September 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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Are associated types synonyms like type classes?
by Brian Smith 17 Nov '06

17 Nov '06

I read the easy parts of the "Associated Types with Class" and "Associated Type Synonyms" papers. An associated type synonym seems to kind of work similarly to a restricted form of class. In what way are the two following examples different? -- define a class with a type synonym, and a set of operations class A a where type B b foo :: a -> B b instance A Int where type B = Bool foo = (==0) -- define two classes, and an function that . class A a, B b where foo :: a -> b instance A Int, B Bool where foo = (==0) Also, has anybody written a paper on the differences between typeclasses + associated types and ML's module system + overloading? Thanks, Brian

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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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A better syntax for qualified operators?
by Brian Hulley 19 Oct '06

19 Oct '06

Hi - Consider the scenario when you want to find a function that returns the i'th element of an array but all you know is that there is a module called Data.Array.IArray that will probably have such a function in it. So you start typing in your program: let ith = Data.Array.IArray. at this point, you'd hope the editor you're using would somehow display a list of avaliable values exported from Data.Array.IArray including the indexing function, so you could select it, thus I would *like* to be able to use the syntax: let ith = Data.Array.IArray.(!) because it's not the user's fault that the person who wrote Data.Array.IArray decided to use a symbol instead of an identifier for this function - the user of Data.Array.IArray in this case just wants to see normal identifiers to use with prefix application so the use of (!) at this point effectively gets rid of the unwanted operatorness associated with the function. However the current syntax of Haskell would not allow this. Instead you have to write: let ith = (Data.Array.IArray.!) The problem is that the user of Data.Array.IArray has to know already in advance, before typing the 'D' of "Data", that the indexing function has been named with a symbol instead of an identifier, but this knowledge is only available later, when the user has typed the '.' after "IArray", so the current syntax would be frustrating for the user because the user then has to go all the way back and insert an opening paren before the 'D'. Also, consider the appearance of: let ith = (Data.Array.IArray.!) arr i b = Data.Array.IArray.bounds arr vs let ith = Data.Array.IArray.(!) arr i b = Data.Array.IArray.bounds arr I'm not sure if I've managed to explain this problem clearly enough, but my proposal is that we might consider changing the lexical syntax of Haskell as follows: varId ::= id varOp ::= symbol varIdOp ::= ` varId varOpId ::= ( varOp ) varOpIdOp ::= ` varOpId qvarId ::= {conId .}+ varId -- { }+ denotes 1 or more times qvarIdOp ::= ` qvarId qvarOp ::= {conId .}+ varOp qvarOpId ::= {conId .}+ varOpId qvarOpIdOp ::= `qvarOpId In other words, to turn an operator symbol into an id, the parentheses would be put immediately around the symbol (with no spaces since this is lexical syntax), and to turn an id into an operator the backquote is put in front of the entire (qualified) id. (Also the trailing backquote in the existing syntax is redundant) The above syntax would have 3 advantages: 1) It allows the client of a module to write code without having to worry if the author of the module used symbols or identifiers to name functions - everything exported from the module can be made to appear as if it was named by an identifier (ie OpId) 2) Moving the parentheses to the lexical syntax makes syntax highlighting easier (because there are no comments to worry about inside the OpId) and also makes parsing simpler because all the mess associated with Ops versus Ids is handled by the lexer 3) It allows an editor to make a distinction between (+) -- an operator turned into an identifier - varOpId ( + ) -- an expression with 2 gaps in it which should be marked as incomplete (+ ) -- a section with 1 gap Some examples of the proposed syntax are: let ith = Data.Array.IArray.(!) arr i foo = k `Math.(+) 6 -- default precendence bar = k Math.+ 6 -- using precedence of + in module Math When you try to write an editor for Haskell (or some subset of it), you quickly discover these areas of Haskell syntax like the above which need to be changed to get an optimum interactive editing experience. I think it *is* possible to adjust the Haskell grammar so that it is LL(1) and the only reason it is not already LL(1) seems to be that the grammar has been designed with compilers (which only need to deal with complete modules) in mind rather than programmers interactively editing in mind. (The other change needed for LL(1) is to give contexts a marker before they appear eg: foo :: {MonadIO m} a -> m a ) By LL(1) I'm really meaning that the grammar for interactive editing needs to be adjusted so that it is possible to maintain the invariant that as code is entered from left to right constructs and identifiers can be highlighted according to their grammatical role and highlighting (modulo incompleteness) must remain unchanged regardless of whatever is typed afterwards to the right otherwise it can become more of a liability than a help, hence my hope that some future revision of Haskell grammar might consider taking the above points into account. Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

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Is Haskell a 5GL?
by Ch. A. Herrmann 10 Oct '06

10 Oct '06

Dear Haskellers, do you think that Haskell is a 3GL (third generation language) or a 5GL or that the hierarchy of programming language generations is useless? I see a problem if the potential of a language is evaluated according to the generation it belongs to. 1GLs: machine languages 2GLs: assembler languages 3GLs: programming based on algorithms (C, Java, ...) 4GSs: domain-specific languages (SQL,...) 5GLs: automatic problem solving (Mathematica, Prolog,...) and/or visual programming (Visual Basic,...) Haskell's laziness would in principle permit programming in a style akin to Prolog, e.g., for parser combinators. The question is: how strong is the influence of laziness in everyday Haskell programming? Cheers -- Christoph

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irrefutable patterns for existential types / GADTs
by oleg＠pobox.com 04 Oct '06

04 Oct '06

It seems that irrefutable pattern match with existentials is safe. The fact that irrefutable pattern match with GADT is unsafe has been demonstrated back in September 2004. Let us consider the following regular existential data type > data TFoo where > Foo :: Show a => a -> TFoo > Bar :: Int -> TFoo Despite the 'where' syntax, it is NOT GADT; it is just a regular data type. We can write > test_foo vf = case vf of ~(Foo x) -> body Now, if 'x' does not occur in `body', we could have just as well written test_foo vf = body If `x' does occur in body and the scrutinee is not of the form `Foo', then 'x' is undefined, and so 'body' bottoms out when it demands the value of 'x'. No surprise, and no concern here. Let is now consider a GADT > data GTFoo a where > GFoo :: GTFoo Int > GBar :: GTFoo Bool > > test_gfoo :: GTFoo a -> a > test_gfoo vf = case vf of GFoo -> (1::Int) It can be faithfully emulated as follows > -- the data constructors Em_GFoo and Em_GBar must be hidden! > data EmulateGTFoo a = Em_GFoo | Em_GBar > > em_gfoo :: EmulateGTFoo Int > em_gfoo = Em_GFoo > em_gbar :: EmulateGTFoo Bool > em_gbar = Em_GBar The constructors Em_GFoo and Em_GBar should be hidden. The user should use `smart' constructors em_gfoo and em_gbar, which not only set the correct type (Int vs Bool) but also produce the witness, viz. Em_GFoo or Em_GBar. Now, the test_gfoo function should be written as > tesd_emulate_gfoo :: EmulateGTFoo a -> a > tesd_emulate_gfoo vf = case vf of Em_GFoo -> unsafeCoerce (1::Int) So, we test for evidence, Em_GFoo, and if it is presented, we proceed with unsafeCoerce, which generalizes Int to the desired type 'a'. We know this generalization is safe because the evidence Em_GFoo told us that 'a' was really Int. The similarity with Dynamics is uncanny. Now, had we used an irrefutable match instead, we would have proceeded with unsafeCoerce without checking for the evidence. The following code, written back in Sep 13, 2004, shows that the above is not an empty concern. The code did indeed typecheck, with the version of GHC (tidt-branch CVS branch) that existed at that time. Running the code produced the result one'd expect when reading an Int as a Bool. I think the code below was the reason GHC prohibited irrefutable GADT pattern matching since. > {-# OPTIONS -fglasgow-exts #-} > > module Main where > > import Data.IORef > > data T a where > Li:: Int -> T Int > Lb:: Bool -> T Bool > La:: a -> T a > > writeInt:: T a -> IORef a -> IO () > writeInt v ref = case v of > ~(Li x) -> writeIORef ref (1::Int) > > readBool:: T a -> IORef a -> IO () > readBool v ref = case v of > ~(Lb x) -> > readIORef ref >>= (print . not) > > tt::T a -> IO () > tt v = case v of > ~(Li x) -> print "OK" > > main = do > tt (La undefined) > ref <- newIORef undefined > writeInt (La undefined) ref > readBool (La undefined) ref

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How can I redirect stdout?
by Matthew Bromberg 02 Oct '06

02 Oct '06

I'm reposting this in it's own new thread since I think it involves a general issue beyond exporting Haskell DLLs. I am having some problems with GHCs stdout when a Haskell program is called from a windows program. As I noted earlier I am calling some Haskell code from C as a bridge to being able to ultimately call Haskell from Matlab 6.5. The Haskell code is compiled into a .DLL file on a windows machine. Matlab calls some C code which then calls the Haskell code. As soon as it gets into the Haskell code I get this run time error in ghcDLL.dll <stdout>: hPutChars: invalid argument (Bad File Descriptor) The Haskell code seems to work correctly if called from a stand-alone C program. Moreover, if I scrub all print statements from the Haskell code, it appears to run correctly from Matlab. Matlab does not properly redirect stdout so any printf calls from the C code simply fail to print. However Haskell's behavior is to halt after generating a runtime error. The C code used to generate the Haskell .dll is of course mingw gcc, but the C code used to create my Matlab mex file is Microsoft VC++ 6.0. I tried to redirect stdout using freopen() in C, but that never seems to work with Microsoft. At any rate it certainly doesn't effect Haskells use of stdout. I think in general for windows programs there is no stdout defined, whereas it's always defined for console programs. My question is, is there some way I can redirect stdout from inside Haskell so that all output is sent to a file? Is there an equivalent of freopen() in Haskell that works?

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non lazy io
by Matt Roberts 02 Oct '06

02 Oct '06

I would like a non-lazy alternative to readFile is there such a thing in the libraries? matt r

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