Re: [Haskell-cafe] Tracer for Haskell showing substitutions
Hi, "Hat": The Haskell Tracer. http://www.haskell.org/hat/ >From the site: Hat helps locating errors in programs. Furthermore, it is useful for understanding how a (correct) program works, especially for teaching and program maintenance. Hat is not a time or space profiler. Hat can be used for programs that terminate normally, that terminate with an error message or that terminate when interrupted by the programmer. "Vital"/"Pivotal": it's dead, but it may be interesting to you anyway. http://www.cs.kent.ac.uk/projects/pivotal/ http://www.cs.kent.ac.uk/projects/vital/ >From the site: Pivotal has similar goals to its predecessor system, Vital. In particular: * Documents are live in the sense that, if a document is changed, the displayed values are automatically re-evaluated. Thus documents are always in a consistent state. * Direct manipulation of ADT values is supported. That is, an end user is able to manipulate the text of a Haskell module simply by point-and-click mouse operations on displayed values. Pen and paper work too. Ezra. Ulrik Rasmussen-2 wrote: > > Hi all, > > I was wondering if someone has written a tracer/debugger that shows you > how a given Haskell expression is evaluated, by generating all the > intermediate states of the expression until it is in normal form? > > For instance, given the following code: > >> take' 0 xs = [] >> take' n (x:xs) = x : take' (n-1) xs >> exp = take' 2 [1,2,3,4,5,6] > > the trace of 'exp' would generate something like this: > >> exp = take' 2 [1,2,3,4,5,6] >> exp = (\n (x:xs) -> x : take' (n-1) xs) 2 [1,2,3,4,5,6] >> exp = 1 : take' (2-1) [2,3,4,5,6] >> exp = 1 : take' 1 [2,3,4,5,6] >> exp = 1 : (\n (x:xs) -> x : take' (n-1) xs) 1 [2,3,4,5,6] >> exp = 1 : 2 : take' (1-1) [3,4,5,6] >> exp = 1 : 2 : take' 0 [3,4,5,6] >> exp = 1 : 2 : (\0 xs -> []) 0 [3,4,5,6] >> exp = 1 : 2 : [] >> exp = [1,2] > > That is, all the substitutions performed when evaluating 'exp' from left > to right. > > I was thinking that something like this could be rather useful when > teaching or learning Haskell. > > > Thanks, > > Ulrik > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- View this message in context: http://old.nabble.com/Tracer-for-Haskell-showing-substitutions-tp27421880p27424789.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[Haskell-cafe] cursive to foldr
Using the same basic structure you did, and foldr, I think below is the simplest method: import Data.Maybe searchList :: (a -> Bool) -> [a] -> Maybe [a] searchList p xs = foldr (\x acc -> if p x then Just (x: fromMaybe [] acc) else acc) Nothing xs ghci> searchList (=='o') "A quick brown fox" Just "oo" ghci> searchList (==' ') "A quick brown fox" Just " " ghci> searchList (=='z') "A quick brown fox" Nothing >From maybe gets rid of the Maybe, so that our recursive call works: ghci> fromMaybe [] (Just [1..3]) [1,2,3] That's why we got the error below when we tried without fromMaybe; on subsequent applications of foldr, the type would have to change. :1:51: Couldn't match expected type `[a]' against inferred type `Maybe [a]' In the expression: if p x then Just (x : acc) else acc In the first argument of `foldr', namely `(\ x acc -> if p x then Just (x : acc) else acc)' In the expression: foldr (\ x acc -> if p x then Just (x : acc) else acc) Nothing xs I have a feeling that using fromMaybe is not the best way, but it gets the job done for now. On that note; if somebody with some more experience would chime in, that'd be awesome. ;) Ezra dima.neg wrote: > > How can I do this using foldr? > > searchList p [] = Nothing > searchList p (x:xs) > | p x = Just (x:filter p xs) > | otherwise = searchList p xs > > > I try this: > searchList p xs = foldr (\x acc -> if p x then Just (x:acc) else acc) > Nothing xs > but don't work. > > Thanks > -- View this message in context: http://old.nabble.com/Recursive-to-foldr-tp26368900p26399795.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help to solve simple problem !
The following program should work:
===compress.hs=
toList :: (Eq a) => [a] -> [[a]]
toList [] = []
toList x = start : toList end
where (start, end) = span (==(head x)) x
toTuple :: [a] -> (a, Int)
toTuple x = (head x, length x)
compress :: Eq a => [a] -> [(a, Int)]
compress x = map toTuple $ toList x
=
The important thing to understand here, is the "span" function from the
Prelude, and apply it recursively.
*Main> span (=='A') "AAABCC"
("AAA","BCC")
*Main> span (=='h') "hhhaskell"
("hhh","askell")
I've used a function "toList" to separate each part of the string into a
separate element of a list:
*Main> toList "AAABCC"
["AAA","B","CC"]
*Main> toList "EEEZZZRR!!"
["EEE","ZZZ","RR","","!!"]
>From there, "toTuple" takes the first letter of the string, and puts it with
its length in a tuple:
*Main> toTuple "EEE"
('E',3)
*Main> toTuple "E"
('E',5)
Because of how it's defined, we also get the following:
*Main> toTuple "Ezra"
('E',4)
But that won't matter if we only use it after "toList", as in "compress".
Also, I've used your type signature for my solution, not your example
output; if you want it to be displayed like that, you'll have to write your
own function for printing.
This solution is probably not optimal, but it's a start.
Hope that helps,
Ezra
Aneto wrote:
>
> Hi ! I am beginner in Haskell and have problems with this problem:
> compress :: Eq a => [a] -> [(a, Int)]
> If you have string "AAABCCC" it transforms it to : {A, 3} {B,1} {C,3}
>
> Could you help me with it ?
> Thank you in advance !
>
--
View this message in context:
http://old.nabble.com/Help-to-solve-simple-problem-%21-tp26249028p26292205.html
Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
