Re: [Haskell-cafe] converting prefixes of CString - String
Let 'c2h' convert CStrings to Haskell Strings, and 'h2c' convert
Haskell Strings to CStrings. (If I understand correctly, c2h . h2c
=== id, but h2c . c2h is not the identity on all inputs;
That is correct. CStrings are 8-bits, and Haskell Strings are 32-bits.
Converting from Haskell to C loses information, unless you use a multi-byte
encoding on the C side (for instance, UTF8).
So actually I am incorrect, and h2c . c2h is the identity but c2h . h2c is not?
I suggest you look at the utf8-string package, for instance
Codec.Binary.UTF8.String.{encode,decode}, which convert Haskell strings
to/from a list of Word8, which can then be transferred via the FFI to
wherever you like.
This package was very helpful; I looked at the source to see how the
utf8 encoding was done. It looks as if the functionality I want is
technically feasible but not implemented yet; it shouldn't be too
much trouble to implement it myself, by imitating the existing
'decode' function but changing its behavior when it runs out of input
in the middle of a utf8-character. Also key is the property
s1 ++ s2 == decode (encode s1)) ++ decode (encode s2))
holds.
Thanks,
Eric
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[Haskell-cafe] converting prefixes of CString - String
I have been reading Foreign.C.String but it does not seem to provide the functionality I was looking for. Let 'c2h' convert CStrings to Haskell Strings, and 'h2c' convert Haskell Strings to CStrings. (If I understand correctly, c2h . h2c === id, but h2c . c2h is not the identity on all inputs; or perhaps c2h is not defined for all CStrings. Probably this is all locale dependent.) I have an infinite Haskell String transferred byte-wise over a network; I would like to convert some prefix of the bytes received into a prefix of the String I started with. However, if I understand correctly, if s is a Haskell String it is not necessarily true that c2h (take n (h2c s)) is a prefix of s for all n. So I have two questions: Given a CString of the form cs = take n (h2c s), how do I know whether c2h cs is a prefix of s or not? Is there a way to recognize whether a CString is valid as opposed to truncated in the middle of a code point, or is this impossible? Better yet, given a CString cs = take n (h2c s), is there a way to find the maximal prefix cs' of cs such that c2h cs' is a prefix of s? If s == s1 ++ s2, is it necessarily true that s == (c2h (h2c s1)) ++ (c2h (h2c s2))? If so, then I can perform my conversion a bit at a time, otherwise I'd need to start from the beginning of the cstring each time I receive additional data. In practice, I think my solution will come down to restricting my program to only using the lower 128 characters, but I'd like to know how to handle this problem in full generality. Thanks, Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] thread safety, IO arrays, and IO refs
Hello, I wish to use a mutable array in multiple threads. Can IO arrays be used in any thread, or only the thread they are created in? (So if I create an IO array in one thread, pass it to another via an MVar, can I read / edit it in that other thread?) Similarly about IORefs... can they be used across threads? (Obviously I will need to manage locking myself if I use an IO ref or array.) Are there any good resources on concurrency in Haskell that would answer questions similar to these? I've been learning by reading the ghc library documentation, but of course it's not comprehensive. The online tutorials I've seen aren't so in-depth, either. Thanks, Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] thread safety, IO arrays, and IO refs
I tried a quick test with an IOArray, so I know that at least one time it works. I don't know enough of the internals of IO and IOArray to be able to extrapolate; there could be some race condition hidden internally if IO is misused. Thanks, Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Not in scope: type constructor or class `Map'
Because Data.Map is imported qualified, any symbols in it (including Map) needs to be qualified: type Bindings = Map.Map String Int A standard idiom is to do import like so: import qualified Data.Map as Map import Map (Map) so that the Map symbol itself does not need qualification. Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] SampleVar semantics, documentation vs. source
Hi, doc: http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/base-4.3.0.0/Control-Concurrent-SampleVar.html source: http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/base-4.3.0.0/src/Control-Concurrent-SampleVar.html The documentation for Control.Concurrent.SampleVar implies that a SampleVar is either filled or empty. The source code suggests that there are three possible states: full (when readers == 1), empty (when readers == 0), and empty with blocked readers (when readers 0). In particular, the isEmptySampleVar function, isEmptySampleVar :: SampleVar a - IO Bool isEmptySampleVar (SampleVar svar) = do (readers, _) - readMVar svar return (readers == 0) returns false in the third state, when readers 0. Can someone clarify the semantics of SampleVar? Thanks, Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] lhs syntax highlighting in vim
Hello, The syntax highlighting file for literate haskell in vim says that its maintainer is haskell-cafe@haskell.org, so hopefully one of you will find this relevant. In literate haskell files, vim optionally highlights the non-code text according to TeX markup. The syntax highlighting file looks for key phrases (e.g., \documentclass) to decide whether to use TeX markup highlighting; but it (erroneously, in my opinion, and at variance with the documentation) will use TeX markup highlighting on any lhs file that contains the '%' character anywhere in it. The bug is in line 74 of version 1.01 of the lhaskell.vim syntax highlighting file. Thanks, Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie: State monad example questions
So, are there any other simple motivating examples that show what state is really good for? Here's an example from some code that I'm (trying to) write; I am writing a DSL for the Povray Scene Description Language. This part of my program creates a `String' which holds a piece of Povray SDL code. I am using the state to keep track of an infinite list of unique identifiers -- when I use an identifier I would like to avoid reusing the same one later. type Identifier = String type Identifiers = [Identifier] all_identifiers :: Identifiers all_identifiers = map (\n - var ++ show n) [0, 1..] next_id :: State Identifiers Identifier next_id = do (a:as) - get put as return a I define a function let_ so that if a user of my code writes something like: let_ value expr For example, if a user said: let_ (vector (0, 0, 0)) (\origin - let_ (vector (1, 2, 3)) (\p - union [box origin p, sphere origin (float 1), cylinder origin p (float 0.5)])) it should be analogous to: union [box (vector (0, 0, 0)) (vector (1, 2, 3)), sphere (vector (0, 0, 0)) (float 1), cylinder (vector (0, 0, 0)) (vector (1, 2, 3)) (float 0.5)] (Cf. http://www.haskell.org/pipermail/haskell-cafe/2008-February/039639.html for details on what I'm trying to do here, but it has nothing to do with my example usage of a state monad.) In my definition of let_, I extract a fresh, unused identifier which is assigned to the value of value. type Code x = State Identifiers String let_ :: Code x - (Code x - Code y) - Code y let_ m_value m_expr = do id - next_id value - m_value expr - m_expr (return id) return (#declare ++ id ++ = ++ value ++ ;\n ++ expr) Either of the expressions m_value or m_expr may require their own unique identifiers, but the State monad takes care of threading my `Identifiers' state so that the same identifier will not be used more than once. Later on, when I made a more sophisticated version of `Identifiers' which kept of track of multiple different namespaces from which identifiers could come, I only had to modify `next_id' without having to worry about whether I would have to make changes in other parts of the program (although I believe further changes would not have been necessary even if I had not used State monads, the modularity of the code is much more obvious when using the State monad instead of explicitly writing out the state that is being passed around). Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Short circuiting and the Maybe monad
Since a tree is a kind of container, yes, it should be a monad. [I'm still
not really sure whether it's useful.]
I have a use for the tree monad -- well, I have a use for the monadic
`join' operation (but I don't use any of the monadic operations other
than `return').
My program is making a file. It stores the lines in the file (I need
the individual lines, for operations like indent all lines by four
spaces):
* type File = [Line]
And each line is a ordered collection of String s, waiting to be concatenated:
* type Line = [String]
At the end of the day I need one long string:
* finish :: File - String
* finish file = concat (concat file)
(I'm eliding dealing with endlines, etc.)
But elsewhere in my program, I would like to have an O(1) (++)
operation, i.e., I would like to be able to do line1 ++ line2 and
file1 ++ file2 in O(1) time. So I use a Tree a instead of [a]:
type File = Tree Line
type Line = Tree String
finish file = concat (tree_to_list (tree_flatten file))
Admittedly, I'm not really doing anything actually monadic here -- I
just happen to be using the `join' operation. I am sure others can
come up with better examples.
While we're at it, an example use for the ((-) a) monad. For any set
S and ring R, the abelian group of functions f :: S - R is a free
module (in the mathematical sense of the word module) over the ring
R. The group and module operations are easily defined in terms of the
monadic operations:
data FreeModule s r = FreeModule (s - r)
instance Functor (FreeModule s) where ...
instance Monad (FreeModule s) where ...
instance Ring r = Group (FreeModule s r) where
(+) m1 m2 = liftM2 (+) m1 m2
negative m = fmap negative m
zero = return zero
instance Ring r = Module r (FreeModule s r) where
r * m = fmap (r *) m
(Here I write `r * m' to mean the module element `m' left-multiplied
by the ring element `r'.)
I have another implementation of FreeModule which specializes S to the
natural numbers: but the set of functions f :: \mathbb{N} - R are
isomorphic with f :: [R] (provided we only permit infinite lists), in
the same way that Dave Menendez describes how f :: Bool - a is
isomorphic to f :: Diag a. Under this isomorphism, we translate the
monadic operations from ((-) \mathbb{N}) to monadic operations on
lists -- but the resulting monad is different from the usual list
monad (where join = concat is the structure flattening operation).
After all, the `concat' operation is pretty boring if you only permit
infinite lists (for then `concat' is the same as `head').
Eric
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Re: [Haskell-cafe] Short circuiting and the Maybe monad
I have another implementation of FreeModule which specializes S to the
natural numbers: but the set of functions f :: \mathbb{N} - R are
isomorphic with f :: [R] (provided we only permit infinite lists), in
the same way that Dave Menendez describes how f :: Bool - a is
isomorphic to f :: Diag a.
This is what I get for reading only the first half of Dave Menendez's
email... I saw the words Here's a more complex example and skipped
the rest. Read his explanation -- it's a lot more coherent than mine.
Eric
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Re: [Haskell-cafe] Re: Endianess
So I've always wondered, if you are writing down a number being dictated (slowly) by someone else, like 234, do you write the 2, then leave space and write the 4, then go back and fill in with 3? Or do you push the 4 onto the stack until the 3 arrives, and write 34 at once. My German professor told us a story, set in WWII: Two British pilots shot down behind enemy lines, who had received very thorough training on German culture, current sports, etc., successfully blended in with the native Germans for some time until someone noticed them in a cafe writing down the sum on their bill in left-to-right order. They were summarily shot. Not that I have any idea whether this is based on truth or not, or whether Germans, in fact, write the last two digits in reverse order. Also, Claus's reply gives me a headache. Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type unions
I have been trying to write a DSL for Povray (see www.povray.org) in Haskell, using the technique of: http://okmij.org/ftp/papers/tagless-final-APLAS.pdf with some inspiration taken from http://okmij.org/ftp/Haskell/DSLSharing.hs The Povray Scene Description Language is a very declarative language, with few high level constructs (even loops take a bit of work) -- which is why I'm putting it in Haskell. At one point, I needed a varargs function for the DSL, a function f :: b - a - b dressed up to take a variable number of 'a's, known at compile time. This was easy enough: data Nil a = Nil data Cons b a = a ::: b a infixr 1 ::: class VarArgs v where apply_args :: (s - a - s) - s - v a - s instance VarArgs Nil where apply_args _ start _ = start instance VarArgs b = VarArgs (Cons b) where apply_args f start (a ::: b) = apply_args f (f start a) b The solution is quite workable: I can simply write the following, and I believe the summation is expanded out at compile-time: apply_args (+) 0 (2 ::: 3 ::: 8 ::: 1 ::: (-3) ::: Nil) But I found I also needed a function to take a union type -- that is, the function would either take an argument of type T1, or of type T2, known at compile time. I tried a similar technique as I tried with varargs, and unfortunately ended up with this: data LeftOf a b = L a data RightOf a b = R b class Union u where apply_union :: (a - c) - (b - c) - (u a b) - c instance Union LeftOf where apply_union f _ (L a) = f a instance Union RightOf where apply_union _ g (R b) = g b type A = Integer type B = String type C = () type Union_ABC u1 u2 = u1 A (u2 B C) f_A = show . (+ 3) f_B = reverse f_C = const unit f :: (Union u1, Union u2) = Union_ABC u1 u2 - String f = apply_union f_A (apply_union f_B f_C) main = do putStrLn $ f $ (L 6 :: Union_ABC LeftOne LeftOne) putStrLn $ f $ R (L hello, world) putStrLn $ f $ R (R ()) Notice a lot of ugliness in my example: e.g., the definition of f, the type signature of f (I can't move the context into the type-synonym Union_ABC), creating objects of the union type, and the unpleasant surprise that I needed to provide the type of 'L 6'. This solution is very not-scalable: the Povray SDL is a messy language, and for my DSL I would need approximately 20 or 30 such unions, each a union of about 20 types (admittedly with a lot of overlap from union to union). I think the solution is to abandon the lofty ideal of statically determining argument types; instead have a universal type with tags to distinguish types dynamically: data Universal = UA A | UB B | UC C f :: Universal - String f (UA a) = f_A a f (UB b) = f_B b f (UC c) = f_C c main2 = do putStrLn $ f $ UA 6 putStrLn $ f $ UB hello, world putStrLn $ f $ UC () ...but I'm not ready to give up hope yet. Suggestions please? Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type unions
Try making a type class for the functions. That will allow you both varargs and unions. Have a look at Text.Printf. Thank you -- looking at Printf was very helpful. My syntax is much happier as a result. I also see now that I am approaching the problem from the wrong direction -- that by writing a whole slew of functions whose behavior depends arbitrarily upon their argument type, I would need to code the desired behavior for each function and each argument type; getting hung up on making the syntax manageable hid this realization from me. The solution is for me to try much harder to extract common behavior across argument types and only code special cases when truly necessary. Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maybe a, The Rationale
IME nullable by default is one of the biggest sources of runtime crashes in high level OOP languages like C#, which is a shame because it really isn't that difficult to statically eliminate the vast majority of them, especially when you're sort of hand-waving the semantics of your language anyway and don't require it to be super rigorous... I definitely agree. After I'd been learning Haskell for 6 months and then wrote a program in Java C++, almost the first thing I did was code up a generic MaybeT class in each language. It is so much clearer and more obvious to _explicitly_ have no value (Maybe.Nothing()) as opposed to _implicitly_ having no value (null). Now I find my Java C++ MaybeT class indispensable when I am programming in those languages. Eric ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
