purescript/purescript

A small strongly typed language that compiles to Javascript

Foad Ardalan

@techla

I have a little category theory background and I really want to make it work with it

I will explore the solution you presented to me

thank you again

Gavin Bisesi

@Daenyth

on the other hand though it's just one line of code, might not be worth it unless this is a widespread thing

Foad Ardalan

@techla

it’s a good exercise anyway

Gavin Bisesi

@Daenyth

Writing naive and straightforward code is often best unless you're sure the fancy stuff pays off enough for how much it costs other developers to read

Foad Ardalan

@techla

@Daenyth it’s actulally as simple as

stringP :: String -> Parser String
stringP = (map (fromCharArray)) <<< sequence <<< (map charP) <<< toCharArray

I get it wrong since the very begining

Sean Yu

@seanyu4296

@techla afaik, Show instances are not derivable. What you can do is derive instance generic then use genericShow

import Data.Generic.Rep as GR
import Data.Generic.Rep.Show (genericShow)

data IntOrBoolean2
  = Int2 Int
  | Boolean2 Boolean

-- note the underscore at the end for the `rep` parameter of class Generic
derive instance genericIntOrBoolean2 :: GR.Generic IntOrBoolean2 _

instance showIntOrBoolean2 :: Show IntOrBoolean2 where
  show = genericShow
  -- now we get a Show

https://purescript-simple-json.readthedocs.io/en/latest/generics-rep.html

Foad Ardalan

@techla

@seanyu4296 thank you

here the Rep stands for Representable ?

ie Functors that are isomorphique to the Home Functor ?

Sean Yu

@seanyu4296

I think so Rep is representable

a is representable in this kind of data structure

ohhh im not sure about the second question.

Foad Ardalan

@techla

Representable in category theory are those Functors

the Rep keyword in haskell meens that

@seanyu4296 it’s a type class that defines 2 functions tabulate and index

Sean Yu

@seanyu4296

ahh no i think it is different. im not well versed with category theory

Foad Ardalan

@techla

Stream are representable (List minus the empty List)

so what does Rep represent ?

in category theory those functors that you can tabulate or index can « represent » the home functor

tabulate and index are actually 2 natural transformations going back and forth to the Home functor

Foad Ardalan

@techla

(understood that the Home Functor is the Reader Functor)

Sean Yu

@seanyu4296

from what i know under the hood if you derive an instance of Generic

rep is a Sum, Product,Constructor, NoConstructor, Arguments, NoArguments

Ill get back to you next time on the functors and category theory since im not familiar yet

Foad Ardalan

@techla

@seanyu4296 https://www.youtube.com/watch?v=KaBz45nZEZw

Clear and accessible

Shubham

@prophet1906

I am stuck with Traversable instance for Tree a from chapter 7 of purescript by example.

instance infixFoldable :: Foldable Tree where
  foldl f acc Leaf = acc
  foldl f acc (Branch left node right) = foldl f (f (foldl f acc left) node) right
  foldr = foldrDefault
  foldMap = foldMapDefaultL

Any idea what I am doing wrong?

Ebmtranceboy

@Ebmtranceboy

@prophet1906 for some reason, you can't rely on the default implementation of either foldl or foldr here. You should define both. In a way, it makes sens since there are so many ways to combine a 2-argument function with a "2-dimensional" structure.

Shubham

@prophet1906

Thanks @Ebmtranceboy, Will try again with implementation of fold & folr.

Shubham

@prophet1906

It is working fine when both foldl & foldr is implemented. Understood issue with 2d structure.

Jun Matsushita

@jmatsushita

Hi there, I'm bumping into a type error I can't make sense of when making a record of natural transformations. I've made a gist which I think is a minimal reproduction : https://try.purescript.org/?gist=404eefaba98d2e42c16fcd333487ce8b

It works with a simple data constructor Nat m n = Nat (m ~> n) but not with a record NatR m n = Nat { nat :: (m ~> n) }

Jun Matsushita

@jmatsushita

use :: forall m n. Nat m n -> m Unit -> n Unit
use (Nat nat) = nat 

useR :: forall m n. NatR m n -> m Unit -> n Unit
useR (NatR { nat }) = nat -- Error!

I get an error with useR:

  Could not match type

    a2

  with type

    Unit


while trying to match type m0 a2
  with type m0 Unit
while checking that expression nat
  has type m0 Unit -> n1 Unit
in value declaration useR

where n1 is a rigid type variable
        bound at (line 0, column 0 - line 0, column 0)
      m0 is a rigid type variable
        bound at (line 0, column 0 - line 0, column 0)
      a2 is a rigid type variable
        bound at (line 17, column 34 - line 17, column 54)

Jun Matsushita

@jmatsushita

Looks like something's up with inference and pattern matching record puns since using dot syntax type checks correctly:

useR3 :: forall m n. NatR m n -> m Unit -> n Unit
useR3 (NatR n) = n.nat

Is that a bug or known behavior?

Ebmtranceboy

@Ebmtranceboy

@jmatsushita Sometimes, it is difficult to predict what the compiler complains about with polymorphic definitions. Often, it is sufficient to provide enough arguments : useR (NatR { nat }) x = nat x

Jun Matsushita

@jmatsushita

@Ebmtranceboy Thanks, neat trick! I had to extend my minimal example because unfortunately, in my case it still didn't work. https://try.purescript.org/?gist=c544c34971cecfd61528fa06c996460b

data Nat m n = Nat (m ~> n)

data NatR m n = NatR { nat :: (m ~> n) }

morphS :: forall m n. Monad m => Monad n => (m ~> n) -> m Unit -> n Unit
morphS t fm = t fm

-- useR :: forall m n. Monad m => Monad n =>  NatR m n -> m Unit -> n Unit
-- useR (NatR { nat }) x = morphS nat x -- Error!
-- Error: Could not match type a4 with a1 
--   a4 rigid type variable bound at line 17, column 34-54...
-- Stays at line 17 even if I move the code around.
-- Actually it's also the same location in my local code base... Wat?   

useR' :: forall m n. Monad m => Monad n =>  NatR m n -> m Unit -> n Unit
useR' (NatR h) x = morphS h.nat x -- Works

useR'' :: forall m n. Monad m => Monad n =>  NatR m n -> m Unit -> n Unit
useR'' (NatR h) = morphS h.nat -- Pointfree also works

There's also some added strangeness going on with the code location reported by the type checker... Should some of this be documented as gotchas somewhere so that they're googleable?

Ebmtranceboy

@Ebmtranceboy

@jmatsushita I'm not sure where is heading your investigations. The compiler error you mention goes away if you replace morphS with identity in the useR definition but you probably have something else in mind :) I advise you to ask on the functional programming Slack, channel #purescript or #purescript-beginners. They're quite crowded

Jun Matsushita

@jmatsushita

Thanks @Ebmtranceboy I'll head there!

Kevin Mungai

@kevinmungai

Hi everyone hope you are well. How do I import Nothing from Data.Maybe I am getting an error as shown above

Jun Matsushita

@jmatsushita

You probably want to import Data.Maybe (Maybe(..))

The (..) asks to not only import the type, but also the type constructors (Just and Nothing)

Kevin Mungai

@kevinmungai

Thanks I will use (..) to import the type constructors as well.

Ketan Shukla

@ketankshukla

Are there any courses out there that teaches purescript with react?

Where communities thrive

People

Repo info

Activity