January 2005 - Haskell-Cafe

lazy readFile
by Henning Thielemann 03 Jan '05

03 Jan '05

I'm also interested, how to get this example working: http://www.haskell.org/pipermail/haskell/2001-November/008336.html This seems to be an old discussion: http://www.haskell.org/pipermail/libraries/2001-April/000358.html Was a strictReadFile implemented in the meantime?

2 1

Hassoal
by Henning Thielemann 03 Jan '05

03 Jan '05

Does someone know more about the Haskell interface to the Structured Audio Orchestra Language as presented at http://sourceforge.net/projects/hassoal/ ? I was not able to contact the author.

1 0

Associative array updating
by Dominic Fox 03 Jan '05

03 Jan '05

Another newbie question. The following is a naive attempt at a word count function, that takes a string and returns a list of word/count pairs. import Char isLetter c = isAlphaNum c || c=='\'' normalize = map toLower . filter isLetter isWord [] = False isWord s = True nwords = filter isWord . map normalize . words update w [] = [(w, 1)] update w ((x, n):xs) = if x==w then (x, n+1) : xs else (x, n) : (update w xs) wordCount = foldr update [] . nwords It works fine, but it's obviously very inefficient. I could make it less inefficient by implementing the word/count dictionary as a hash table or binary tree, but that's what standard libraries are for. So - what is there in the standard libraries that I could use in this scenario? I've looked at the Array module, but don't think it's quite suitable (we don't know the array bounds ahead of time, and the indices are strings). Data.HashTable looks better, but involves IO as it mutates the dictionary entries in place. Is there an alternative that could be used with a fold as in the code above? Dominic

2 2

Typed Lambda-Expressions with GADTs
by Ashley Yakeley 01 Jan '05

01 Jan '05

This compiled with today's CVS GHC 6.3. I don't think you can do this without GADTs. module Expression where { data Sym env r where { FirstSym :: Sym (r,xx) r; NextSym :: Sym env r -> Sym (a,env) r }; runSym :: Sym env r -> env -> r; runSym FirstSym (r,xx) = r; runSym (NextSym var) (a,env) = runSym var env; data OpenExpression env r where { Lambda :: OpenExpression (a,env) r -> OpenExpression env (a -> r); Symbol :: Sym env r -> OpenExpression env r; Constant :: r -> OpenExpression env r; Application :: OpenExpression env (a -> r) -> OpenExpression env a -> OpenExpression env r }; newtype Expression r = MkExpression (forall env. OpenExpression env r); runExpression :: Expression r -> r; runExpression (MkExpression expr) = runOpenExpression expr () where { runOpenExpression :: OpenExpression env a -> env -> a; runOpenExpression (Constant r) _ = r; runOpenExpression (Application ar a) env = (runOpenExpression ar env) (runOpenExpression a env); runOpenExpression (Lambda exp) env = \a -> runOpenExpression exp (a,env); runOpenExpression (Symbol var) env = runSym var env; }; sym0 :: OpenExpression (a,env) a; sym0 = Symbol FirstSym; sym1 :: OpenExpression (a,(b,env)) b; sym1 = Symbol (NextSym FirstSym); sym2 :: OpenExpression (a,(b,(c,env))) c; sym2 = Symbol (NextSym (NextSym FirstSym)); k :: Expression (a -> b -> a); -- k = \x.\y -> x = \.\.1 k = MkExpression (Lambda (Lambda sym1)); s :: Expression ((a -> b -> c) -> (a -> b) -> a -> c); -- s = \x.\y.\z -> x z (y z) = \.\.\.2 0 (1 0) s = MkExpression (Lambda (Lambda (Lambda (Application (Application sym2 sym0) (Application sym1 sym0))))); } -- Ashley Yakeley, Seattle WA

1 0

Haskell Pangolins
by Dominic Fox 01 Jan '05

01 Jan '05

Hi all, This is just a request for comment - I've written a short Haskell program, intended to demonstrate some of the basic features of the language (how to create a data structure, transform it, and run a user interaction loop). I would like to know if there are any obvious respects in which this program could be simplified, clarified or made more idiomatic. The code may be found here: http://codepoetics.com/code/pangolin.hs thanks, Dominic

4 3

re: Associative Array Updating
by Dominic Fox 01 Jan '05

01 Jan '05

On the other hand, given that a version of this using a btree just crunched through the whole of Joyce's Ulysses in a few seconds, maybe it doesn't matter that much... Dominic

1 0