1
00:00:01,750 --> 00:00:03,590
hi everybody a gum

2
00:00:03,590 --> 00:00:06,890
to another app is out of at the

3
00:00:06,890 --> 00:00:11,179
100 max and what we're going to do today

4
00:00:11,179 --> 00:00:14,750
is we're going to look how to define

5
00:00:14,750 --> 00:00:18,260
functions in Haskell nom

6
00:00:18,260 --> 00:00:21,660
I think everybody %uh view that

7
00:00:21,660 --> 00:00:25,240
has been to college in a different town

8
00:00:25,240 --> 00:00:28,420
them where they grew up has had

9
00:00:28,420 --> 00:00:31,800
this experience you know in one way or
another

10
00:00:31,800 --> 00:00:35,690
you leave your own down where you grew
up he went to

11
00:00:35,690 --> 00:00:40,420
elementary school you went to highschool
you go off for college

12
00:00:40,420 --> 00:00:43,570
and then ten years later you come back

13
00:00:43,570 --> 00:00:46,980
and thinks have changed slightly

14
00:00:46,980 --> 00:00:50,020
but they're also a little bit the same

15
00:00:50,020 --> 00:00:53,460
and that can be a little bit
disorientating because you know

16
00:00:53,460 --> 00:00:56,809
your memory thanks you know were

17
00:00:56,809 --> 00:01:00,500
like they were but a reality they have
changed

18
00:01:00,500 --> 00:01:04,070
slightly and well

19
00:01:04,070 --> 00:01:07,720
you whilst get the same experience when
you look

20
00:01:07,720 --> 00:01:11,939
at haskell haskell looks very familiar

21
00:01:11,939 --> 00:01:16,229
to other programming languages again
differs in subtle ways

22
00:01:16,229 --> 00:01:19,409
and goal of this lecture is to

23
00:01:19,409 --> 00:01:23,020
walked through some of the syntax of has
goal

24
00:01:23,020 --> 00:01:26,119
and function definition and expressions

25
00:01:26,119 --> 00:01:29,759
and compare and contrast the wit

26
00:01:29,759 --> 00:01:33,979
a.m. how you do these things and other
programming languages

27
00:01:33,979 --> 00:01:37,000
and once you're aware of these
differences

28
00:01:37,000 --> 00:01:40,650
it will be fairly easy to adopted them
but

29
00:01:40,650 --> 00:01:45,070
there are some things where haskell is
definitely a little bit

30
00:01:45,070 --> 00:01:49,600
different than an what

31
00:01:49,600 --> 00:01:52,740
most people are used to in Java or

32
00:01:52,740 --> 00:01:58,159
in BHP or whatever your favorite
languages

33
00:01:58,159 --> 00:02:01,530
so on

34
00:02:01,530 --> 00:02:06,370
the first thing is conditional
expressions and every language has

35
00:02:06,370 --> 00:02:12,160
conditional expressions am and often
there's a difference between conditional

36
00:02:12,160 --> 00:02:13,170
statements

37
00:02:13,170 --> 00:02:18,080
that are yet ready using if they
announce and conditional expressions

38
00:02:18,080 --> 00:02:21,160
turn their expressions that are written
in red question mark

39
00:02:21,160 --> 00:02:26,300
golan and other languages do have if
their nails for expressions at all for

40
00:02:26,300 --> 00:02:27,520
example in Scala

41
00:02:27,520 --> 00:02:30,700
but if you're using C sharp or

42
00:02:30,700 --> 00:02:34,970
job on there there's the conditional
statements

43
00:02:34,970 --> 00:02:38,570
and then for conditional expressions
there's

44
00:02:38,570 --> 00:02:42,540
different index right haskell since
there's no statements

45
00:02:42,540 --> 00:02:46,160
an when you want to ride a conditional
you ride

46
00:02:46,160 --> 00:02:49,930
if than else nation this

47
00:02:49,930 --> 00:02:53,580
gauge or defining a function addiction
integer

48
00:02:53,580 --> 00:02:57,250
and breeders another integer and its
defiant

49
00:02:57,250 --> 00:03:00,470
Adonis is the absolute value sure it
takes

50
00:03:00,470 --> 00:03:04,330
value as as if that value is greater
than 0

51
00:03:04,330 --> 00:03:07,989
then you return an else you return

52
00:03:07,989 --> 00:03:13,570
minus and you'd negate value so here you
see that we're using the familiar

53
00:03:13,570 --> 00:03:20,370
if the house as an expression for an

54
00:03:20,370 --> 00:03:23,880
of course since it's just an expression

55
00:03:23,880 --> 00:03:27,000
they can be and nested as well

56
00:03:27,000 --> 00:03:30,870
and sensed conditions conditions are
expressions

57
00:03:30,870 --> 00:03:34,160
you always have to have a Dan and else
part

58
00:03:34,160 --> 00:03:38,370
a ride so in this case you can define

59
00:03:38,370 --> 00:03:42,700
and the sign of a number save and is
less than 0

60
00:03:42,700 --> 00:03:46,360
its -1 it and is zero

61
00:03:46,360 --> 00:03:52,130
and assigned 0 and else and and is
greater than zero then the sign

62
00:03:52,130 --> 00:03:56,590
is one alright so you always have to
have both

63
00:03:56,590 --> 00:04:00,760
then and else branch just like in turney
expressions

64
00:04:00,760 --> 00:04:04,340
I'll an

65
00:04:04,340 --> 00:04:07,600
because has killed people like brevity

66
00:04:07,600 --> 00:04:12,170
they often don't use conditional
expressions but the you show called

67
00:04:12,170 --> 00:04:14,069
guarded equations

68
00:04:14,069 --> 00:04:16,530
and the guard integration is

69
00:04:16,530 --> 00:04:21,590
kept her common pattern where a function
starts with a conditional saw

70
00:04:21,590 --> 00:04:26,620
if the right hand side over function
definition immediately has a conditional

71
00:04:26,620 --> 00:04:32,030
it's often nicer to use this form with
guard integration sure you write your

72
00:04:32,030 --> 00:04:34,090
function definition your left hand side

73
00:04:34,090 --> 00:04:37,370
and then you're right you're conditions
here so

74
00:04:37,370 --> 00:04:40,760
the absolute value of an is

75
00:04:40,760 --> 00:04:44,750
an when and is greater than equal r0

76
00:04:44,750 --> 00:04:49,840
arch minus an otherwise and otherwise is
just

77
00:04:49,840 --> 00:04:53,300
another alias for false now let's

78
00:04:53,300 --> 00:04:57,280
go back to and let's go back to

79
00:04:57,280 --> 00:05:03,610
definition here up the absolute value
using conditional

80
00:05:03,610 --> 00:05:07,060
and here you see what I said is that
this is a function

81
00:05:07,060 --> 00:05:10,630
this defiant immediately using

82
00:05:10,630 --> 00:05:13,680
a conditional and

83
00:05:13,680 --> 00:05:18,479
and haskell people prefer to ride it
like this and I see that the right hand

84
00:05:18,479 --> 00:05:19,110
side

85
00:05:19,110 --> 00:05:22,330
is simpler and the cases are

86
00:05:22,330 --> 00:05:26,020
more distinguished you going i'm easily
reason to buy this

87
00:05:26,020 --> 00:05:29,150
its greater than 0 if it's not

88
00:05:29,150 --> 00:05:36,150
it its am less than zero and the other
function especially when you have nested

89
00:05:36,440 --> 00:05:37,380
conditionals

90
00:05:37,380 --> 00:05:40,780
this becomes even more worthwhile

91
00:05:40,780 --> 00:05:44,919
so in order to define the sign up a
number average less than zero

92
00:05:44,919 --> 00:05:48,160
-1 its equal 200

93
00:05:48,160 --> 00:05:52,160
an otherwise is Wong here you see

94
00:05:52,160 --> 00:05:55,770
they got mathematical heritage of has
call

95
00:05:55,770 --> 00:05:58,970
where we like to write things in a very

96
00:05:58,970 --> 00:06:02,970
concise way and these guarded equations

97
00:06:02,970 --> 00:06:06,020
are something that are very

98
00:06:06,020 --> 00:06:09,729
idiomatic has so typically you don't
ride

99
00:06:09,729 --> 00:06:12,770
if they announce you ride an

100
00:06:12,770 --> 00:06:18,280
these guarded equations

101
00:06:18,280 --> 00:06:20,720
another thing that and

102
00:06:20,720 --> 00:06:24,210
is very heavily used in Haskell

103
00:06:24,210 --> 00:06:27,500
and that few contemporary languages

104
00:06:27,500 --> 00:06:30,970
have its better matching an

105
00:06:30,970 --> 00:06:34,310
and scholar for example has better
matching

106
00:06:34,310 --> 00:06:38,520
there you have to use a a case statement
to do to better matching

107
00:06:38,520 --> 00:06:42,650
and she sharp will get better matching
is uses the switch statement

108
00:06:42,650 --> 00:06:46,420
but it has cool you can use better
matching directly do when you define

109
00:06:46,420 --> 00:06:48,750
function so for example when we define

110
00:06:48,750 --> 00:06:52,550
want to define negation function from
ball

111
00:06:52,550 --> 00:06:57,160
tubal we can define a bit better
matching directly showing against not a

112
00:06:57,160 --> 00:06:57,890
false

113
00:06:57,890 --> 00:07:01,880
it was true and not of through it was
false

114
00:07:01,880 --> 00:07:05,390
John that is very very concise

115
00:07:05,390 --> 00:07:10,530
definition negation let's look at
another example

116
00:07:10,530 --> 00:07:13,800
say that we want to define a.m. and

117
00:07:13,800 --> 00:07:18,100
of two billion show ditch a function for
bald eagle

118
00:07:18,100 --> 00:07:21,930
to belong and we just ride on

119
00:07:21,930 --> 00:07:25,640
that through table for and so this is

120
00:07:25,640 --> 00:07:29,420
exactly how are you if you if you look
up the definition

121
00:07:29,420 --> 00:07:35,020
of and and any book on digital logic you
will see the same through table here

122
00:07:35,020 --> 00:07:38,040
so we can ride a dime but if you look at

123
00:07:38,040 --> 00:07:41,680
all of this you see that it can be
written

124
00:07:41,680 --> 00:07:45,090
even more compactly because the only
case

125
00:07:45,090 --> 00:07:48,170
where this thing is true is run

126
00:07:48,170 --> 00:07:52,150
both arguments here are true again

127
00:07:52,150 --> 00:07:56,670
and in all other cases it's false so you
can take tish

128
00:07:56,670 --> 00:07:59,910
these four cases here and collapse them
down to DC do

129
00:07:59,910 --> 00:08:03,460
through and through it goes through and
otherwise

130
00:08:03,460 --> 00:08:07,260
whatever and whatever it was false

131
00:08:07,260 --> 00:08:11,220
and this underscore here is used

132
00:08:11,220 --> 00:08:14,450
very very often has car and it means

133
00:08:14,450 --> 00:08:18,840
this is a very bold I don't care about
is just use as a placeholder

134
00:08:18,840 --> 00:08:24,230
and doing better match again

135
00:08:24,230 --> 00:08:28,450
and now if you look at this definition

136
00:08:28,450 --> 00:08:31,960
an it well evaluates

137
00:08:31,960 --> 00:08:35,590
both arguments and

138
00:08:35,590 --> 00:08:39,029
in order to do this match because we're
putting a pattern

139
00:08:39,029 --> 00:08:42,580
on both sides of D and

140
00:08:42,580 --> 00:08:47,720
has good people are very sensitive to
this because has collision lazy language

141
00:08:47,720 --> 00:08:48,050
in

142
00:08:48,050 --> 00:08:52,040
they want you want to do as little a
valuation as possible

143
00:08:52,040 --> 00:08:55,260
in order to an finds

144
00:08:55,260 --> 00:08:58,320
the answer and to

145
00:08:58,320 --> 00:09:02,430
dysfunction call alright so I a nicer
definition

146
00:09:02,430 --> 00:09:05,740
according to like haskell ATM is a
full-line

147
00:09:05,740 --> 00:09:09,180
if if it's true then

148
00:09:09,180 --> 00:09:12,690
through and something is the same as B

149
00:09:12,690 --> 00:09:16,570
and false and whatever is false

150
00:09:16,570 --> 00:09:20,170
let's see if this is true through

151
00:09:20,170 --> 00:09:25,260
and threw it goes through through and
false

152
00:09:25,260 --> 00:09:29,660
Eagles fall so through and whatever's
there

153
00:09:29,660 --> 00:09:32,760
is whatever was there an average

154
00:09:32,760 --> 00:09:37,250
falls on the left hand side it doesn't
matter what's here on the right inside

155
00:09:37,250 --> 00:09:37,680
edge

156
00:09:37,680 --> 00:09:41,360
always falls saw this is actually

157
00:09:41,360 --> 00:09:44,839
they gonna your them most

158
00:09:44,839 --> 00:09:47,920
up the mall definition of

159
00:09:47,920 --> 00:09:51,010
and because it only

160
00:09:51,010 --> 00:09:54,300
evaluates the first re-invent and

161
00:09:54,300 --> 00:09:58,430
as necessary

162
00:09:58,430 --> 00:10:01,340
not haskell patterns are matched in
order

163
00:10:01,340 --> 00:10:06,610
saw there let matched top to bottom left
to right or left to right

164
00:10:06,610 --> 00:10:10,750
top to bottom if I'm really precise and

165
00:10:10,750 --> 00:10:14,110
and so the compiler does not try to
rearrange

166
00:10:14,110 --> 00:10:17,570
the patterns necessarily to am

167
00:10:17,570 --> 00:10:20,670
make tomorrow the mall biggest the order

168
00:10:20,670 --> 00:10:25,370
is important and you can enter makes a
better matching

169
00:10:25,370 --> 00:10:29,700
red-carded equations and so

170
00:10:29,700 --> 00:10:33,850
the easiest thing to think about is that
that there's are always matched

171
00:10:33,850 --> 00:10:37,400
left to right top to bottom

172
00:10:37,400 --> 00:10:40,650
you cannot 3b variables for example

173
00:10:40,650 --> 00:10:44,270
you cannot say B&B egos be

174
00:10:44,270 --> 00:10:47,960
and assured that this implicitly doing
equality

175
00:10:47,960 --> 00:10:51,100
overby Asia although variables

176
00:10:51,100 --> 00:10:55,980
inside a pattern must be unique an

177
00:10:55,980 --> 00:11:01,470
one thing that we have seen we have
written lists using describe records

178
00:11:01,470 --> 00:11:06,340
and describe records really are just
indexing sugar for

179
00:11:06,340 --> 00:11:09,430
and writing the list using explicit

180
00:11:09,430 --> 00:11:13,160
gongs and and the list

181
00:11:13,160 --> 00:11:17,020
right so analyst appears in a in

182
00:11:17,020 --> 00:11:20,200
a constant list appear some someplace

183
00:11:20,200 --> 00:11:24,340
it will mean this but inside a better
matching

184
00:11:24,340 --> 00:11:30,240
we often have to write lists in this for
so for example when we want to define

185
00:11:30,240 --> 00:11:34,700
at the head of a list redefined using
better matching as follows:

186
00:11:34,700 --> 00:11:38,510
in the right to list in Gaunce had are
in had

187
00:11:38,510 --> 00:11:42,540
still form thats and call it like that
so

188
00:11:42,540 --> 00:11:47,460
the hat of analysts redhead action and
Dale

189
00:11:47,460 --> 00:11:51,210
I don't care is the hat and detail

190
00:11:51,210 --> 00:11:54,790
up a list with hats that I don't care
about and deal access

191
00:11:54,790 --> 00:11:58,340
is excess I'm so again

192
00:11:58,340 --> 00:12:01,580
very concise definitions and just for
fun

193
00:12:01,580 --> 00:12:05,270
let's look at the types they'll take
Celeste

194
00:12:05,270 --> 00:12:08,730
and since that day Lord blesses Celeste
it

195
00:12:08,730 --> 00:12:10,620
diapers list

196
00:12:10,620 --> 00:12:13,610
to list hat take Celeste

197
00:12:13,610 --> 00:12:16,699
but that value of the head

198
00:12:16,699 --> 00:12:21,190
is the element type of the less its a
this thing goes from list

199
00:12:21,190 --> 00:12:24,410
day to a now of course

200
00:12:24,410 --> 00:12:29,130
these are not total functions biggest
had until

201
00:12:29,130 --> 00:12:34,019
are not defined on the empty list so
when I apply had stood empty list

202
00:12:34,019 --> 00:12:35,779
there's no pattern that matches

203
00:12:35,779 --> 00:12:39,500
and so what you will get as you'll get
an error and the same for tail

204
00:12:39,500 --> 00:12:46,500
another way to say that

205
00:12:46,899 --> 00:12:51,860
is that the pattern like this ex-cons
excess and only matches

206
00:12:51,860 --> 00:12:55,240
a non-empty sure if you go ahead with
the empty list

207
00:12:55,240 --> 00:12:58,800
this pattern here

208
00:12:58,800 --> 00:13:02,220
doesn't match because it's not the form

209
00:13:02,220 --> 00:13:05,360
ex-cons anything okay

210
00:13:05,360 --> 00:13:09,649
so an easy had a venti less there's no
case that matches

211
00:13:09,649 --> 00:13:13,370
you get an error the other thing

212
00:13:13,370 --> 00:13:17,980
is that you have to and brent is sized
is less

213
00:13:17,980 --> 00:13:22,290
patterns because remember application by
its strongest

214
00:13:22,290 --> 00:13:26,019
so this would mean has of axe calms

215
00:13:26,019 --> 00:13:30,500
yarn a.m. anything so you have to put
brands

216
00:13:30,500 --> 00:13:34,540
am in these batters to make it work
alright

217
00:13:34,540 --> 00:13:39,329
that was our first part of defining
functions national

218
00:13:39,329 --> 00:13:40,550
see you after the break