Of Monads and Men

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flawr

20:44

I just noticed ghci's :i does not like some expressions, e.g. :i (^2) does not work, while let f = (^2) then :i f works.

Laikoni

@flawr As far as I see :info consistently works with identifiers only.

E.g. :i 5 also fails, but :i x after x=5 works.

> :info name ...

> Displays information about the given name(s). For example, if name is a class, then the class methods and their types will be printed; if name is a type constructor, then its definition will be printed; if name is a function, then its type will be printed. If name has been loaded from a source file, then GHCi will also display the location of its definition in the source.

downloads.haskell.org/~ghc/6.12.2/docs/html/users_guide/…

flawr

21:10

I thought it must be something like that, but I would have loved it to work on all expressions like :t

Laikoni

@flawr Like display type and location if it is a name, and just the type otherwise?

You could define your own command with :def [name] [expr]

flawr

@Laikoni oh!

Is there a function like (a->b->c) -> (e->a) -> (e->b) -> (e->c)?

Just noticed for the function monad, <$> is the same as .

and (>>=) is the "mirror image" of (<*>)

Laikoni

21:29

@flawr I think that's liftM2 in the function monad.

flawr

@Laikoni exactly, thanks!

Too bat Control.Monad is so long =/

Laikoni

@flawr I found it by asking pointfree.io to convert f g a b e=g(a e)(b e) and it just answered f = liftM2.

flawr

Oh that is neat:)

H.PWiz

@flawr I think you have asked this before

Oct 11 at 21:01, by flawr

Something like (a->b) -> (a->c) -> (b->c->d) -> (a -> d)

You can use f.g<*>h for f (g e) (h e)

flawr

In my defense: the arguments are in the other order:D

@H.PWiz Ah that makes things easier, thanks!

I need to start to write this stuff down=/

Laikoni

21:41

@flawr f.g<*>h is already on the star board :D

flawr

how am I supposed to understand this cryptic stuff ^^

for context: I'm working on this challenge: codegolf.stackexchange.com/questions/150936/…

(I'm here so far: Try it online!)

Laikoni

@flawr I have a 86 byte solution

flawr

dang, I'm on 96

Laikoni

Without imports, if that helps.

flawr

21:58

whaat

this is my solution now: Try it online!

Laikoni

OP allowed input as string, so you don't need show

My solution: Try it online!

flawr

ah read.pure.maximum<$> span(==x)r is clever!

Took me a while to realize you're using the (,)e functor/monad/applicative/whatever

Laikoni

I think xnor gave me this tip once and since then it has stuck.

flawr

I mean if I see it I can figure out what is going on, but there are so many instances of those typeclasses that I never remember when writing my own code

Laikoni

85 bytes with n^(1+length b) -> n^length(x:b): Try it online!

@flawr I think most of them are rarely useful for golfing.

flawr

22:11

when are you gonna post it? when you're at 10 bytes? :)

@Laikoni This is the same as (f.g)<*>h, right?

Laikoni

When I finished writing the explanation. :)

@flawr Yes

Prelude> :i (.)
(.) :: (b -> c) -> (a -> b) -> a -> c   -- Defined in `GHC.Base'
infixr 9 .
Prelude> :i <$>
(<$>) :: Functor f => (a -> b) -> f a -> f b
        -- Defined in `Data.Functor'
infixl 4 <$>

The infix level of (.) is higher than the one of (<*>).

Laikoni

22:29

@flawr Posted

@flawr There is a problem with the solution: As far as I see 88000 should be True because of 2^3=8, but your solution checks 2^3=88 and returns False.

Ok, nimi already posted a comment five minutes ago.

With the exact same example o.O

flawr

Fixed it

flawr

23:10

good night guys

Laikoni

good night

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