August 2006 - Haskell-Cafe

Separate a string into a list of strings
by Sara Kenedy 18 Apr '08

18 Apr '08

Hi all, I want to write a function to separate a string into a list of strings separated by commas. Example: separate :: String -> [String] separate "Haskell, Haskell, and Haskell" = ["Haskell", "Haskell", "and Haskell"] If anyone has some ideas, please share with me. Thanks. S.

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Creating static libraries using GHC
by Tom Hawkins 16 Mar '07

16 Mar '07

I have a chunk of Haskell code I would like wrap up and distribute as a library. Is there a way to build a static library (*.a) that includes my code plus the Haskell runtime, which C programs can easily link against? Here is what I have tried so far... ghc --make -fffi MyLib # Builds MyLib.o and MyLib_stub.o. gcc -c -I/usr/local/lib/ghc-6.4.1/include MyLibWrapper.c ar -r libMyLib.a MyLib.o MyLib_stub.o MyLibWrapper.o This works fine when I use ghc to compile and link a C program... ghc main.c libMyLib.a But if I use gcc, it throws a lot of unresolved references. I have no problem compiling with ghc, but the folks using my library probably won't have it installed. I added -v to ghc to see how it calls gcc; it seems to link in different libraries based on what Haskell libraries are being used, and it undefines a bunch is symbol references. Are there any switches to have ghc return a single archive with everything included? Please excuse my ignorance of the linking process. Thanks for any help! -Tom

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forall and a parse error
by Neil Mitchell 15 Nov '06

15 Nov '06

Hi, I was experimenting with forall and higher rank types briefly, in particular: x :: [forall a . a] This is illegal because of: http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.htm… Which is fine, however its surprising to compare the error messages: [forall a . a] parse error on input `forall' [(forall a . a)] Illegal polymorphic or qualified type: forall a. a In the type signature: lst :: [(forall a. a)] In normal Haskell, I tend to view [x] as equivalent to [(x)] (provided that x is not a tuple) but in this case it has a different meaning - albeit both are erronous meanings. When running the example with Hugs, they both come out as syntax errors - the first on the forall, the second on the closing square bracket. Thanks Neil

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A restricted subset of CPP included in a revision of Haskell 98
by Brian Smith 05 Nov '06

05 Nov '06

Hi, I find it strange that right now almost every Haskell program directly or indirectly (through FPTOOLS) depends on CPP, yet there is no effort to replace CPP with something better or standardize its usage in Haskell. According to the following document, and my own limited experience in reading Haskell code, CPP is the most frequently used extension: http://hackage.haskell.org/trac/haskell-prime/wiki/HaskellExtensions I think that if we accepted that CPP was part of the language, we could then place some restrictions on its use to facilitate easier parsing. Here are some suggestions, off the top of my head: * #define can only be used for parameterless definitions * #define'd symbols are only visible to the preprocessor * #define can only give a symbol a value that is a valid preprocessor expression * #define can only appear above the module declaration * a preprocessor symbol, once defined, cannot be undefined or redefined * #include and #undef are prohibited * The preprocessor can only be used at the top level. In particular, a prepropcessor conditional, #error, #warn, #line would not be allowed within the export list or within a top-level binding. * A Haskell program must assume that any top-level symbol definitions are constant over the entire program. For example, a program must not depend on having one module compiled with one set of command-line preprocessor symbol bindings and another module defined with a different set of bindings. * preprocessor directives must obey Haskell's layout rules. For example, an #if cannot be indented more than the bindings it contains. The result would be: * Syntax can be fully checked without knowing the values of any preprocessor symbols. * Preprocessor syntax can be added easily to a Haskell parser's BNF description of Haskell. * No tool will need to support per-file/module preprocessor symbol bindings. Again, all this is just off the top of my head. I am curious about what problems these restrictions might cause, especially for existing programs. I know that GHC itself uses some features that would be prohibited here. But, GHC is really difficult for tools to handle even with these restrictions on its source code. For now, I am more interested in the libraries in FPTOOLS and users' programs. What libraries/programs cannot easily be reorganizated to meet these restrictions? I suspect "#define'd symbols are only visible to the preprocessor" would be the most troublesome one. Thanks, Brian

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source code for haskell web server?
by mvanier 28 Sep '06

28 Sep '06

Does anyone know where I could find the source code for the Haskell web server described in the papers "Tackling the Awkward Squad" by SPJ and "Writing High-Performance Server Applications in Haskell, Case Study: A Haskell Web Server" by Simon Marlow? Thanks in advance, Mike

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Computing lazy and strict list operations at the same time
by C Rodrigues 27 Sep '06

27 Sep '06

Here's a puzzle I haven't been able to solve. Is it possible to write the initlast function? There are functions "init" and "last" that take constant stack space and traverse the list at most once. You can think of traversing the list as deconstructing all the (:) [] constructors in list. init (x:xs) = init' x xs where init' x (y:ys) = x:init' y ys init' _ [] = [] last (x:xs) = last' x xs where last' _ (y:ys) = last' y ys last' x [] = x Now, is there a way to write initlast :: [a] -> ([a], a) that returns the result of init and the result of last, takes constant stack space, and traverses the list only once? Calling reverse traverses the list again. I couldn't think of a way to do it, but I couldn't figure out why it would be impossible.

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map (-2) [1..5]
by Tamas K Papp 19 Sep '06

19 Sep '06

The code in the subject generates an error. I understand why this is (- is treated as part of the number), but I don't know how to solve it, ie how to tell Haskell that - is a function/binary operator? Thanks, Tamas

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Nice cygwin application to be used with ghci
by Maurício 10 Sep '06

10 Sep '06

Hi, I've just uploaded a package ('rlwrap') to cygwin that I like to use with ghci. You can use it like this: rlwrap ghcii.sh and then you will use ghci as if it were readline aware (i.e., you can press up arrow to get last typed lines etc.). 'rlwrap' is very stable and I never had unexpected results while using it. Since the issue of ghci integration with terminals has been raised here sometimes, I thought some guys here would be interested (actually, I found rlwrap looking for a better way to use ghci). Best, Maurício

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Derived Read instance for types with infix constructors (ghc 6.4.1)
by Misha Aizatulin 09 Sep '06

09 Sep '06

hi, the Haskell Report 10.4 says that "The result of show is readable by read if all component types are readable" however if I define a type like data T = A | T `And` T deriving (Read, Show) then *Main> show $ A `And` A "A And A" *Main> (read "A And A") :: T *** Exception: Prelude.read: no parse *Main> In fact, I wasn't able to guess, what I should type so that the value (A `And` A) gets parsed. I have ghc 6.4.1. Looking into the code of the derived instance I see that it expects Text.Read.Lex.lex to return (Symbol "And") for the constructor. If I understand the code for lex correctly, then it parses things as Symbol if they consist only of "!(a)#$%&*+./<=>?\\^|:-~" How then do I read values of type T defined above? Thanks in advance for any directions. Cheers, Misha

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A free monad theorem?
by Benjamin Franksen 04 Sep '06

04 Sep '06

I'd like to know if the following reasoning can be made more precise: As we all know, the monadic bind operation has type: bind :: Monad m => m a -> (a -> m b) -> m b My intuition says that in order to apply the second argument to some non-trivial (i.e. non-bottom) value of type a, the bind operator needs to be able to somehow 'extract' a value (of type a) from the first argument (of type m a). I would like to argue that this is because bind is polymorphic in a, which makes it impossible to construct values of type a 'out of thin air' (besides bottom). Thus, however bind is defined, the only place where it can obtain a 'real' value of type a is from its first argument. Although one might think that this implies the existence of some function extract :: Monad m => m a -> a it is obvious that this is too limiting, as the IO monad plainly shows. Even monads that can be implemented in Haskell itself (the vast majority, it seems) usually need some additional (monad-specific) argument to their 'extract' (or 'run') function. However, even a value of type IO a /does/ produce a value of type a, only the 'value producing computation' is not a (pure) function. But how can all this be made more precise with less handwaiving? The background for my question is an argument I had some time ago with someone about what the 'real nature' of monads is. He argued that monads are mainly about 'chaining' (somehow wrapped up) values in an associative way, refering to the monad laws. I argued that monadic values get 'chained' in a very specific way and that in order to get an intuition about what this monadic chaining really means on the most general level, the standard model of 'computation that returns a value of type a' is the appropriate one. I tried to use the above (handwaving) argument to convince him (and myself) that this model is indeed 'the right one'. Furthermore, if it really is, then one might conjecture that for /every/ monad there must be some natural interpretation as the representation of some kind of computation (effectful or not). So my second question is: Are there known 'counter-examples' where this intuition breaks down, i.e. monads for which a computational interpretation is unknown or at least obscure enough not to qalify as 'natural'? Cheers, Ben

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