November 2008 - Beginners

(fwd) Haskell Quick Reference (1-page PDF) - QuickReference.pdf (0/1)
by Benjamin L.Russell 07 Nov '08

07 Nov '08

The following one-page Haskell Quick Reference was posted earlier today on Haskell-Cafe; I am forwarding it to this list as a reference for any interested beginners. -- Benjamin L. Russell On Thu, 6 Nov 2008 13:41:13 +0000, in gmane.comp.lang.haskell.cafe Malcolm Wallace <Malcolm.Wallace(a)cs.york.ac.uk> wrote: >Some time ago, there was a thread about a "CheatSheet" for Haskell >beginners. As I recall, the CheatSheet was more than 12 pages long. > >For a Haskell tutorial I was running at a conference recently, I needed >a "Quick Reference Guide" that would fit onto a single side of A4. So I >knocked one together quickly, and it is attached as a PDF. I send it to >this list, with permission for anyone to distribute it more widely as >they wish, in the hope that it might be useful. > >Doubtless it is incomplete, and I have no particular desire to fix >errors or maintain this document, so if anyone is interested and would >like to adopt it, I can pass on the editable sources. It was originally >created as an Apple Numbers spreadsheet (simply for speed of creation) >but could be converted to Excel or CSV, for import into other tools. > >Regards, > Malcolm

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Tab complement
by gerry xiao 06 Nov '08

06 Nov '08

Hello,i'm gerry,new to haskell Why i can't do tab complement in GHCI? Best Regards gerry

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Weighted average
by Michael Snoyman 04 Nov '08

04 Nov '08

Hi everyone, I'm trying to set up some type safe functions for doing weighted averages and sum of products. The example I give below is to try and calculate the average miles per gallon for a collections of vehicles. Unfortunately, I am unable to get my weightedAverage function to work, seemingly due to an ambiguity for which instance to use. I think that the issue is that my "class Multiplicable" should only have two parameters, as opposed to the three it currently has. However, I can't figure out how to get that to work. Any help is greatly appreciated. Thank you, Michael ---------------------------------- {-# LANGUAGE MultiParamTypeClasses #-} import Prelude hiding (sum, product) class Addable a where add :: a -> a -> a zero :: a sum :: [a] -> a sum = foldr add zero class Multiplicable a b c where mult :: a -> b -> c product :: [a] -> [b] -> [c] product x y = map (\(x1, y1) -> x1 `mult` y1) $ zip x y sumProduct :: (Addable c, Multiplicable a b c) => [a] -> [b] -> c sumProduct x y = sum $ product x y weightedAverage x y = (sumProduct y x) `divide` (sum y) class Dividable a b c where divide :: c -> a -> b newtype MilesPerGallon = MilesPerGallon Double deriving Show newtype Gallon = Gallon Double deriving Show newtype Mile = Mile Double deriving Show instance Addable Gallon where add (Gallon x) (Gallon y) = Gallon $ x + y zero = Gallon 0 instance Addable Mile where add (Mile x) (Mile y) = Mile $ x + y zero = Mile 0 instance Multiplicable Gallon MilesPerGallon Mile where mult (Gallon x) (MilesPerGallon y) = Mile $ x * y instance Dividable Gallon MilesPerGallon Mile where divide (Mile x) (Gallon y) = MilesPerGallon $ x / y milesPerGallon :: [MilesPerGallon] milesPerGallon = map MilesPerGallon [35, 25, 29, 20, 52] gallons :: [Gallon] gallons = map Gallon [500, 190, 240, 100, 600] totalGallons :: Gallon totalGallons = sum gallons totalMiles :: Mile totalMiles = sumProduct gallons milesPerGallon totalMilesPerGallon :: MilesPerGallon totalMilesPerGallon = totalMiles `divide` totalGallons -- I would like some way to get the following line to replace the previous --totalMilesPerGallon = weightedAverage milesPerGallon gallons main = do putStrLn $ "Total gallons of gas used: " ++ show totalGallons putStrLn $ "Total miles traveled: " ++ show totalMiles putStrLn $ "Average miles per gallon: " ++ show totalMilesPerGallon

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Control.Monad.State: State CTOR unneeded to create a State?
by Larry Evans 01 Nov '08

01 Nov '08

Page: http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mon… suggests, with the content: Constructors State runState :: (s -> (a, s)) that State has a labeled constructor: http://www.haskell.org/onlinereport/exps.html#sect3.15.2 yet the Examples section: http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mon… shows, AFAICT, no call to the State CTOR: tick :: State Int Int tick = do n <- get put (n+1) return n I assume the n must be an Int (since it's in n+1), but what are the get and put. There is a get and put described here: http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mon… however, I don't see how they are relevant since they are defined for an already existing MonadState, and AFAICT, tick hasn't been defined yet since it's on the lhs of the =. Please, what am I missing? TIA -regards Larry

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