Starting out with FP

7:58 PM

@boatSoap I think Javascript has some pretty functional features

@Caridorc Thanks for offering this, I was unaware SO had chat. Do you often write scala and what language did you transition from?

Caridorc

Scala does not work on my system but I often play with Ruby that is similar on the surface

I know some Haskell that I would suggest if you want to make the leap to FP

Writing imperative code in scala is possible, and that makes Functional thinking harder to achieve

Haskell is absolutely without imperative support

Scala is more practical in other ways though. I suggest I bit of Haskell and going back to Scala when you are confortable with it

Are you in for a practical example of Haskell? A mind bender one?

boatSoap

That does make sense. One issue is the sheer number of built in functions. I was planning on going with Haskell and Scala was suggested by a friend

Sure, lets see the example

Caridorc

@boatSoap Haskell built-ins are super easy to find

just Hoogle them

haskell.org/hoogle

Scala has a weak type system, Java-like types

In Haskell all you need to write a function is figure out its type

then you can search it in hoogle by type

The practical example I had promised you: map

map goes from a function and a list to a list

perfect, on a first approximation, the type is

boatSoap

yes

or am looking at it in hoogle now

Caridorc

8:13 PM

map :: (a -> b) -> [a] -> [b]

Sooo

boatSoap

yes

Caridorc

(a -> b)

is a function from a to b

manyA n = replicate n 'a'

^ a simple example

a in this case is Int

and b is String

map is very general

a may be anything

and b may be anything

This generality is fundamental to writing general functions in Haskell

@boatSoap Is what I mean by generality clear?

boatSoap

yes it is

how would you go about invoking your example

Caridorc

map manyA [1, 5, 6, 10]

boatSoap

List a.map.toString()

Caridorc

8:18 PM

wait, How can you call map with no argument?

boatSoap

You need a function as an argument

Caridorc

Exactly, but you called map with no arg in your example

Interactive Haskell shell:

Prelude> let manyA n = replicate n 'a'
Prelude> map manyA [1, 5, 6, 10]
["a","aaaaa","aaaaaa","aaaaaaaaaa"]

let is necessary because it is Haskell way of faking imperativeness in a REPL

Is my interactive shell session clear?

boatSoap

a lot goes on behind the scenes

yes it is clear, thank you

Caridorc

Another thing is that Haskell is executable math

How many arguments can a function have in Math?

1

And function in Haskell can only ever take one argument

Prelude> (map manyA) [1, 5, 6, 10]
["a","aaaaa","aaaaaa","aaaaaaaaaa"]

boatSoap

i see

Caridorc

8:23 PM

Prelude> (map manyA)

<interactive>:5:1:
    No instance for (Show ([Int] -> [[Char]]))
      arising from a use of `print'
    Possible fix:
      add an instance declaration for (Show ([Int] -> [[Char]]))
    In a stmt of an interactive GHCi command: print it

Error, a function from a list of ints to a list of strings cannot be printed

Haskell errors are hard, maybe the hardest part of Haskell

All my previous languages had duck typing

Haskell is type-driven

Almost every Haskell error is a type Error, Haskellers are proud of this

@boatSoap Is there any specific part of Haskell or FP you are interested in?

boatSoap

FP for me is a way to learn something new. I don't think I will be using it work anytime soon, but who knows

I really appreciate the time you put into this

And understand how much I must change my approach to a problem when shifting from imperative to functional

Caridorc

@boatSoap You are welcome, Haskell is interesting for me, but it is still weird

main = print "Welcome to Haskell and FP"

boatSoap

To me, it reads as a string of verbs

Caridorc

Exactly!

boatSoap

but, it is still foreign

and I can't infer what happens between them yet

Caridorc

8:29 PM

OO programming is a list of nouns with some verb here and there

FP is a list of verbs with possibly no nouns at all

(people still use nouns to make code easier to read, do not worry)

Look at how much this seems imperative:

main = do
  putStrLn "Please enter your name:"
  name <- getLine
  putStrLn ("Hello, " ++ name ++ ", how are you?")

But there is a trick, what you see are really only functions and 'monads` (things that say what should be done between the functions)

@Hosch250 hi

Hosch250

Hi.

Caridorc

It is remarkable that the above code just pretends to be imperative but really is Haskell

Also Haskell favors pure functions

Quiz:

Which function of this is pure?

foo = print 1
bar = 1

(Yes bar is a function, a function that takes no arguments, but still a function)

pure = Mathematical

boatSoap

does pure have nothing to do with only "doing one thing"

Caridorc

@boatSoap pure means Mathematical

boatSoap

i guess foo only does one thing as well

it doesn't return a value, it simply prints

Caridorc

8:38 PM

Having a strong math background really helps with Haskell

Imagine your math class

boatSoap

yea..... the days I wish I studied more math are countless

Hence Euler and other mathematical algorithm problems

Caridorc

Is the function LaunchARocket purely mathematical and abstract?

boatSoap

likely not

Caridorc

Exactly

Because it interacts with the real word

In Haskell, >90% of the functions should be pure

with no communication to the real world, just pure math

@boatSoap In your opinion, why this reccomandation?

boatSoap

side effects will be minimized, troubleshooting will be localized

Caridorc

8:46 PM

@boatSoap Exactly. A functional will give the same output for the same input everytime, just like, yes a Mathematical function

f x = x + 1

f x = 1 + x

By the way In haskell you can simplify

f = (+ 1)

works as well

boatSoap

functions are methods?

1.(+1)

Caridorc

no way

. is the composition operator

boatSoap

does that work?

Caridorc

Prelude> let f = (+ 1)
Prelude> f 5
6

f . g means applying g and then f

Prelude> let last' = head . reverse
Prelude> last' [1,2,3]
3

@boatSoap there are no Objects in Haskell, Objects means to be Noun-centric

The error your code triggers is kind of verbose:

Prelude> 1.(+1)

<interactive>:26:1:
    No instance for (Num (b0 -> c0)) arising from the literal `1'
    Possible fix: add an instance declaration for (Num (b0 -> c0))
    In the first argument of `(.)', namely `1'
    In the expression: 1 . (+ 1)
    In an equation for `it': it = 1 . (+ 1)

<interactive>:26:4:
    No instance for (Num b0) arising from a use of `+'
    The type variable `b0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:

You can actually define . (DOT) if you want, it is easy

And I consider myself a beginner in Haskell

so I am just scratching the surface

@boatSoap as in Math, it is all about understanding it, playing with it and gaining familiarity with it.

@boatSoap I wish you good luck with it, and of course you are welcome to post your programs to CodeReview for feedback

boatSoap

9:07 PM

@Caridorc Thanks again for all the information!

Caridorc

@boatSoap You are welcome.

Transcript for

Starting out with FP