harpocrates

module Memoize (memoize) where

import Data.Map as M
import Data.IORef
import System.IO.Unsafe

-- | Memoize a function. The use of unsafePerformIO is fine since referential transparency is maintained.
-- `memoize f` will return a new function which behaves the same as `f` but memoizes its results everytime
-- it is called with an argument it hasn't seen yet.
memoize :: Ord a => (a -> b) -> (a -> b)

harpocrates

module PriorityQueue (
    PQ,
    size, priority, peek,
    singleton, branch, push,
    pop
  ) where

{- Priority queue with ordered keys `k` and values `v`. A branch stores the
   minimum key in its subtrees as well as the value associated (for fast peek)
   as well as its size (for easily maintaining balance). -}

harpocrates

{-# LANGUAGE TypeFamilies #-}

-- Subsitute type `x` for type `y` in type `a`
-- Works only for type constructors having up to 4 type arguments...
type family Substitute x y a where
  Substitute x y x = y
  Substitute x y (k a b c d) = k (Substitute x y a) (Substitute x y b) (Substitute x y c) (Substitute x y d)
  Substitute x y (k a b c) = k (Substitute x y a) (Substitute x y b) (Substitute x y c)  
  Substitute x y (k a b) = k (Substitute x y a) (Substitute x y b)
  Substitute x y (k a) = k (Substitute x y a)

harpocrates

import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context

object Macro

// inspect how scala has desugared your code
def traceSugar[T](t: T): T = macro impl
def impl(c: Context)(t: c.Tree): c.Tree = { println(t); t }

harpocrates

module ImperativeQuickSort (quicksort) where

import Prelude hiding (take,drop,length,tail,init)
import Data.Vector (modify,Vector)
import Data.Vector.Generic.Mutable hiding (modify)

-- | Imperative quicksort with O(log n) auxilary space (on top of returned vector) 
quicksort :: Ord a => Vector a -> Vector a
quicksort = modify qsort' 
  where

harpocrates

-- Not for the weak at heart.
-- If you use this, you should feel bad about yourself!

import Control.Exception
import System.IO.Unsafe

-- Given a value, check if it has anything undefined in WNHF. If so, return the exception message, else the value
fromError :: a -> Either String a
fromError x = unsafePerformIO $ catch (Right x <$ evaluate x) getMessage
  where

harpocrates

In a recent discussion I had with a friend about Haskell and Scala, they brought up the fact that they sometimes miss Scala's partial functions. In Scala, these are a trait of their own somewhat different from what Haskellers usually understand by "partial function". In particular, you can check if a value is in the domain of the partial function before applying it to the function.

Interestingly enough, partial functions are also supported in Haskell - they just happen to be hidden away in some more obscure parts of the base library. What follows is my attempt to make a module that brings this functionality out and makes it more accessible. Since this is meant to be a literate Haskell source, let's start with some preamble.

{-# LANGUAGE TypeOperators, NoImplicitPrelude, GeneralizedNewtypeDeriving #-}

module Data.Function.Partial where

import Prelude hiding (id, (.), ($))

harpocrates

{-# LANGUAGE GADTs, PolyKinds, DataKinds, TypeOperators, FlexibleInstances, FlexibleContexts #-}

import Data.Foldable
import Text.PrettyPrint.HughesPJClass

data Nat = Z | S Nat

type N0 = Z
type N1 = S N0
type N2 = S N1

harpocrates

{-# LANGUAGE TypeFamilies #-}

import Data.Void

class Foo k where
  data Bar k
  
instance {-# OVERLAPPING #-} Foo () where
  data Bar () = UnitBar Void
  

harpocrates

{-# LANGUAGE
      FlexibleInstances, FlexibleContexts, UndecidableInstances,
      TypeFamilies, DataKinds, GADTs, ConstraintKinds, TypeOperators,
      MultiParamTypeClasses, FunctionalDependencies 
  #-}

module VarArgs (
  -- * Core
  Args(..), args,
  -- * Utility