# Notes from Book: Introduction to Functional Programming - Philip Wadler

This is an old book but it is one of the best programming books ever written.

# Table of Contents

## ch01: Fundamental Concepts

### 1.1 Functional Programming

```
programming in fp =
building definitions and evaluating expressions
programmer's role
construct a function to solve a problem
computer's role
act as an evaluator (calculator)
expressions
contain instances of names of functions
evaluated by using definitions as simplification (reduction)
feature of fp
order of evaluation doesn't affect outcome =
meaning of expression is its value
computer's role: to obtain it
thus: expressions can be constructed, reasoned about using algebraic laws
like other mathematical expressions
result: a conceptual framework
very simple, concise, flexible and powerful
```

#### 1.1.1 Sessions and scripts

```
session: sequence of interactions bw user and computer
building definitions
script: a list of definitions
ex
square x = x * x
min x y = x, if x <= y
= y, if x > y
two functions: min, square
square: x as argument
ex
square (min 3 4) =
square 3 =
9
purpose of definition:
to introduce a binding between a name and a value
ex: name: square, value: function that squares its argument
environment (context): set of bindings
expressions
evaluated in some context
contain instances of names found in that context
evaluator
uses definitions of these names
to simplify expressions
```

### 1.2 Expressions and values

```
most important feature of mathematical notation:
referential transparency
expression is used only to describe a value
there are no other effects
value of expression depends only on values of its subexpressions
variables: names that stand for unknown values
```

#### 1.2.1 Reduction

```
evaluation = simplification = reduction
=> means "reduces to"
ex
square (3+) => square 7
=> 7 * 7
=> 49
49 cannot be further reduced
printed by computer
canonical: expression is canonical if it cannot be further reduced
@mine: concept map
[definition] consists of -> n [line]
[line] -<>1 [expression]
[line] -<>1 [guard]
[definition] ^- [conditional_definition]
[definition] introduces -> 1 [binding]
[definition] has inner -> 1 [context]
[context] has -> n [definition]
[binding] -<>1 [name]
[binding] -<>1 [value]
[expression] contain instances of -> n [name]
[evaluator] uses [definition] of [name] to simplify [expression]
[expression] means -> [value]
[variable] is [name] with unknown [value]
```

### 1.3 Types

```
types: set of values
2 kinds of types
basic types: values are given as primitive
compound types: values are constructed from other types
strong typing:
every expression can be assigned a type
that can be deduced from the constituents alone
consequence:
any expression that cannot be assigned a type
is not well-formed
ex: not well-formed definition
ay x = 'A'
bee x = x + ay x
2 stages of analysis before evaluation
syntax: does expression conform to correct syntax
type: does expression have sensible type
@mine: concept map
[type] is a set of -> n [value]
[expression] has -> [type]
[type] of [expression] can be deduced from its sub [expression]
[type] ^- [basic_type]
[type] ^- [compound_type]
[compound_type] constructed from other -> n [type]
```

### 1.4 Functions and definitions

```
most important value kind: function value
function f:
a rule of mapping with each element of type A to a unique element of type B
source type: A
target type: B
f::A->B
functions are values exactly as other values
```

#### 1.4.1 Type information

```
ex:
square:: num -> num
type information of function
square x = x * x
definition of function
type information is optional
can be inferred from its defining equation alone
very general source/target types
ex
id x = x
type variables: α -> α
α denotes a type variable
polymorphic type: id has polymorphic type
expression contains type variables
```

#### 1.4.2 Forms of definition

```
case analysis
ex
min x y = x, if x<=y
= y, if x>y
two expressions:
actual function definition: = x
guard: x <= y
local definition:
ex:
f(x,y) = a +1 where a = x / 2
where: introduces a local definition
whose context (scope) is rhs of definition of f
```

#### 1.4.3 Currying

```
ex:
min x y = x, if x<=y
= y, if x>y
min' (x,y) = x, if x<=y
=y, if x>y
difference between min and min':
min: takes two arguments one at a time
min': takes a single argument which is a structured value consisting of a pair of numbers
min'::(num,num) -> num
min::num->(num->num)
(min x) is a function which takes y
ex:
(add 1): successor function
(add 0): identity function
currying: replacing structured arguments by a sequence of simple ones
benefits of currying
reduces number of brackets to be written in expressions
associates to left:
operation of functional application
ex:
min x y
means: (min x) y
not: min (x y)
@mine: concept map
[function] is a rule of [mapping] between elements of [type] A and [type] B
[function] is a -> [value]
[function] is a kind of [value]
[expression] is a -> [value]
[type_variable]
[polymorphic_type] is a -> [type]
[expression] has [polymorphic_type] if it contains [type_variable]
[currying] replaces multiple [argument] one by one
[functional_application] is a kind of -> [operation]
[operation] has -> [associativity]
[left_associative] is a kind of -> [associativity]
[right_associative] is a kind of -> [associativity]
[specification] is a description of a [program]
[implementation] is a [program] that satisfies [specification]
```

### 1.5 Specifications and implementations

```
specification: mathematical description of the task a program is to perform
implementation: a program that satisfies the specification
different in nature
specs: expressions of programmer's intent or client's expectation
purpose: to be brief and clear
impl: expressions for execution by computer
purpose: to be efficient
link between the two: implementation satisfies the specification
programmer: provide a proof that this is the case
ex
spec:
increase x > square x for all x >= 0
result of increase should be greater than square of its argument
impl:
increase x = square (x + 1)
```

## ch02: Basic Data Types

### 2.1 Numbers

#### 2.1.1 Precedence

```
higher precedence first:
functional application
^
x / div mod
+ -
ex:
square 3 * 4 =
(square 3) * 4
```

#### 2.1.2 Order of association

```
functional application:
operator denoted by a space
associates to the left
ex:
5-4-2 =
(5-4)-2
associates to the right
exponentiation:
3^4^5 =
3^(4^5)
function type operator:
α -> β -> γ =
α -> (β -> γ)
associative: something different
an operator ⊕ is associative if:
(x ⊕ y) ⊕ z = x ⊕ (y ⊕ z)
+ and x are associative, ^ not
then order of association has no effect
```

#### 2.1.3 div and mod

```
(+):: num -> num -> num
section: bracketed operator
converts it to ordinary prefix function
(+) x y = x + y
bracketed operators can be used in expressions
both f x = f x x
both (+) 3 = (+) 3 3 = 3 + 3 = 6
double = both (+)
section: an argument can also be enclosed with operator
ex
(x⊕) y = x ⊕ y
(⊕x) y = y ⊕ x
```

#### 2.2.2 The logical operators

```
∧ conjunction (and)
∨ disjunction (or)
¬ negation (not)
ex: leap year
must be divisible by 4
if divisible by 100, then it must be divisible by 400
opt1:
leap y = (y mod 4 = 0) ∧ (y mod 100 ≠ 0 ∨ y mod 400 = 0)
opt2: by cases
leap y = (y mod 4 = 0) , if y mod 100 = 0
= (y mod 400 = 0), otherwise
ex: do the lines form a triangle
analyse a b c = 0, if a + b ≤ c
= 1, if a + b > c ∧ a = c
= 2, if a + b > c ∧ a ≠ c ∧ (a = b ∨ b = c)
ex: undefined value
if no guard evaluates to True
all alternatives in a conditional definition must have same type
@mine: concept map: operators
operators:
functional application (space)
arithmetic
function type ->
[operator] is [associative]
if (x ⊕ y) ⊕ z = x ⊕ (y ⊕ z)
[operator] is a [infix_function]
[bracketed_operator] is a [prefix_function]
(+) x y = x + y
[section] is a [curried_function]
(x⊕) y = x ⊕ y
[undefined_value] is a [value]
[expression] has [undefined_value] if no [guard] evaluates to True
```

### 2.3 Characters and strings

#### 2.3.1 Strings

```
list of characters
```

#### 2.3.2 Layout

```
to produce tables, pictures, formatted text
show:: α -> string
show 42 = "42"
justification functions
ljustify, cjustify, rjustify :: num -> string -> string
```

### 2.4 Tuples

```
ex:
quotrem :: (num,num) -> (num,num)
quotrem (x,y) = (x div y, x mod y)
```

#### 2.4.1 Example: rational arithmetic

### 2.5 Patterns

```
ex:
defined by patterns
cond True x y = x
cond False x y = y
equivalent to
cond p x y = x, if p = True
= y, if p = False
ex:
by pattern
permute 0 = 1
permute 1 = 2
permute 2 = 0
equivalent to
permute n = 1, if n = 0
= 2, if n = 1
= 0, if n = 2
patterns containing variables
ex:
pred 0 = 0
pred (n + 1) = n
```

### 2.6 Functions

```
higher-order function: function which takes a function as argument, or delivers one as result
```

#### 2.6.1 Functional composition

```
h x = f(g x)
(f · g) x = f(g x)
(·)::(β -> γ) -> (α -> β) -> (α -> γ)
associative operation
(f·g)·h = f·(g·h)
thus no need to put in brackets when writing sequences of compositions
benefit: some definitions become shorter
soln x = f1 (f2 (f3 x))
same as:
soln = f1 · f2 · f3
```

#### 2.6.2 Operators

```
operators: functions used in infix form
all non-primitive operators
associate to the right
have the same binding power
when to define a function as operator?
if operator is associative
ex
x ⊕ y ⊕ z
same as
radd (radd x y) z
radd x (radd y z)
```

#### 2.6.3 Inverse Functions

```
suppose for f::A->B
distinct values in A are mapped to distinct values in B
f x = f y iff x = y
injective: such a function is injective
inverse of f: f⁻¹
f⁻¹(f x) = x
ex:
f:: num -> (num, num)
f x = (sign x, abs x)
f⁻¹(s,a) = s * a
common in mathematical specifications: specify a function f by requirement that it is inverse of a given function g known to be injective
ex:
sqrt (square x) = x
this gives no hint as to how to compute
not executable (constructive) definition
surjective: for every y in B there exists some x in A st f x = y
f(f⁻¹y) = y
bijective: f is both injective and surjective
```

#### 2.6.4 Strict and non-strict functions

```
strict if f NaN = NaN
ex:
three :: num -> num
three x = 3
evaluations
three (1/0) = 3
three NaN ≠ NaN
non-strict semantics is preferable to strict
reasons
reasoning about equality easier
we can define new control structures by defining new functions
ex
cond p x y = x, if p
= y, otherwise
normal definition:
recip x = 0, if x = 0
= 1/x, otherwise
instead use non-strict definition:
recip x = cond (x = 0) 0 (1/x)
evaluation:
recip 0 = cond (0 = 0) 0 (1/0) = cond True 0 NaN = 0
with strict semantics:
recip 0 = cond (0 = 0) 0 (1/0) = cond True 0 NaN = NaN
tuples are non-strict:
(x,y)
fst(1+0, 1/0) = 1
eager vs. lazy evaluation:
fst(x,y)
eager
evaluate x, y first
lazy
evaluate fst first
```

### 2.7 Type synonyms

```
sometimes it is inconvenient to declare types of functions
type synonym: enables programmer to introduce a new name for a given type
ex:
position == (num,num)
angle == num
distance == num
definition of move:
move :: distance -> angle -> position -> position
move d a (x,y) = (x + d * cos a,y + d * sin a)
ex:
string == [char]
can be generic: parameterised by type variables
ex:
pairs α == (α,α)
automorph α == α -> α
```

### 2.8 Type inference

```
(·)f g x = f(g x)
f :: t1
g :: t2
x :: t3
f(g x) :: t4
(·) :: t1 -> t2 -> t3 -> t4
three rules
application rule
if f x :: t, then x::t' and f::t'->t for some new type t'
equality rule
function rule
@mine: concept map
[definition] - n [pattern]
[function] ^- [higher_order_function]
[higher_order_function] has a [function] as [argument] or returns one
[functional_composition] is [associative]
nonprimitive [operator] are [right_associative]
[function] is [injective]
if f x = f y iff x = y
[inverse] of a [function]
[function] is [surjective]
if ∀ y ∈ B => ∃ x ∈ A: f x = y
[bijective] if both [injective] and [surjective]
[strict]
if f Na = Na
[nonstrict]
if f Na ≠ Na
[nonstrict] is preferable to [strict]
[tuple] are [nonstrict]
[eager] vs. [lazy] evaluation
[type_synonym] is a new [name] for a given [type]
[type_synonym] can be [generic]
[generic] means that it can be parameterised by [type_variable]
```

## ch03: Lists

### 3.1 List notation

### 3.2 List comprehensions

```
uses a syntax adapted from conventional mathematics for describing sets
[〈expression〉| 〈qualifier〉;...;〈qualifier〉]
〈qualifier〉is either a boolean valued expression or a generator
〈variable〉<- 〈list〉
ex:
? [x*x|x <- [1..10]; even x]
[4,16,36,64,100]
? [(a,b)| a <- [1..3]; b <- [1..2]]
[(1,1),(1,2),(2,1)..]
multiple generators:
later generators vary more quickly than their predecessors
later generators can depend on earlier ones
ex:
?[(i,j)| i<-[1..2]; j <-[i+1..2]]
we can freely add generators with boolean valued expressions
ex:
?[(i,j)| i<-[1..2]; even i; j <-[i+1..2]]
ex: using comprehensions in other comprehensions
pairs = [(i,j)| i<-[1..2]; even i; j <-[i+1..2]]
[i+j | (i,j) <- pairs]
ex: using list comprehensions in definitions
spaces n = [' ' | j <- [1..n]]
ex: greatest common divisor
divisors n = [d | d <- [1..n]; n mod d = 0]
gcd a b = max[d | d <- divisors a; b mod d = 0]
ex: whether a number is prime
prime n = (divisors n = [1,n])
```

### 3.3 Operations on lists

```
concatenation: ++
(++) :: [α] -> [α] -> [α]
associative operations
has identity element: []
naming conventions:
x: element
xs: list of x
xss: list of xs
concat function: concatenates a list of lists into one long list
concat :: [[α]] -> [α]
concat xss = [x | xs <- xss; x <- xs]
length: #
(#)::[α]->num
head and tail: hd, tl
hd: first element
tl: select remaining portion
hd([x]++xs) = x
tl([x]++xs) = xs
hd[] = NaN
hd :: [α]->α
tl :: [α] -> [α]
relationship between hd and tl:
xs = [hd xs] ++ tl xs
init and last: break a list at the end
init(xs++[x]) = xs
last(xs++[x]) = x
take and drop:
?take 3 [1..10]
[1,2,3]
?drop 3 [1..10]
[4,5,6,7,8,9,10]
take n xs ++ drop n xs = xs
takewhile and dropwhile
reverse
zip
zip ::([α],[β]) -> [(α,β)]
?zip([0..2],"hel")
[(0,'h'),..]
use cases:
scalar product: vectors x and y
sp(xs,ys) = sum[x*y|(x,y)<-zip(xs,ys)]
alternative definition:
sp = sum · zipwith(*)
zipwith f (xs,ys) = [f x y|(x,y) <- zip(xs,ys)]
non-decreasing sequences: nondec function determines whether a sequence is in non-decreasing order
nondec xs = and[x≤y|(x,y) <- zip(xs, tl xs)]
position: position function returns position of first occurrence of x in xs
position :: [α] -> α -> num
positions xs x = [i | (i,y) <- zip([0..#xs-1],xs); x = y]
position xs x = hd(positions xs x ++ [-1])
list indexing: !
(!)::[α]->num->α
define (!) with zip
xs ! n = hd[y|(i,y) <- zip([0..#xs-1],xs); i = n]
define nondec with (!)
nondec xs = and[xs!k≤xs!(k+1)|k<-[0..#xs-2]]
list-difference: --
?[1,2,3,1]--[1,3]
[2,1]
concatenation and list-difference are related:
(xs ++ ys) -- xs = ys
tl xs = xs -- [hd xs]
drop n xs = xs -- take n xs
we can define the condition that one list is a permutation of another list:
permutation xs ys = (xs -- ys = []) ∧ (ys -- xs = [])
@mine: concept map
concatenation: ++
(++) :: [α] -> [α] -> [α]
concat:
concat :: [[α]] -> [α]
concat xss = [x | xs <- xss; x <- xs]
```

### 3.4 Map and filter

```
map: applies a function to each element of a list
map :: (α->β) -> [α] -> [β]
map f xs = [f x | x <- xs]
benefit of map: very simple and direct description
sum of squares of first 100 positive integers
sum(map square [1..100])
ex: mathematical notation of sigma
sigma f m n = sum(map f [m..n])
sumsquares = sigma square 1 100
useful algebraic identities concerning map:
map(f·g) = (map f)·(map g)
if we apply g to every element of a list, and then apply f to each element of the result, then the same effect is obtained by applying (f·g) to original list
ie. map distributes through functional composition
consequence: if f has an inverse f⁻¹:
(map f)⁻¹ = map f⁻¹
filter: takes a predicate p and a list xs and returns sublist xs whose elements satisfy p
define by list comprehension:
filter :: (α->bool) -> [α] -> [α]
filter p xs = [x|x<-xs; p x]
ex:
filter even [1,4,3]
(sum · map square · filter even) [1..10]
benefits:
concise and simple: sums of squares of even numbers in range 1 to 10
(sum · map square · filter even) [1..10]
useful identities:
filter p (xs ++ ys) = filter p xs ++ filter p ys
filter p · concat = concat · map (filter p)
filter p · filter q = filter q · filter p
first law: filter distributes through concatenation
second law: generalises this distributive property to lists of lists
third law: filters can be applied in any order
ex: second law
xss = [[1,2],[4,3]]
p = even
filter p · concat xss = filter p [1,2,4,3]
= [2,4]
concat · map (filter p) xss = concat [[2],[4]]
= [2,4]
translating comprehensions:
there is a close relationship between functions map and filter and notation of list comprehensions
we can define list comprehensions in terms of map, filter and concat
4 rules for conversion:
(1) [x| x <- xs] = xs
(2) [f x| x <- xs] = map f xs
(3) [e| x <- xs; p x; ...] = [e| x <- filter p xs; ...]
(4) [e| x <- xs; y <- ys; ...] = concat [[e| y <- ys; ...] | x <- xs]
ex: translate: [1| x <- xs]
introduce function const
const k x = k
[1| x <- xs] = [const 1 x| x <- xs]
= map (const 1) xs
ex: translate: [x| x <- xs; x = min xs]
introduce:
minof xs x = (x = min xs)
[x| x <- xs; x = min xs]
= [x| x <- xs; minof xs x]
= [x| x <- filter (minof xs) xs]
= filter (minof xs) xs
ex: translate: [x| xs <- xss; x <- xs]
[x| xs <- xss; x <- xs]
= concat [[x | x <- xs] | xs <- xss] '(4)
= concat [xs | xs <- xss] '(1)
= concat xss '(1)
ex: translate: [(i,j)|i <- [1..n]; j <- [i+1..n]]
[(i,j)|i <- [1..n]; j <- [i+1..n]]
= concat [[(i,j) | j <- [i+1..n]] | i <- [1..n]] '(4)
= concat [map (pair i) [i+1..n]] | i <- [1..n]] '(2)
= concat (map mpair [1..n]) '(2)
where
pair i j = (i,j)
mpair i = map (pair i)[i+1..n]
comparison:
list comprehensions: clear and simple
no need to invent special names for subsidiary functions that arise with map and filter
higher order functions:
easier to state algebraic properties
use these properties in the manipulation of expressions
@mine: concept map
[map] distributes over [composition]
like: [multiplication] distributes over [addition]
map(f·g) = (map f)·(map g)
z*(x+y) = z*x + z*y
```

### 3.5 The fold operators

```
previous operations: return lists
fold operators: they can convert lists into other kinds of values as well
foldr:
foldr f a [x1, x2, ..., xn] = f x1 (f x2 (...(f xn a) ...))
ex:
foldr (⊕) a [x1,..,xn] = x1 ⊕ ( x2 ⊕ (...(xn ⊕ a)...))
second arg of ⊕ must have same type as the result of ⊕
first arg may have a different type
foldr :: (α -> β -> β) -> β -> [α] -> β
if first arg is of different type then foldr operator can't be associative
using foldr we can define:
sum = foldr (+) 0
product = foldr (x) 1
concat = foldr (++) []
and = foldr (∧) True
or = foldr (∨) False
common in all examples:
function ⊕ is associative
has identity element a
ie.
x ⊕ (y ⊕ z) = (x ⊕ y) ⊕ z
x ⊕ a = x = a ⊕ x
monoid: ⊕ and a form a monoid
if ⊕ and a form a monoid, then:
foldr (⊕) a [] = a
foldr (⊕) a [x1,...,xn] = x1 ⊕ ... ⊕ xn
ex where args to foldr is not a monoid:
ex: #
(#) = foldr oneplus 0
where oneplus x n = 1 + n
this defines the length of a list by counting one for each element
oneplus :: α -> num -> num
so it cannot be associative
ex:
reverse = foldr postfix []
where postfix x xs = xs ++ [x]
takewhile p = foldr (⊕) []
where x ⊕ xs = [x] ++ xs, if p x
= [], otherwise
ex:
takewhile (<3) [1..4] = 1 ⊕ (2 ⊕ (3 ⊕ (4 ⊕ [])))
= [1] ++ ([2] ++ ([]))
= [1,2]
fold left: foldl
foldl (⊕) a [x1, ..., xn] = (...((a⊕x1)⊕x2)...)⊕ xn
ex:
foldl (⊕) a [x1,x2] = (a ⊕ x1) ⊕ x2
foldl :: (β->α->β)->β->[α]->β
uses:
pack xs = foldl (⊕) 0 xs
where n ⊕ x = 10 * n + x
this codes a sequence of digits as a single number, most significant digit comes first
difference between foldr and foldl
Both fold-left and fold-right reduces one or more list with a reducing procedure from first to last element, but the order it is applied is kept in fold-right while it is reversed in fold-left. It's
@mine: concept map
[foldr] is for right-associative [operator]
[foldr] applies [function] in reverse order of the list argument
[foldl] is for left-associative [operator]
[foldl] applies [function] in same order of the list argument
foldr :: (α -> β -> β) -> β -> [α] -> β
second [argument] has same [type] as result of [function]
because foldr (⊕) a [x1,..,xn] = x1 ⊕ ( x2 ⊕ (...(xn ⊕ a)...))
a is the last argument in the open form
@mine: mnemonics
symmetry of reverse functions
opt1: foldr
reverse = foldr postfix []
where postfix x xs = xs ++ [x]
opt2: foldl
reverse = foldl prefix []
where prefix xs x = [x] ++ xs
opt1 case
foldr reverses order of operations on xs
(x1 ++ (x2 ++ [])) = (x1 ++ [x2]) = [x2 x1]
because of right association
xs is second argument
xs contains last first
postfix puts elements to the end of the resulting list
opt2 case
foldl keeps order of operations on xs
(([] ++ x1) ++ x2) = ([x1] ++ x2) == [x2 x1]
because of left association
xs is first argument
xs contains first first
```

#### 3.5.1 Laws

```
important laws:
three duality theorems
first duality theorem
foldr (⊕) a xs = foldl (⊕) a xs
whenever ⊕ and a form a monoid and xs is a finite list
second duality theorem
generalisation of the first
suppose:
x ⊕ (y ⨂ z) = (x ⊕ y) ⨂ z
⊕ and ⨂ associate with each other
x ⊕ a = a ⨂ x
foldr (⊕) a xs = foldl (⨂ ) a xs
special case: (⊕) = (⨂ )
first duality theorem follows
uses:
(#) = foldl plusone 0
where plusone n x = n + 1
reverse:
reverse = foldl prefix []
where prefix xs x = [x] ++ xs
foldr postfix [] xs = foldl prefix [] xs
third duality theorem:
foldr (⊕) a xs = foldl (⊕') a (reverse xs)
⊕' is defined by:
x ⊕' y = y ⊕ x
ex: reversing prefix: cons function
prefix xs x = [x] ++ xs
cons x xs = [x] ++ xs
thus:
xs = foldr cons [] xs
foldr cons [] xs = foldl prefix [] (reverse xs)
xs = reverse (reverse xs)
other uses:
if ⊕ and a form a monoid:
foldr (⊕) a (xs ++ ys) = (foldr (⊕) a xs) ++ (foldr (⊕) a ys)
foldl f a (xs ++ ys) = foldl f (foldl f a xs) ys
```

#### 3.5.2 Fold over non-empty lists

```
say: find max element of a list
max = foldl (max) a
question: what should a be?
better if a is identity element
if xs contains negative numbers, there is no such a
introduce new functions: foldl1, foldr1
foldl1 (⊕) [x1 .. xn] = (..((x1 ⊕ x2) ⊕ x3) ..) ⊕ xn
foldr1 (⊕) [x1, x2, ..., xn] = (x1 ⊕ ( .. (xn-1 ⊕ xn) ..))
remember foldl:
foldl (⊕) a [x1, ..., xn] = (...((a⊕x1)⊕x2)...)⊕ xn
in particular:
foldl1 (⊕) [x1] = x1
foldl1 (⊕) [x1, x2] = x1 ⊕ x2
foldl1 (⊕) [x1, x2, x3] = (x1 ⊕ x2) ⊕ x3
foldr1 (⊕) [x1, x2, x3] = x1 ⊕ (x2 ⊕ x3)
foldl1, foldr1 :: (α -> α -> α) -> [α] -> α
max = foldl1 (max)
defining foldl1 in terms of foldl:
foldl1 (⊕) xs = foldl (⊕) (hd xs) (tl xs)
foldr1 (⊕) xs = foldr (⊕) (last xs) (init xs)
```

#### 3.5.3 Scan

```
scan: apply fold left to every initial segment of a list
scan (⊕) a [x1, x2, ...] = [a, a ⊕ x1, (a ⊕ x1) ⊕ x2, ...]
last value is: foldl (⊕) a xs
thus: foldl (⊕) a = last · scan (⊕) a
scan (+) 0 [1,2,3,4,5] = [0,1,3,6,10,15]
```

### 3.6 List patterns

```
pattern matching with list arguments
introduce: (:) cons (construct)
(:) :: α -> [α] -> [α]
inserts a value as a new first element of a list
? 1 : []
[1]
? 1:2:[3,4]
[1,2,3,4]
(:) associates to the right
x:xs = [x] ++ xs
difference from ++:
(:) every list can be expressed in exactly one way
that is why it can do pattern matching
hd(x:xs) = x
tl(x:xs) = xs
define: null test
null[] = True
null(x:xs) = False
define: single if list contains a single element
single [] = False
single [x] = True
single (x:y:xs) = False
note that: [x] is abbreviation for x:[]
note that: three cases are exhaustive and disjoint
```

## ch04: Examples

### 4.1 Converting numbers to words

```
ex: (convert): translate number to its English formulation
```

### 4.2 Variable-length arithmetic

### 4.3 Text processing

### 4.4 Turtle graphics

### 4.5 Printing a calendar

## ch05: Recursion and Induction

```
every natural number is
either 0
or (n + 1)
every list is
either []
or (x:xs)
operator (#) defined by recursion
#[] = 0
#(x:xs) = 1 + (#xs)
concatenation by recursion
[] ++ ys = ys
(x:xs) ++ ys = x : (xs ++ ys)
prove by induction for lists:
P(xs) holds if
case []. P([]) holds
case (x:xs). if P(xs) => P(x:xs) for every x
proof for: concatenation is associative:
claim: xs ++ (ys ++ zs) = (xs ++ ys) ++ zs
proof: induction on xs
case [].
[] ++ (ys ++ zs) = ys ++ zs
= ([] ++ ys) ++ zs
establishes the case
case (x:xs).
(x:xs) ++ (ys ++ zs) = x : (xs ++ (ys ++ zs)) (++.2)
= x: ((xs++ys) ++ zs) (hypothesis)
= (x : (xs ++ ys)) ++ zs (++.2)
= ((x:xs) ++ ys) ++ zs (++.2)
```

### 5.3 Operations on lists

#### 5.3.1 Zip

```
recursive definition of zip:
zip ([], ys] = []
zip (x:xs, []) = []
zip (x:xs, y:ys) = (x,y) : zip (xs, ys)
show that:
#zip (xs,ys) = (#xs) min (#ys)
proof.
case [], ys.
#zip ([],ys) = #[] (zip.1)
= 0 (#.1)
= 0 min (#ys)
establishes the case
case (x:xs), [].
case (x:xs), (y:ys).
...
```

#### 5.3.5 Map and filter

```
recursive definition of map:
map f [] = []
map f (x:xs) = f x : map f xs
recursive filter:
filter p [] = []
filter p (x:xs) = x:filter p xs, if p x
= filter p xs, other wise
ex: filter odd (map square [2,3])
= filter odd (4:map square [3]) (map.2)
= filter odd (4:9) (map.2)
= filter odd (9) (filter.3)
= 9: filter odd [] (filter.2)
= 9:[] (filter.1)
= [9]
one law:
filter p (map f xs) = map f (filter (p·f) xs)
```

### 5.4 Auxiliaries and generalisation

#### 5.4.1 List difference

```
recursive definition of -- (list difference):
definition:
xs -- [] = xs
xs -- (y:ys) = remove xs y -- ys
remove [] y = []
remove (x:xs) y = xs, if x = y
= x:remove xs y, otherwise
remove is auxiliary function for (--)
```

#### 5.4.2 Reverse

```
definition:
reverse [] = []
reverse (x:xs) = reverse xs ++ [x]
```

#### 5.4.3 Second duality theorem

```
recursive definitions:
foldr (⊕) a [] = a
foldr (⊕) a (x:xs) = x ⊕ (foldr (⊕) a xs)
foldl (⊕) a [] = a
foldl (⊕) a (x:xs) = foldl (⊕) (a ⊕ x) xs
second duality theorem:
let (⊕) and (⨂) be binary operators and a a value st:
x ⊕ (y ⨂ z) = (x ⊕ y) ⨂ z
⊕ and ⨂ associate with each other
x ⊕ a = a ⨂ x
then:
foldr (⊕) a xs = foldl (⨂ ) a xs
```

### 5.6 Combinatorial functions

```
combinatorial:
they involve selecting or permuting elements of a list in some manner
initial segments:
inits returns all initial segments of a list
ex
inits "era"
["", "e", "er", "era"]
definition
inits [] = [[]]
inits (x:xs) = [[]] ++ map (x :) (inits xs)
subsequences:
ex:
subs "era"
["", "a", "r", "ra", "e", "ea", "er", "era"]
definition:
subs [] = [[]]
subs (x:xs) = subs xs ++ map (x :) (subs xs)
interleave:
all possible ways of inserting an element into a list
ex:
interleave "e" "ar"
["ear", "aer", "are"]
definition
interleave x [] = [[x]]
interleave x (y:ys) = [x:y:ys] ++ map (y :) (interleave x ys)
permutations:
ex:
perms "era"
["era", "rea", "rae", "ear", "aer", "are"]
definition:
perms [] = [[]]
perms (x:xs) = concat (map (interleave x) (perms xs))
partitions:
xss is a partition of xs if:
concat xss = xs
ex: ["era"] and ["e","ra"] are partitions of "era"
parts: returns all partitions of a list
ex:
parts "era"
[["era"], ["er","a"],...]
```

## ch07: Infinite Lists

```
applications:
infinite lists can be used to model sequences of events ordered in time
ex: interactive processes
```

### 7.1 Infinite lists

### 7.1 Infinite lists

```
[1..]
take n [1..] = [1..n]
[m..]!n = m + n
map factorial [1..] = scan (*) 1 [1..]
filter (<10) (map (^2) [1..])
[1,4,9
doesn't finish
takewhile (<10) (map (^2) [1..])
[1,4,9]
```

### 7.2 Iterate

```
fⁿ: a function composed with itself n times
f⁰ = id
f¹ = f
f² = f · f
write fⁿ as (power f n)
definition of power:
power f 0 = id
power f (n+1) = f · power f n
iterate:
iterate f x = [x, f x, f² x, ...]
ex
iterate (+1) 1 = [1,2,3,...]
iterate (*2) 1 = [1,2,4,...]
iterate (div10) 2718 = [
uses
[m..] = iterate (+1) m
[m..n] = takewhile (≤n) (iterate (+1) m)
digits = reverse · map (mod10) · takewhile (≠ 0) · iterate (div10)
extract digits of a positive integer
ex
digits 2718
= reverse · map (mod10) · takewhile (≠ 0) [2718,271,27,2,0,0...]
= reverse · map (mod10) · takewhile (≠ 0) [2718,271,27,2]
= reverse [8,1,7,2]
= [2,7,1,8]
group n = map (take n) · takewhile (≠ []) · iterate (drop n)
break a list into segments of length n
uses with map, takewhile, iterate
corresponds to
while (arg != [])
arg = drop n arg
result += take n arg
capture this patterns as a generic function: unfold
unfold h p t = map h · takewhile p · iterate t
corresponding operations:
hd (unfold h p t x) = h x
thus h of unfold corresponds to hd
tl (unfold h p t x) = unfold h p t (t x)
nonnull (unfold h p t x) = p x
definition of iterate:
opt1: inefficient
iterate f x = [power f i x | i <- [0..]]
computes each of (power f 0 x), (power f 1 x) independently
opt2: recursive
iterate f x = x : iterate f (f x)
ex:
iterate (*2) 1 = 1 : iterate (*2) ((*2) 1)
= 1:2:iterate (*2) ((*2) 2)
similar to scan:
both compute each element of the output list in terms of preceding element
```

### 7.3 Example: generating primes

### 7.4 Infinite lists as limits

```
example problem:
filter (<10) (map (^2) [1..])
[1:4:9:Na]
why is this so?
a theory of infinite lists is required
limits: one way of dealing with infinite objects
infinite object: limit of an infinite sequence of approximations
ex: infinite list [1..] is limit of the following sequence:
Na
1:Na
1:2:Na
...
```

### 7.5 Reasoning about infinite lists

```
lists:
partial list: any list ending in bottom
any list of the form x1:x2:...:xn:Na
thus: every infinite list is the limit of an infinite sequence of partial lists
finite list: ends in []
ex: 1:2:3:[] = [1,2,3]
partial list: ends in Na
infinite list: does not end at all
ex: [1,2,3,...]
continuity property:
if xs1,xs2,xs3,... is an infinite sequence
with limit xs and f is a computable function
then f xs1, f xs2, ... is an infinite sequence whose limit is f xs
ex: [1..] is the limit of the sequence given above
we can compute map (*2) [1..] as:
map (*2) Na = Na
map (*2) (1:Na) = 2:Na
map (*2) (1:2:Na) = 2:4:Na
..
limit of this sequence is infinite list: [2,4,6,...]
note that:
no equation defining map matches map (*2) Na because Na does not match either [] or (x:xs)
this follows from case exhaustion on map
thus we have:
map (*2) (1:Na) = (1*2):(map (*2) Na) (map.2)
= 2:Na (map.0)
(map.0) indicate case exhaustion on map
consider expression: filter (<10) (map (^2) [1..])
applying filter (<10) gives sequence:
filter (<10) Na = Na
filter (<10) (1:Na) = 1:Na
filter (<10) (1:4:Na) = 1:4:Na
filter (<10) (1:4:9:Na) = 1:4:9:Na
filter (<10) (1:4:9:16:Na) = 1:4:9:Na
filter (<10) (1:4:9:16:25:Na) = 1:4:9:Na
...
limit is: 1:4:9:Na
applying takewhile (<10) gives sequence:
takewhile (<10) Na = Na
takewhile (<10) (1:Na) = 1:Na
takewhile (<10) (1:4:Na) = 1:4:Na
takewhile (<10) (1:4:9:Na) = 1:4:9:Na
takewhile (<10) (1:4:9:16:Na) = 1:4:9:[]
takewhile (<10) (1:4:9:16:25:Na) = 1:4:9:[]
...
limit: 1:4:9:[]
```

### 7.6 Cyclic structures

```
data structures can be defined recursively
consider: ones = 1:ones
ones = 1:ones
= 1:1:ones
= 1:1:1:ones
...
so ones is bound to infinite list [1,1,1,...]
memory representation:
1:->1
consider: more
more = "More " ++ andmore
where andmore = "and more " ++ andmore
```

#### 7.6.1 Forever

```
let forever x be infinite list [x,x,...]
then:
ones = forever 1
definition:
opt1:
forever x = x:forever x
correct but doesn't create a cyclic structure
opt2:
forever x = zs
where zs = x:zs
this produces cyclic structure
```

#### 7.6.2 Iterate

```
definition that uses cyclic structure:
iterate f x = zs
where zs = x : map f zs
```

### 7.8 Interactive programs

```
interactive program:
f::[char] -> [char]
ex: echo user input by capitalising
capitalises = map capitalise
capitalise x = decode (code x + offset), if 'a' ≤ x ∧ x ≤ 'z'
= x, otherwise
where offset = code 'A' - code 'a'
ex: terminating an interactive program
capitalises' = takewhile (≠ '#') · capitalises
```

#### 7.8.1 Modelling interaction

```
consider sequences of partial lists:
capitalises Na = Na
capitalises ('H':Na) = 'H':Na
capitalises ('H':'e':Na) = 'H':'e':Na
techniques used to model infinite lists are suitable for modelling interactive programs
```

#### 7.8.2 Hangman

## ch08: New Types

```
3 ways to combine types:
function space operator -> to form the type of functions
tupling operator (α, ..., β) to form tuples of types
list construction operator [α] to form lists of values of a given type
4. way: construct new types directly
```

### 8.1 Enumerated types

```
type definition: explicit enumeration of the values of a new type:
ex: type day:
day ::= Sun | Mon | Tue | Wed | Thu | Fri | Sat
ubiquitous Na: assumed to be a value of every type
constructor function: 7 new constants of the type day
convention
constructor: first letter capital
variable, value, type: first letter small
```

### 8.2 Composite types

```
types whose values depend on those of other types
ex:
tag ::= Tagn num | Tagb bool
constructors denote a function:
Tagn :: num -> tag
Tagb :: bool -> tag
ex: (Tagn 3) is a value of type tag
differences of constructors from other functions:
there is no definition associated with constructor functions
they are primitive conversion functions whose role is just to change values of one type into another
expressions of the form (Tagn n) can appear as patterns on the lhs of definitions
ex:
numval :: tag ->
numval (Tagn n) = n
boolval :: tag -> bool
boolval (Tagb b) = b
ex: functions discriminating between alternatives of the type
isNum (Tagn n) = True
isNum (Tagb b) = False
properties:
isNum Na = Na
isNum (Tagn Na) = True
isNum (Tagb Na) = False
ex: temp
opt1: union type
temp ::= Celsius num | Fahrenheit num | Kelvin num
three alternatives
one component type: num
comparing temparatures: requires an explicit equality test
eqtemp :: temp -> temp -> bool
opt2: type synonym
temp == (tag, num)
tag ::= Celsius | Fahrenheit | Kelvin
opt3: non-union type with multi argument constructor
temp ::= Temp tag num
tag ::= Celsius | Fahrenheit | Kelvin
temp: new type
non-union type: has only one alternative
ex: file type
file ::= File [record]
values have form (File rs) where rs is a list of records
to delete a record from a file:
delete :: record -> file -> file
delete r (File rs) = File (rs -- [r])
other file processing functions:
empty :: file
insert :: record -> file -> file
merge :: file -> file -> file
advantage of defining file as a special type:
additional measure of security
synonym definition:
file == [record]
then every operation on lists is valid for files, no protection against misuse
```

#### 8.2.1 Polymorphic types

```
type definitions can be parameterised by type variables
ex:
file α ::= File [α]
previous type file: (file record)
ex:
pair α β ::= Pair α β
ex:
Pair 3 True :: pair num bool
Pair [3] 0 :: pair [num] num
```

#### 8.2.2 General form

```
t α₁ α₂ ... ::= C1 t11 ... t1k1 | ... | Cn tn1 ... tnkn
C1, ... Cn are constructor names
tij: type expression
```

### 8.3 Recursive types

much of the power of the type mechanism comes from the ability to define recursive types

#### 8.3.1 Natural numbers

```
nat ::= Zero | Succ nat
ex: values of nat
Zero
Succ Zero
Succ (Succ Zero)
every natural number is represented by a unique value of nat
arithmetic functions can be defined in terms of nat
n ⊕ Zero = n
n ⊕ (Succ m) = Succ (n ⊕ m)
```

#### 8.3.2 Lists as a recursive type

```
recursive type definition of lists:
list α ::= Nil | Cons α (list α)
Nil is a value of (list α), and (Cons x xs) is a value of (list α) whenever x is a value of α and xs is a value of (list α)
ex:
Na
Nil
Cons Na Nil
values of type (list α) have 1-1 correspondence with values of type [α]
thus: lists need not be primitive type
partial and infinite lists:
Na
Cons 1 Na
Cons 1 (Cons 2 Na)
...
```

#### 8.3.3 Arithmetic expressions as a recursive type

```
different from previous examples:
this is non-linear
non-linear types: trees
if e is an arithmetic expression, then either:
e is a number; or
e is an expression of the form: e1 ⊕ e2
where e1 and e2 are arithmetic expressions
⊕ is one of + - * /
type definition:
aexp ::= Num num | Exp aexp aop aexp
aop ::= Add | Sub | Mul | Div
two constructors: Num and Exp
aexp is non-linear
because aexp appears in two places
ex:
(3+4)
Exp (Num 3) Add (Num 4)
3+4*5
Exp (Num 3) Add (Exp (Num 4) Mul (Num 5))
convert values of type aexp to numbers:
eval :: aexp -> num
eval (Num n) = n
eval (Exp e1 op e2) = apply op (eval e1) (eval e2)
subsidiary function: apply is defined by cases:
apply Add = (+)
apply Sub = (-)
apply Mul = (*)
apply Div = (/)
```

### 8.4 Abstract types

```
introducing a new type
= naming its values
except functions
each value = unique expression in terms of constructors
Using definition by pattern matching
expressions can be generated
there is no need to name the operations associated with the type
concrete types:
types in which values are prescribed but the operations are not
abstract types:
reverse
defined by naming its operations
the meaning of each operation has to be described
approach opt1: algebraic specification
states relationships between operations as a set of algebraic laws
approach opt2: describe each operation in terms of the most abstract representation
```

#### 8.4.1 Abstraction functions

```
representing one class of values by another:
abstraction function abstr
abstr:: B -> A
if abstr b = a, then b represents the value a
requirement:
abstr is surjective
every value in A has one representation b
ex: correspondence between [α] and (list α)
abstr :: (list α) -> [α]
abstr Nil = []
abstr (Cons x xs) = x : abstr xs
ex: between num and nat
abstr :: nat -> num
abstr Zero = 0
abstr (Succ x) = abstr x + 1
not bijective:
every natural number has corresponding nat
ex: mathematical sets and lists
abstr :: [α] -> set α
abstr [] = {}
abstr (x:xs) = {x} ∪ abstr xs
```

#### 8.4.2 Valid representations

```
the function abstr:: B -> A gives complete information
about how elements of A are represented by elements of B
in particular if (abstr b) is well-defined, then b is said to be a valid representation
explicit predicate: valid
determines what representations are valid
intention:
arguments to abstr to be restricted for which (valid x) is true
ex: representing sets by lists
abstr :: [α] -> set α
abstr [] = {}
abstr (x:xs) = {x} ∪ abstr xs
the following 3 definitions of a valid representation:
valid1 xs = True
valid2 xs = nodups xs
valid3 xs = nondec xs
pairs (abstr, valid1), (abstr, valid2), (abstr, valid3) describe 3 different representations
```

#### 8.4.3 Specifying operations

```
ex: addset adds an element x to a set s
addset :: α -> set α -> set α
addset x s = {x} ∪ s
insert implements this operation in terms of lists
insert :: α -> [α] -> [α]
specified by the equation:
abstr (insert x xs) = addset x (abstr xs)
specifications are pictured as commuting diagrams
set---------------->set
^ addset x ^
| |
abstr | | abstr
| |
| insert x |
list--------------->list
diagram commutes
both ways lea to the same result
thus we require:
abstr · (insert x) = (addset x) · abstr
possible implementations of insert:
insert1 x xs = [x] ++ xs
insert2 x xs = [x] ++ [y| y <- xs; x ≠ y]
insert3 x xs = takewhile (<x) xs ++ [x] ++ dropwhile (≤x) xs
implementations for:
insert1: (abstr,valid1)
insert2: (abstr,valid2)
insert3: (abstr,valid3)
```

#### 8.4.4 Queues

```
suppose (queue α) is a collection of values of type α with following operations:
start :: queue α
join :: queue α -> α -> queue α
front :: queue α -> α
reduce :: queue α -> queue α
relationships between operations:
front (join start x) = x
front (join (join q x) y) = front (join q x)
reduce (join start x) = start
these equations constitute algebraic specification
```

#### 8.4.5 Arrays

```
abstract type defined by the following for operations:
mkarray :: [α] -> array α
length :: array α -> num
lookup :: array α -> num -> α
update :: array α -> num -> α -> array α
relationships between operations:
length (mkarray xs) = #xs
length (update A k x) = length A
lookup (mkarray xs) k = xs ! k
lookup (update A j x) = x, if j = k
= lookup A k, otherwise
```

## ch09: Trees

```
any data-type that exhibits a non-linear (branching) structure is called a tree
```

### 9.1 Binary trees

```
two-way branching structure
btree α ::= Tip α | Bin (btree α) (btree α)
ex:
Bin (Tip 1)
(Bin (Tip 2) (Tip 3))
```