Arnau Abella monadplus

Where is <nixpkgs> ?

Nix looks for expressions at NIX_PATH.

NIX_PATH is very similar to PATH but used by nix expressions (an expression is search on the NIX_PATH from left to right)

$ nix-instantiate --eval -E '<nixpkgs>'
/nix/var/nix/profiles/per-user/root/channels/nixos
$ echo $NIX_PATH

Source: https://nixos.wiki/wiki/Packaging/Binaries#Extra_Dynamic_Libraries

# First, you need to index your channel packages
$ nix-index

# Then you can locate the file:
$ nix-locate -1 -w lib/libsane.so.1

(xsane.out)

Updated version can be found on my website.

Running a haskell script without GHC

Given the following haskell script generate-random-samples.hs that requires mwc-random ...

{-# LANGUAGE ScopedTypeVariables #-}

import System.Random.MWC

All credit to O. Charles and his wonderful blog "24 of GHC extensions"

Expressing relations between types using type classes

Suppose that we want to model a generic mutable variable container using type classes.

This first approach works fine for IORef and MVar...

class IOStore store where

"unsafe" c ffi (doesn't mean what you think) takes 10-15 ns when i measured it on my computer mroe than 5 years ago
[23:03] carter: safe which does some scheduler stuff took an extra 100-200 ns
[23:03] carter: so for c stuff that takes < 1-10 microseconds, unsafe is fine and safe
[23:04] carter: for stuff that runs longer, use safe c calls
[23:05] carter: (reason is: safe calls dont pause / block gc)
[23:07] carter: https://bitbucket.org/carter/who-ya-gonna-call-talk-may-2013-ny-haskell/src/master/ might have some code


	\usepackage{xcolor}

	\definecolor{codegreen}{rgb}{0,0.5,0}
	\definecolor{codegray}{rgb}{0.5,0.5,0.5}
	\definecolor{backcolour}{rgb}{0.95,0.95,0.92}

	\usepackage{listings}
	\lstset{
	backgroundcolor=\color{backcolour},

	{-# LANGUAGE MagicHash #-}
	module Main where

	-- More on https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#unboxed-types-and-primitive-operations

	import GHC.Exts

	showUnboxedInt :: Int# -> String
	showUnboxedInt n = (show $ I# n) ++ "#"

	import Data.IntMap.Lazy as Map

	{-
	Notice that the smallest is not evaluated under 'go' function,
	otherwise the computation would be ⊥
	-}
	attachMin :: Ord a => [a] -> [(a, a)]
	attachMin l =
	let (l', smallest) = go smallest l
	in l'

	{-# LANGUAGE ScopedTypeVariables #-}
	module Main where

	import GHC.TypeLits
	import Data.Proxy
	import Data.Maybe
	import System.IO

	main :: IO ()
	main = do