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hello everybody welcome back

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at the next episode up functional
programming 101

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and today we're going to talk about

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interactive programs I'm but before we
start

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I want to share a story that I
originally heard from Simon Peyton Jones

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and I'm in bed Jones and everybody
should know

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is one of the main drivers behind

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has car an also am

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one of the founders of the Glasgow
Haskell Compiler that

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we have been using in this course an

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here's the story so in this no box

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there is a super powerful a quantum
computer

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it's extremely powerful it can go to a
break every encryption scheme

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no known to man it can are sold and the
problem it kind of cures cancer

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everything the only problem is that

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it has no I L so we will never know

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whether this quantum computer works

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are not the moral of the story is dead

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if your program is not able to
communicate

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with the outside world it might as well

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not exist and if you go back to the
early days of has gone

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it was actually really hard to have a
Haskell program

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communicate with the outside world
where's if you use

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and the other programming language it's
trivial

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the first go to be alright hello world
rich Prince hello world

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on the standard output whereas in
Haskell

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that hello world Walsh super complicated

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first we had stream i/o then we had
continuation based I L and

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finally and that you're going to talk
about today

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we have the I O market and with the i/o
Mon at

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an

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as call has gotten its I O ports

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an on the box such that they can easily
communicate

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an with the outside world alright so

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so far we have only reading programs
that

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take some and food and return a string
or are an integer our top all our list

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and we have printed that value

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an on the out to adopt the

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and rebel above the of the GATT
interpreter

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but what we really want to do is we want
to ride

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programs like most programs like every

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Java program like every see sharp
program like

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any impaired program that takes

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inputs from the actual world does
something with it

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and produces output which and

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in fact what we want is that it produces
aren't food

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as it's getting new Enbridge

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alright so we want to produce our bridge
as forgetting

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UN which so what this program has to do

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it has two interact with the keyboard

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and discreet so and if we

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look at our program as a function

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and some says now it becomes dependent
on the keyboard

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and b-sides just producing not good

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it also will communicate with discrete

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the problem

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is dead haskell programs are

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your functions which means they cannot
have side effect

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and maybe it's good to go I'll

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informally and talk about

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what a mathematical function as dramatic
a magmatic 'em

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law that contractor

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a mathematical function is

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a function that when you give it this
same

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argument well always give you the same
result

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am for example vectorial

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when I gave it an

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it will return factorial an when I give
it and again

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it will return factorial and again now
look at a common function

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that we use in impaired programming for
example

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redline redline takes no arguments are

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and a Mt Dew bottle and readers a string
when I call redline again

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with the same arguments the same empty
to bomb federal return different string

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sure redline is not a mathematical
function

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and the reason is is because it has side
effects

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so it will return strain

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but there's something there that is not
apparent in the dock

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and that's what's the i/o loan at an

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souls the i/o monette gonna expresses

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in the type over function that this
function

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has a side effect Asia interactive
programs programs that

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indirect with the outside world have
side effects

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and we need a way to incorporate side
effects

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in Haskell programs an not

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I don't think that side effect are bad
am

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actually they are and super useful
because you know without side effects is

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really hard to ride these

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interactive programs an but the thing is
that you have to make sure

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again it's a balance it's a question of
taste

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how you used shot those side effects
it's a little bit like salt or spices

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and food

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you can overdo it your food is too salty
and it doesn't taste well

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but if you don't use any salt your
program your car

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your program your food is also not tasty

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a ride George up to you as a developer

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to decide how much side effects to use

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an but it's never good answer to say

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up I'm never going to use side-effect

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orange also never good the other extreme
to say

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I don't care about purity a it is

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that's exactly what shows monster if
programming

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that you can decide where do you side
effect

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and where to avoid them but if I hear
you say

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you know

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we have to program without any side
effects you have not

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understood the passions of discourse are
gay

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so as I said mentioned the solution to
the problem is to introduce a new type

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and we really don't care how this type
is defined

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an for for now we'll just issue

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its own abstract type I love a

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and this type denotes and action

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that will perform some side effects and
as a result

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will return a value of type A

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and

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maybe a good way to

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remembered the difference is that in
Haskell so far we've only used

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expressions

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but we didn't mention statements writers
and bird

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language there are expressions and there
are statements are statements

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mostly communicate via side effects and

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a.m. in some sense by entered using the
i/o Mon at

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we are also injured using statements in
Haskell

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or another way to say that this
reintroduce the notion of

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actions in Haskell a but these actions
themselves

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are also expression so they can be
compost as you get some concrete

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examples

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of the i/o monette years I O of jar

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adjusted typeof in action that will
perform some side effects

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and then return a character and then

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the and most comparative

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of impaired if types that is I O of unit

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that is a function that only perform
side

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the only interesting thing it does is
performing side effects

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because it has no real result value its
readers the empty tubal

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am and in a typical imperative language

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this would be the type void so if you
have a function that returns void

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in nashville that becomes function that
returns

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type I O unit K

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and void is not think are you

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this Mt Dew ball is nothing special

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it's just a two ball and has no
components now

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that we have this type in again we
really don't care how diol more night is

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defined

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as long as we have operations

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defined on them okay so this is like a
little bit like

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in object-oriented programming we have a
class

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we have met the John that class in in
order to use

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a glass you don't need to know how that
class is implemented

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just you now used the mat and that's the
same thing here

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with the i/o mod and the first operation
that we have

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on the eye on it here is get jar in

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a typical imperative language GetJar

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has died unit arrow

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I O of jock unit arrow charge sorry

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and as go because we're lazy we don't
need the unit arrow

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and so instead GetJar is just

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something it has a side effect and then

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returns a character now if if this would
be strict

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it would kinda you know rear a
immediately return a character right

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there

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but since has caused lazy in our we
don't have to

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have this explicit dunk that d first
evaluation

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let another and dressy thing

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in typical imperative languages whenever
you see a function that has to be unit

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arrow something it means that you're
trying to simulate

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laziness here's another

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an example the function

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board jar which rides a character to
standard output

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it takes a character and edge returns
void

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it has side effects but it doesn't
return and the

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usual value and then an

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we have just like for parsers we have
the

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action that immediately returns without
doing anything

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okay so it takes a value and immediately
readers that value

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without performing

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any for dinner side effects and then

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real me using the same kind of
sequencing that were used for barger

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and again in the exercises you well

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see how an did this do notation

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is translated in terms of the binds

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function and but right now

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well just used to do notation because it
looks very much like

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traditional impaired it got so here's a
function

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that reach a character region other
character but

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yeah ignores the result and then region
third character

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and breeders the barrel does too so if
you remember departures from the last

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lecture

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we had more or less exactly the same
example here

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except that we used the I'd impartial
three times and here

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for reading a character from the
Standard & Poor's weekend

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define a function using GetJar

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that reads a string from the Standard &
boot as follows: every

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she reread the character we call that
character axe

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am notice that GetJar has died I O of
jar

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and then X here has type char

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so the arrow is very similar to the
arrow in generators in list

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comprehension show there's an

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I O of tea on the right and then there's
a

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the on the left of the arrow and that's
where why

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we use the arrow symbol and not shady

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ecosystem a bigoted type on this side is
different than the dye

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on that well if the character that we
just read is it

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and newline then were done so we return

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the empty string otherwise we
recursively

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read a.m. Hong to read the rest of this
character

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and then we return the previous
character that we're at

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concatenated on the string that we're
eager

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so another gonna recursive an

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thing here where we an just

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goal this thing itself recursively until
we had the new line

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and then re return all values here's an
example

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that takes a string righted do this
standard

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all if we have the empty string reacted
we cannot do anything so we just

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return unit immediately if we have a
string that starts with the character

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00:13:35,040 --> 00:13:37,380
axed followed by a string access

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be printed their first character and
then recursively

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we print the rest of the string on the
screen an

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this guy here readers I O unit so the
whole thing readers i IO

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unit and everything Fitch now

220
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weekend am the fine

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00:13:58,680 --> 00:14:02,700
another function that simply combines
put string it puts our

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00:14:02,700 --> 00:14:06,170
to ride the new line behind it so we
first print the string

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and then reprinted newline the beauty of
all this

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00:14:13,170 --> 00:14:17,450
is that we can mix and match impaired if
code or I O

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00:14:17,450 --> 00:14:21,690
caught red all the functions that we
have defined before

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and particular functions over less
biggest strings are just less

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so weekend comparatively read

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string from the standard input dan

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you wish regular list

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00:14:37,270 --> 00:14:40,690
functions on that strength and then

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and comparatively

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windy I Oman at ride this to the
standard I

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which are large walk turn as example we
a

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Brent and their strength be reached this

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line from the & Boch and then we print

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that deadline has so many characters
okay

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very simple but you see here you combine
there's an

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island of purity in the sea of impurity

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00:15:09,600 --> 00:15:14,199
and dishes a perfect perfect perfect way
to program

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so you have little parts of pure code
embed

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00:15:18,279 --> 00:15:23,589
in a context of impure and when we

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I'm run this string length in are nghe I

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ever well and print and torturing me
type string

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and it will tell us that this string has
five characters

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00:15:36,130 --> 00:15:39,699
K and note that this thing is only

246
00:15:39,699 --> 00:15:43,259
evaluated for the side effects it
doesn't return

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00:15:43,259 --> 00:15:46,279
an useful value and we see that

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00:15:46,279 --> 00:15:51,800
because it has type I'll unit this is
the end of Part one

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00:15:51,800 --> 00:15:57,680
see you in a few minutes when we're
continuing to talk about the eye on it

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thank you