “evaluates” to a value.
We “evaluate” an expression to obtain a value.

11. Integer Evaluation
1 + 1 2
1 - 1
1 * 1 1 …

12. Boolean Evaluation
1 = 1
true
1 <> 1
false
1 <> 0
true …

13. An Integer Expression if 1 = ~1 then 10 else ~10
if false then else ~10
~10

14. Values as Expressions 1
???

15. Outline Expressions and values V Variables Functions Types Recursion
Datatypes
Pattern matching
Higher-order functions
Exceptions
Modules

16. Variable Declaration - val x = 1 + 1; val x = 2 : int
A variable x is “bound” to value 2.
From now on, any occurrence of x is replaced by 2.
- val y = x + x;
val y = 4 : int

17. Local Declaration let val x = 1 val y = x + x val z = y + y in z + z
end 8

18. Nested Local Declaration
let
val x = 1 in
x + x
end
let
val y = <expression> in
y + y
end
let
val y = let val x = 1 in x + x end in
y + y
end

19. Why “Local”?
let
val y = let
val x = 1 in
x + x
end
x + y
okay???

20. Variables are NOT variable.
The contents of a variable never change.
immutability of variables
Surprise?
That’s because you are thinking about variables in imperative programming.
variables in SML <> variables in C

21. Then Why Variables? Any advantage in using variables at all? let
val x = 1
val y = x + x
val z = y + y in
z + z
end
((1 + 1) + (1 + 1)) + ((1 + 1) + (1 + 1))
VS.
What if it takes 10 hours to evaluate 1?

22. Outline Expressions and values V Variables V Functions Types Recursion
Datatypes
Pattern matching
Higher-order functions
Exceptions
Modules

23. When is the first time you learned the concept of function?

24. 즐거운 낱말 공부 시간
함수(函數): 두 변수 x, y간에 어떤 관계가 있어 x의 변화에 따라 y가 일정한 법칙으로 변화할 때 y를 x의 함수라 함. (function) (동아 마스타 국어사전)

25. 函 즐거운 한자 공부 시간 1. 함(함). 2. 편지(함) 3. 갑옷(함) 4. 넣을, 들일(함) 예:
(書函) 서함: 책을 넣는 상자

26. Function = 函數 = Box Number!

27. Using a Box Number

28. Using a Box Number - Generalized… …

29. Function in SML = Box Number
(fn x => x + 1) n =

30. Function Application (fn x => x + 1) n…
(fn x => x + 1) n
We “apply” (fn x => x + 1) to n.
x is called a formal argument/parameter.
n is called an actual argument/parameter.

31. Evaluating a Function Application…
(fn x => x + 1) n
n + 1

32. Functions in SML Nameless function fn x => x + 1;
Storing a nameless function to a variable
val incr = fn x => x + 1;
Function declaration
fun incr x = x + 1;

33. Function Applications
incr 1
(fn x => x + 1) 1
1 + 1 2

34. First-Class Stamps

35. First-class Objects First-class objects = primitive objects
can be stored in a variable.
can be passed as an argument to a function.
can be returned as a return value of a function.
Examples:
integers
booleans
characters
floating-point numbers …

36. First-class Objects in C
integers
characters
floating-point numbers
pointers
structures …
Functions?
Function pointers are first-class objects.
But functions are not.
Why? You cannot create new functions on the fly!

37. Functions = First-class Objects in SML
can be passed as an argument to a function.
can be returned as a return value of a function.

38. Box Number as Output … …
such that

39. Box Number as Output x … +x …

40. Box Number as Output x y
y+x

41. Box Number as Output x y
fn y => y+x
y+x

42. Box Number as Output x y
fn y => y+x
y+x
fn y => y+x

43. Box Number as Output x y y+x fn x => (fn y => y+x)

44. In SML Recall the following declarations are equivalent:
val incr = fn x => x + 1;
fun incr x = x + 1;
Then:
val add = fn x => (fn y => y + x);
fun add x = fn y => y + x;
fun add x y = y + x;
add takes only one argument, not two!
In fact, every function in SML takes only one argument.

45. (fn x => (fn y => y + x)) 1 2
Adding Two Integers
add 1 2
(fn x => (fn y => y + x)) 1 2
(fn y => y + 1) 2
2 + 1 3

46. Box Number as Input (
true,
false)

47. Box Number as Input f
fn f => (f true, f false) ( f
true, f
false)

48. Outline Expressions and values V Variables V Functions V Types
Recursion
Datatypes
Pattern matching
Higher-order functions
Exceptions
Modules

49. Types
A type specifies what kind of value a given expression evaluates to.
1 + 1 : int
true andalso false : bool
#”A” : char
”hello” : string
(1, true) : int * bool
(1, ~1, true) : int * int * bool
1.0 : real
() : unit

50. Type Preservation Expression : T
Value : T
An evaluation preserves the type of a given expression.

51. Example let val x = 1 val y = x + x val z = y + y in z + z end : int

52. Function Types T -> T’ type of functions:
taking arguments of type T
returning values of type T’
Example:
val incr = fn x => x + 1; val incr = fn : int -> int
fun incr x = x + 1; val incr = fn : int -> int
Explicit type annotation
val incr = fn (x:int) => x + 1;
val incr = fn (x:int) : int => x + 1;
fun incr (x:int) = x + 1;
fun incr (x:int) : int = x + 1;

53. Type of add x
fn y => y+x
fn x => (fn y => y+x)

54. Type of add
int
fn x => (fn y => y+x)
fn y => y+x

55. Type of add
int
fn x => (fn y => y+x)
int -> int

56. Type of add
int
int -> (int -> int)
int -> int

57. What is the Type? f
fn f => (f true, f false) ( f
true, f
false)

58. f : bool -> int ? ( true, false) f : (bool -> int) ->
int * int
fn f => (f true, f false) ( f
true, f
false)

59. But why is it
f : bool -> int ?

60. Why not f : bool -> char ?
char * char
fn f => (f true, f false) ( f
true, f
false)

61. f : bool -> int * string ? f : bool -> unit ?
Then why not
f : bool -> string ?
f : bool -> int * string ?
f : bool -> unit ?
f : bool -> (int -> int) ?
f : bool -> <crazy type> ? …

62. So we need
Polymorphism.

63. What is the Type of ? true, false) ( fn f => (f true, f false) f
All we know about f is that it takes booleans as arguments.

64. All we know about f is that it takes booleans as arguments.?

65. f : bool -> ?
fn f => (f true, f false) : (bool -> ?) -> ? * ?

66. f : bool -> 'a
fn f => (f true, f false) : (bool -> 'a) -> 'a * 'a 'a
type variable
usually read as alpha
means 'for any type alpha'

67. Polymorphic Types Types involving type variables 'a, 'b, 'c, ... E.g.
fn x => x :
'a -> 'a
fn x => fn y => (x, y) :
'a -> 'b -> ('a * 'b)
fn (x : 'a) => fn (y : 'a) => x = y :
'a -> 'a -> bool
(* actually does not typecheck! *)

68. Equality Types Motivation Equality (=) is not defined on every type.
E.g.
comparing two functions for equality?
Type variables with equality
''a, ''b, ''c, ...
''a means 'for any type alpha for which equality is defined'
fn (x : ''a) => fn (y : ''a) => x = y :
''a -> ''a -> bool

69. Outline Expressions and values V Variables V Functions V Types V
Recursion
Datatypes
Pattern matching
Higher-order functions
Exceptions
Modules

70. Recursion vs. Iteration
Recursion in SML
fun sum n = if n = 0 then 0 else sum (n - 1) + n
Iteration in C
int i, sum;
for (i = 0, sum = 0; i <= n;
i++) sum += n;
Recursion is not an awkward tool if you are used to functional programming.
Recursion seems elegant but inefficient!

71. Recursion in Action fun sum n = if n = 0 then 0 else sum (n - 1) + n
call stack
f 0
0 + 1
further computation
f 1
1 + 2
...
...
f 8
36 + 9
f 9
f 10 55
evaluation

72. Funny Recursion fun zero n = if n = 0 then 0 else zero (n - 1)
call stack
f 10
f 9
f 8
...
f 1
f 0
no further computation
evaluation

73. Funny Recursion Optimized
fun zero n = if n = 0 then 0 else zero (n - 1)
call stack
f 0
f 1
...
...
f 8
f 9
f 10
evaluation

74. Funny Recursion Further Optimized
fun zero n = if n = 0 then 0 else zero (n - 1)
call stack
f 10
f 9
f 8
...
f 1
f 0
evaluation

75. Tail Recursive Function
A tail recursive function f:
A recursive call to f is the last step in evaluating the function body.
That is, no more computation remains after calling f itself.
A tail recursive call needs no stack!
A tail recursive call is as efficient as iteration!

76. Example Non-tail recursive sum
fun sum n = if n = 0 then 0 else sum (n - 1) + n
Tail recursive sum
fun sum' accum k = if k = 0 then accum else
sum' (accum + k) (k - 1)
fun sum n = sum' 0 n
Think about the invariant of sum':
given:
sum' accum k
invariant:
accum = n + (n - 1) (k + 1)

77. Outline Expressions and values V Variables V Functions V Types V
Recursion V
Datatypes
Pattern matching
Higher-order functions
Exceptions
Modules

78. Enumeration Types in C enum shape { Circle, Rectangle, Triangle};
Great flexibility
e.g.
Circle + 1 == Rectangle
Triangle - 1 == Rectangle
(Circle + Triangle) / 2 == Rectangle
But is this good or bad?

79. Datatypes in SML datatype shape = Circle | Rectangle | Triangle
No flexibility
e.g.
Circle (x)
Triangle - 1 (x)
Circle + Triangle (x)
But high safety.

80. Set datatype set = Empty | Many - Empty; val it = Empty : set - Many;
val it = Many : set

81. Set with Arguments datatype set = Empty | Many of int - Many 5;
val it = Many 5 : set

82. Set with Type Parameters
datatype 'a set =
Empty |
Singleton of 'a |
Pair of 'a * 'a
- Singleton 0;
val it = Singleton 0 : int set
- Pair (0, 1);
val it = Pair (0, 1) : int set

83. Set with Type Parameters
datatype 'a set =
Empty |
Singleton of 'a |
Pair of 'a * 'a
- Pair (Singleton 0, Pair (0, 1))
val it = Pair (Singleton 0, Pair (0, 1)) : int set set

84. Set with Type Parameters
datatype 'a set =
Empty |
Singleton of 'a |
Pair of 'a * 'a
- Pair (0, true);
stdIn: Error: operator and operand don't agree [literal]
operator domain: int * int
operand: int * bool
in expression:
Pair (0,true)

85. Recursive Set with Type Parameters
datatype 'a set =
Empty |
NonEmpty of 'a * 'a set
This is essentially the definition of datatype list.

86. Datatype list datatype 'a list = nil | :: of 'a * 'a list - nil;
val it = [] : 'a list
- 2 :: nil; (* :: infix *)
val it = [2] : int list
- 1 :: (2 :: nil);
val it = [1,2] : int list
- 1 :: 2 :: nil; (* :: right associative *)

87. Datatype list datatype 'a list = nil | :: of 'a * 'a list
val it = [[1],[2]] : int list list
- [1] :: [[2]];
(X)
(X)

88. Using Datatypes We know how to create values of various datatypes:
shape
set
int set
int list
...
But how do we use them in programming?
What is the point of creating datatype values that are never used?

89. Outline Expressions and values V Variables V Functions V Types V
Recursion V
Datatypes V
Pattern matching
Higher-order functions
Exceptions
Modules

90. Simple Pattern datatype shape = Circle | Rectangle | Triangle
(* convertToEnum : shape -> int *)
fun convertToEnum (x : shape) : int =
case x of
Circle => 0
| Rectangle => 1
| Triangle => 2

91. Pattern with Arguments
datatype set =
Empty |
Many of int
fun size (x : set) : int =
case x of
Empty => 0
| Many n => n

92. Wildcard Pattern _ : "don't care"
datatype 'a set =
Empty |
Singleton of 'a |
Pair of 'a * 'a
fun isEmpty (x : 'a set) : bool =
case x of
Empty => true
| _ => false

93. Pattern with Type Annotation
datatype 'a list =
nil |
:: of 'a * 'a list
fun length (x : 'a list) : int =
case x of
(nil : 'a list) => 0
| (_ : 'a) :: (tail : 'a list) =>
1 + length tail

94. Outline Expressions and values V Variables V Functions V Types V
Recursion V
Datatypes V
Pattern matching V
Higher-order functions
Exceptions
Modules

95. Higher-order Functions
Take functions as arguments.
Return functions as the result.

96. Why "Higher-order"? T0 ::= int | bool | real | unit | ...
T1 ::= T0 -> T0 | T (* 1st order *)
T2 ::= T1 -> T1 | T1 (* 2nd order *)
T3 ::= T2 -> T2 | T (* higher order *)
...

97. Higher-order Functions in List
val exists : ('a -> bool) -> 'a list -> bool
val all : ('a -> bool) -> 'a list -> bool
val map : ('a -> 'b) -> 'a list -> 'b list
val filter : ('a -> bool) -> 'a list -> 'a list
val app : ('a -> unit) -> 'a list -> unit
(* printInt : int -> unit *)
fun printInt i =
TextIO.print ((Int.toString i) ^ "\n" );
List.app printInt [1, 2, 3];

98. List Fold Function
foldl : ('a * 'b -> 'b) -> 'b -> 'a list -> 'b
foldr : ('a * 'b -> 'b) -> 'b -> 'a list -> 'b
foldl f b0 [a0, a1, a2, ..., an-1] a0 a1 a2
an-2
an-1
... f b1 f b2 f b3 f
bn-1 f bn b0
...
bn-2

99. Summation fun sum (l : int list) =
List.foldl (fn (a, accum) => a + accum) 0 l
foldl op+ 0 [a0, a1, a2, ..., an-1]
op+ a0 a1 a2
an-2
an-1
... + + + + + 
...

100. List Reversal fun reverse (l : 'a list) =
List.foldl (fn (a, rev) => a :: rev) nil l
List.foldl op:: nil l
Whenever you need iterations over lists, first consider foldl and foldr.

101. More Examples List.exists f l =
List.foldl (fn (a, b) => b orelse f a) false l
List.all f l =
List.foldl (fn (a, b) => b andalso f a) true l
List.app f l =
List.foldl (fn (a, _) => f a) () l
List.map f l =
List.foldr (fn (a, b) => f a :: b) nil l
List.filter f l =
List.foldr (fn (a, b) => if f a then a :: b
else b) nil l

102. Outline Expressions and values V Variables V Functions V Types V
Recursion V
Datatypes V
Pattern matching V
Higher-order functions V
Exceptions
exception: please see the course notes.
Modules

103. Outline Expressions and values V Variables V Functions V Types V
Recursion V
Datatypes V
Pattern matching V
Higher-order functions V
Exceptions V
Modules

104. Structures and Signatures
collection of type declarations, exceptions, values, and so on.
structure Set =
struct
type 'a set = 'a list
val emptySet = nil
fun singleton x = [x]
fun union s1 s2 = s2
end
Signature
conceptually type of structures.
signature SET =
sig
type 'a set
val emptySet : 'a set
val singleton : 'a -> 'a set
val union :
'a set -> 'a set -> 'a set
end

105. Structures + Signatures
Transparent constraint
Type definition is exported to the outside.
structure Set : SET =
struct
type 'a set = 'a list
val emptySet = nil
fun singleton x = [x]
fun union s1 s2 = s2
end
- Set.singleton 1 = [1];
val it = true : bool
Opaque constraint
No type definition is exported.
structure Set :> SET =
struct
type 'a set = 'a list
val emptySet = nil
fun singleton x = [x]
fun union s1 s2 = s2
end
- Set.singleton 1 = [1];
(* Error! *)

106. Functors Functions on structures takes a structure as an argument
returns a structure as the result
structure
structure