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welcome back am

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and the first part up this a lecture

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we at seen some simple examples of

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how to define functions and has gone
especially how that

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differs from most contemporary language
saw

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we have seen that in Haskell
conditionals are expressions not

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statements

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but thats haskell programmers rather
ride

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guarded equations than conditional
expressions

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the other thing we have seen is how did
he find

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how to define functions using better
matching

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and we have seen in particular how to
define functions over lists

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using pattern matching well we're going
to look at now

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is how to define functions biggest has
caused a fun show language

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so we have to have a way a concise way

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to define functions and that Islam the
expressions

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and you probably know /url on the
expressions

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a.m. from the language that you're using
a pure Java programmer

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in Java eights and along the expressions
were injured used

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if you're sheesh our program error you
no longer expressions

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if your hack or PHP programmer

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you have along the expressions if your
javascript you have loved expression

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lund expressions are

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everywhere she plus plus you name it d

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and swift every mother language has long
expressions and as we saw on the ferry

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first lecture

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edition inventions from along to church

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in the 1930s and

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most languages lund expressions look
kinda similar

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and has called a look as follows: so

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day am looked like this

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on the axe arrow body of the lawn
expression

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now and actual codes that you die you
don't write alumni

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deride the slash the right slash X

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and is set up the arrow derides and

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minus sign and greater than but since we
want these lights to look fancy

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where

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actually riding along than and a real

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bank but when you ride this thing called
the right slash

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for the London and you just right the
arrow

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as you would expect K so what is this
thing

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this is a function addiction X and ad
said

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to itself and that function has no name
so it's an expression

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a function type thats Rutland
expressions are

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they are expressions that the note
functions

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Joanne

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the in in mathematics and and

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as church designed them are the ideal

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read along the darkness let richard eyre
international on that but we tied them

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as

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backslash and instead of the arrow
symbol

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we ride the aris him again

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but for the rest haskell I think it's a
language that is closest to the lawn the

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cartilage

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because it's based on your functions
version

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and other languages lund expressions are

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I think a better way would be to call
them closures

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and we will see why because they can
close over variables

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in the outer scope but the the slum the
expressions can have side effects

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and but as we said it doesn't really
matter

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we are really emphasizing the fact that
functional programming is

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programming using expressions so why are
London expressions useful

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well they're useful because they allow
you to express your intent better

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when you're currying functions let's
look at this first example here

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and but SL arriving at XY

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because X plus why should this occur
redefinition and

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this is a function that they should
parameter axe

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readers a function that takes a
barometer why

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and then returns X plus Y really

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the intent is better expressed by this

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at is a function

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it takes bram interact readers a
function that takes a brother why

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and then add them together okay

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so this is actually syntactic sugar

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fordice and when I write haskell I often
write them in this for

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even though it's a little bit longer

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at means that I have to type more
characters

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but I think this for is more readable
than that for

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am and in the course of

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of this these lectures we'll switch
between

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both of these and but again if you look
at my haskell God

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I prefer to write things using a splash
at long last to convey

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the actual meaning of the current
function and expressions

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are also useful when you want to and

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make explicit that you're returning a
function that's

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a camp some sense the same scurrying but
here

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Ikea if we define a constant function

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so this is a function and given a will
return a function that will never be

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you gave it will return that day so %eh
defined goals backs

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readers a function that will whatever
you give its just ignore it

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and return X okay so that constant
function there

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is very concisely defined using along
the expression

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where's here I think it's a little bit
less clear

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gone still backs underscore equals x

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so this round doesn't make explicit
defect

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that you're returning a function so he
is also emphasized by putting the

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parentheses

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around died and here you don't do that

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so I'll this is how I were dry
dysfunction I would

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with the brand she's on the type and I
puts the

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writer function as a lawn the expression
am

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here's another you shove using on the
expressions

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and that's to avoid inventing names

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inventing names is

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one of the hardest things that we have
to do it programming in

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coming up meaningful names because often

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you know their language forces you to
name something where you just don't want

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to name it just want to bash the thing
directly

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so for example if I want to map a
function

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over a list there's no reason I should
get that function here

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name and what this have say anyway

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so instead to what I can do again just

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Obama's love the expression do the map

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and that will it be used to map this
function

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over the list right show this

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not idiomatic national this

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idiomatic really the only way

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place are you want to name something is
when you use it more than once

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or if it becomes clear but thats

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yeah up to interpretation here's another

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all the tea of Haskell and that's called
sections

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so with sections you can take

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a binary operator in and fix operate
like plus

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and use it as a function so by putting

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a ranch around it you can use plus as a
function so one plus two

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it was three and plus in brands

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12 is also tree now this is a nice trick

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because we can also foods Wong

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a brand one-plus and we can do

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plus upper and blessed to show weekend
section began

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partially apply plus by putting

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Iran's around the plush so here we see

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we can do one-plus which is a function

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that expect second argument I will

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at 12 it and here we have to function

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section plus do which is a function that
expects

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value here and then added

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to ensure this is one place to equals
three show in general

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F

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is a by a plus funny place is a binary
operator

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then you can ride plus and brands

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to use it as a normal function

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you can left or right section head by

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partially applying it to an argument K

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sewer this is something that you will
see while all the National

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specially here if you want to have a
function that anchormen something

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just right one-plus as a section to wire
sections useful

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well here are a couple of examples you
on the function that God fiction

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argument and hot shit to all

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you just ride opened brand divided by
two

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and you don't have to ride lumber axe

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axe divided by two so sectioning allows
you to write something

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read-out inventing an and thats

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said the whole goal of this exercise

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giving names is hard so we want to avoid
that

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and Section allows you to do that
alright

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so I am here's a preview of the
exercises the exercises will be

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available on the website

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and these exercises real ask you

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to use am some of these constructs that

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a.m. we have seen in this lecture to
define

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functions and as I promise you this will
be the last

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and exercise that's very haskell
specific

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but since you now we we have been
talking about how haskell

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differs from other languages I think
it's quite useful

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an to follow the rest of the lectures to
do these exercises

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to get a little bit familiar an

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with the Haskell syntex am and so happy
hacking

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and see you next week bye bye