All the side effects were never mentioned to me
I am innocent of uncontrolled abuse

  • 117 Posts
  • 37 Comments
Joined 1 year ago
Cake day: June 17th, 2023
  • jaror@kbin.socialOPtoHaskell@kbin.social•TIL: wrapping list comprehension in a function can prevent fusion
    1·
    7 months ago

    Stream fusion does work:

    data Stream a = forall s. Stream !(s -> Step s a) !s
data Step s a = Yield a !s | Skip !s | Done

data Tup a b = Tup !a !b

cartesianProduct :: Stream a -> Stream b -> Stream (a, b)
cartesianProduct (Stream step1 s01) (Stream step2 s02) = Stream step' s' where
  s' = Tup s01 s02
  step' (Tup s1 s2) =
    case step1 s1 of
      Yield x s1' ->
        case step2 s2 of
          Yield y s2' -> Yield (x, y) (Tup s1 s2')
          Skip s2' -> Skip (Tup s1 s2')
          Done -> Skip (Tup s1' s02)
      Skip s1' -> Skip (Tup s1' s2)
      Done -> Done

eft :: Int -> Int -> Stream Int
eft x y = Stream step x where
  step s
    | s > y = Done
    | otherwise = Yield s (s + 1)

fooS :: Stream (Int, Int)
fooS = cartesianProduct (eft 0 10) (eft 0 10)

toList :: Stream a -> [a]
toList (Stream step s0) = go s0 where
  go !s =
    case step s of
      Yield x s' -> x : go s'
      Skip s' -> go s'
      Done -> []

foo :: [(Int,Int)]
foo = toList fooS
  • jaror@kbin.socialOPtoHaskell@kbin.social•Declarative vs imperative:
    1·
    7 months ago

    Admittedly it gets more complicated when summing two things at the same time:

    let Pair dnormMean dnormNormMean =
      fold (Pair <$> dimap (\(Pair _ dnormI) -> dnormI) (/ fromIntegral cc) sum
                 <*> dimap (\(Pair normBti dnormI) -> normBti * dnormI) (/ fromIntegral cc) sum)
        $ map (\i -> Pair (((inp ! (off + i)) - meanBt) * rstdBt)
                          ((weight ! i) * (dout ! (off + i))))
          [0 .. cc - 1]


    float dnorm_mean = 0.0f;
float dnorm_norm_mean = 0.0f;
for (int i = 0; i < C; i++) {
    float norm_bti = (inp_bt[i] - mean_bt) * rstd_bt;
    float dnorm_i = weight[i] * dout_bt[i];
    dnorm_mean += dnorm_i;
    dnorm_norm_mean += dnorm_i * norm_bti;
}
dnorm_mean = dnorm_mean / C;
dnorm_norm_mean = dnorm_norm_mean / C;
  • jaror@kbin.socialOPtoHaskell@kbin.social•I enjoyed the simplicity of this recent Computerphile video on web servers: https://www.youtube.com/watch?v=7GBlCinu9yg
    1·
    9 months ago

    We can make it a lot more performant, shorter, and also safer by using lazy byte strings:

    {- cabal:
build-depends: base, network, network-run, bytestring
-}

{-# LANGUAGE OverloadedStrings #-}

import Network.Run.TCP (runTCPServer)
import qualified Network.Socket.ByteString.Lazy as Net
import qualified Data.ByteString.Lazy.Char8 as Str

main = runTCPServer (Just "127.0.0.1") "9999" $ \s -> do
  request <- Net.getContents s
  case Str.words (Str.takeWhile (/= '\r') request) of
    ["GET", resource, "HTTP/1.1"] -> do
      let path = Str.concat
            [ "htdocs/"
            , Str.dropWhile (== '/') resource
            , if Str.last resource == '/' then "index.html" else ""
            ]
      page <- Str.readFile (Str.unpack path)
      Net.sendAll s ("HTTP/1.1 200 OK\r\n\r\n" <> page)
    _ -> error "todo"
  • jaror@kbin.socialOPtoHaskell@kbin.social•Haskell Interlude 38: Edwin Brady
    1·
    11 months ago

    I think Idris’ bang notation for performing effects in a do-block is pretty, it could look like this:

    main = do putStrLn ("You said: " ++ !getLine)

    Today, you’d have to come up with a new variable name or figure out the right combinator names:

    main = do line &lt;- getLine; putStrLn ("You said: " ++ line)

main = putStrLn . ("You said: " ++) =&lt;&lt; getLine

    But unfortunately there are more complicated cases:

    main = do print (True || !getLine == "foo")

    In a strict language with built-in short-circuiting logical operations the getLine would never be performed, but in Haskell || is just a normal function that happens to be lazy in its second argument. The only reasonable way to implement it seems to be to treat every function as if it was strict and always perform the getLine:

    main = do line &lt;- getLine; print (True || line == "foo")

    Do you think this is confusing? Or is the bang notation useful enough that you can live with these odd cases? I’m not very happy with this naive desugaring.