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suggestions for re-implementing mapM
by Vale Cofer-Shabica 23 Mar '15

23 Mar '15

Hello all, I've been reading through "Tackling the Awkward Squad" [1] and am implementing the definitions "left as exercises" as I go. For section 2.2, I define: putLine :: [Char] -> IO () putLine :: mapM_ putChar where mapM_ :: Monad m => (a -> m b) -> a -> m () mapM_ f [] = return () mapM_ f (x:xs) = (f x) >> (mapM_ f xs) which works without difficulty. For the sake of learning, I decided to implement mapM as well. My definition (below) works, but seems really in-elegant. I checked the prelude source and found mapM defined in terms of sequence, which has some do-notation I'm not so clear on. I'm trying to avoid using do notation in my implementation because it still feels like magic. I'll work on de-sugaring do notation again once I have a solid handle on (>>=), (>>), and return. Any suggestions for cleaning this up would be much appreciated! mapM :: Monad m => (a -> m b) -> [a] -> m [b] mapM f [] = return [] mapM f (x:xs) = consMM (f x) (mapM f xs) consMM :: Monad m => m a -> m [a] -> m [a] consMM mx mxs = mx >>= ((flip consM) mxs) where consM x mxs = mxs>>=(\xs -> return (x:xs)) Thank you, vale [1] Suggested by apfelmus in a recent email to the list: http://research.microsoft.com/en-us/um/people/simonpj/papers/marktoberdorf/

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Code review: Go challenge #1 in Haskell
by Ramakrishnan Muthukrishnan 22 Mar '15

22 Mar '15

Hello Haskellers: A few weeks ago, there was this thing called "Go challenge" and a problem was posted¹ on decoding a binary format and printing out the drum sequences in the input binary file. I made a Haskell solution and would love to get some code reviews. <https://github.com/vu3rdd/drum> One of the input files given does not have the full bitstream and my program cannot parse that yet. I had been thinking about using a Monad transformer to make a `Get (Maybe) a' and output whatever it could parse and leave the rest behind, instead of failing completely. ¹ http://golang-challenge.com/go-challenge1/ (fwiw, the challenge is over.) Thanks in advance, Ramakrishnan

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Support for Maildir
by Elias Diem 22 Mar '15

22 Mar '15

Hi there Am I right that there are no up-to-date packages/libraries to process email in Maildir format? -- Greetings Elias

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Re: [Haskell-beginners] [Haskell-cafe] Multi-parameter type classes and ambiguous type variables...
by Stuart Hungerford 21 Mar '15

21 Mar '15

On Sat, Mar 21, 2015 at 7:31 PM, Chris Wong <lambda.fairy(a)gmail.com> wrote: > [...] >>> check :: (Eq g, M r g) => g -> Bool >>> check x = sca one x == x >>> >>> The problem I have is that GHC is finding the "r" type variable in the >>> "check" function ambiguous. Given my still limited Haskell knowledge >>> I'm not surprised this is happening. What I would like to know is how >>> experienced Haskellers handle this situation in practice: is there an >>> idiomatic way of disambiguating "r" or is it a sign of poor type class >>> design? > > In the type signature: > > check :: (Eq g, M r g) => g -> Bool > > you fix the type `g`, but not the type `r`. This causes an ambiguity > in the program because if you had e.g. > > instance M Float Vector where ... > instance M Int Vector where ... > > both in the same program, and you passed a Vector to `check`, GHC > won't know which instance to choose. > > To solve this ambiguity, either fix `r` with an extra parameter: > > check :: (Eq g, M r g) => r -> g -> Bool > check one' x = sca one' x == x > > Or declare that `r` is uniquely determined by `g` using a *functional > dependency*: > > class (R r, AG g) => M r g | g -> r where > sca :: r -> g -> g > > Or equivalently, using *associated types*: > > class (AG g, R (Scalar g)) => M g where > type Scalar g :: * > sca :: Scalar g -> g -> g > > check :: (Eq g, M g) => g -> Bool > check x = -- as before > > A Google search for these two terms should yield plenty of tutorials > and examples. Thanks for this excellent explanation. > (By the way, I'd suggest using longer names like "Ring" and > "AdditiveGroup" instead, as they're easier to read.) Yes -- I stripped the example down, including the full names of the type classes, for explaining the issue. I could well have left the full names though. Stu

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Multi-parameter type classes and ambiguous type variables...
by Stuart Hungerford 21 Mar '15

21 Mar '15

Hi, As a learning exercise I'm modelling some algebraic structures as Haskell typeclasses. Some of these are multi-parameter type classes. Here's a very simplified version of the type class relationships: class MM a where one :: a class AM a where zero :: a class (AM a, MM a) => SR a class (AM a) => AG a where inv :: a -> a class (SR a) => R a where neg :: a -> a class (R r, AG g) => M r g where sca :: r -> g -> g check :: (Eq g, M r g) => g -> Bool check x = sca one x == x The problem I have is that GHC is finding the "r" type variable in the "check" function ambiguous. Given my still limited Haskell knowledge I'm not surprised this is happening. What I would like to know is how experienced Haskellers handle this situation in practice: is there an idiomatic way of disambiguating "r" or is it a sign of poor type class design? Thanks, Stu

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Finding the lowest element greater or equal than
by martin 18 Mar '15

18 Mar '15

Hello all, I want to find the lowest element in a collection of items which is greatet or equal to a given element. There can be more than one such element in which case I wouldn't care which one I get. I still want to be able to iterate over the elements following the found element. In a way I want the equivalent of select ... from table where table.x > n How would I do this efficiently? I tried using an ordered list, but I would still have to traverse half the list on average. I suppose I can do this with some sort of tree, but i don't want to write all the code myself. But I couldn't find anything off the shelf.

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Have I just broken Haskell?
by Mark Carter 18 Mar '15

18 Mar '15

Persuant to my previous question, I have made some progress, but there is something odd going on. Suppose we want to implement the following algorithm, which is a little simpler than the original problem: The user types the name of a file as input. Haskell then puts the text "it works" to that file. I tried this: import System.IO let openFileWrite name = openFile name WriteMode h :: IO Handle h <- fmap openFileWrite getLine -- you need to type in a file name This looks wrong, because h presumably needs to be a fixed Handle. Being an IO Handle seems to imply it is mutable. Haskell doesn't complain, though. We also have the problem of how we are going to use hPutStr to put a string to the file. We can't do hPutStr h "it works" because h doesn't have the right type. I tried getting around it by creating a function let put s h = hPutStr h s fmap (put "it works") h But that doesn't work, it says *** Exception: wow: openFile: resource busy (file is locked) I'm guessing what I've done is create a stream OUT OF the file handle, rather than FOR THE file handle. Urgh! Any ideas what the fix is?

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Is it possible to check list length at compile time?
by Kelong Cong 17 Mar '15

17 Mar '15

Hello, I have a function that takes a list as its parameter. The list must always have the same length, for example 16. Instead of producing a "Nothing" or bottom out when an invalid list is supplied, are there ways to verify this at compile time? If not, are there alternative data types that has this kind of functionality? Thanks, Kelong

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There has been discussion on matrix operations and cache thrashing; does management think using Haskell would lead to cash thrashing?
by KC 17 Mar '15

17 Mar '15

There has been discussion on matrix operations and cache thrashing; does management think using Haskell would lead to cash thrashing? What could change management's mind? -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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Equivalent of IO Monad in other functional languages?
by Simon Kitching 17 Mar '15

17 Mar '15

[Note: learning haskell, not familiar with other functional languages] I have a question about handling impure operations in functional or semi-functional languages other than Haskell (eg OCaml, F#, Scala, ..). Haskell uses the IO monad to (a) cleanly separate pure and impure code, and (b) prevent function-calls from being reordered when they have side-effects that must happen in the right order. How do other languages handle this? AIUI, whether a Haskell function is 'pure' or not can be seen from its method signature : if it returns IO then it is impure. All standard library functions that have side-effects (read/write/etc) return IO, and any function that uses an IO value must return type IO. Do other functional languages (1) use a similar approach, or (2) not provide a way to be certain that a function is 'pure' (side-effect-free)? And AIUI the Haskell compiler/runtime can postpone evaluation of any function (laziness), or reorder function calls whever it thinks this good. However using the monad design pattern (deliberately) prevents this (each operation passes its value as a parameter to the subsequent operation, thus forcing an order of evaluation). Do other languages (a) not support compiler/runtime reordering of functions, or (b) have some other way of preventing functions with side-effects from being reordered? Thanks.. Simon

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