November 2010 - Haskell-Cafe

save the dates: SF Bay Area Haskell Hackathon: Feb 11~13, 2011
by Mark Lentczner 24 Nov '10

24 Nov '10

Bryan O'Sullivan and I are hosting a SF Bay Area Haskell Hackathon at the Hacker Dojo in Mountain View, California. Details are all sketchy at this point, but we plan on two components: 1) Haskell Project Hackathon 2) Learn Haskell Workshop See: http://wiki.hackerdojo.com/w/page/Haskell-Hackathon-2011 At this stage we're trying to gauge interest, size, and volunteers to help with organizing and running it. Either add yourself to that wiki page, or e-mail me. - Mark Lentczner Mark Lentczner http://www.ozonehouse.com/mark/ IRC: mtnviewmark

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Another Quasi Quotation question
by jean-christophe mincke 23 Nov '10

23 Nov '10

Hello, I am still playing with template-haskell... I wonder, is there any reason why a quasiquoter cannot create haskell statements and declarations in addition to expressions and patterns? Or more generally create any legal Haskell syntax tree. I.e Suppose I would like to create a quasiquoter for the C language (please, imagine that it could be useful). I could write sth such as : [$c| int a = 6; int b = 7; int c = a +b; struct S { int x; int y} |] It could be nice to generate the appropriate haskell code: a = 6 b= 7 c= a + b data S = S { x::Int; y:Int} etc That would allow to really embed any kind of language in a more or less easy way into haskell, provided that code can be translated into legal haskell. Is there anything that prevent these features. Thank you Regards J-C

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Musings on type systems
by Andrew Coppin 23 Nov '10

23 Nov '10

OK, so how do types actually work? Musing on this for a moment, it seems that declaring a variable to have a certain type constrains the possible values that the variable can have. You could say that a type is some sort of "set", and that by issuing a type declaration, the compiler statically guarantees that the variable's value will always be an element of this set. Now here's an interesting thought. Haskell has "algebraic data types". "Algebraic" because they are sum types of product types (or, equivilently, product types of sum types). Now I don't actually know what those terms mean, but proceeding by logical inferrence, it seems that Either X Y defines a set that could be described as the union or "sum" of the types X and Y. So is Either what is meant by a "sum type"? Similarly, (X, Y) is a set that could be considered the Cartesian product of the sets X and Y. It even has "product" in the name. So is this a "product type"? So not only do we have "types" which denote "sets of possible values", but we have operators for constructing new sets from existing ones. (Mostly just applying type constructors to arguments.) Notionally (->) is just another type constructor, so functions aren't fundamentally different to any other types - at least, as far as the type system goes. Now, what about type variables? What do they do? Well now, that seems to be slightly interesting, since a type variable holds an entire type (whereas normal program variables just hold a single value), and each occurrance of the same variable is statically guaranteed to hold the same thing at all times. It's sort of like how every instance of a normal program variable holds the same value, except that you don't explicitly say what that value is; the compiler infers it. So what about classes? Well, restricting ourselves to single-parameter classes, it seems to me that a class is like a *set of types*. Now, interestingly, an enumeration type is a set of values where you explicitly list all possible values. But once defined, it is impossible to add new values to the set. You could say this is a "closed" set. A type, on the other hand, is an "open" set of types; you can add new types at any time. I honestly can't think of a useful intuition for MPTCs right now. Now what happens if you add associated types? For example, the canonical class Container c where type Element c :: * We already have type constructor functions such as Maybe with takes a type and constructs a new type. But here we seem to have a general function, Element, which takes some type and returns a new, arbitrary, type. And we define it by a series of equations: instance Container [x] where Element [x] = x instance Container ByteString where Element ByteString = Word8 instance (Ord x) => Container (Set x) where Element (Set x) = x instance Container IntSet where Element IntSet = Int ... Further, the inputs to this function are statically guaranteed to be types from the set (class) "Container". So it's /almost/ like a regular value-level function, just with weird definition syntax. Where *the hell* do GADTs fit in here? Well, they're usually used with phantom types, so I guess we need to figure out where phantom types fit in. To the type checker, Foo Int and Foo Bool are two totally seperate types. In the phantom type case, the set of possible values for both types are actually identical. So really we just have two names for the same set. The same thing goes for a type alias (the "type" keyword). It's not quite the same as a "newtype", since then the value expressions do actually change. So it seems that a GADT is an ADT where the elements of the set are assigned to sets of different names, or the elements are partitioned into disjoint sets with different names. Hmm, interesting. At the same type, values have types, and types have "kinds". As best as I can tell, kinds exist only to distinguish between /types/ and /type functions/. For type constructors, this is the whole story. But for associated types, it's more complicated. For example, Element :: * -> *, however the type argument is statically guaranteed to belong to the Container set (class). In other news... my head hurts! >_< So what would happen if some crazy person decided to make the kind system more like the type system? Or make the type system more like the value system? Do we end up with another layer to our cake? Is it possible to generalise it to an infinite number of layers? Or to a circular hierachy? Is any of this /useful/ in any way? Have aliens contacted Earth in living member? OK, I should probably stop typing now. [So you see what I did there?] Also, I have a sinking feeling that if anybody actually replies to this email, I'm not going to comprehend a single word of what they're talking about...

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About "Fun with type functions" example
by Arnaud Bailly 23 Nov '10

23 Nov '10

Hello, I am trying to understand and use the Nat n type defined in the aforementioned article. Unfortunately, the given code does not compile properly: Here is the code: module Naturals where data Zero data Succ a class Nat n where toInt :: n -> Int instance Nat Zero where toInt _ = 0 instance (Nat n) => Nat (Succ n) where toInt _ = 1 + toInt (undefined :: n) type One = Succ Zero type Two = Succ One And here is the error: D:\projets\sequencer>ghc Naturals.hs Naturals.hs:16:18: Ambiguous type variable `n' in the constraint: `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 Probable fix: add a type signature that fixes these type variable(s) I use 6.12.3 on Windows XP. I am most probably missing an option but does not know which one. Thanks in advance for any advice, Arnaud

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Cabal-install is broken
by Permjacov Evgeniy 23 Nov '10

23 Nov '10

current cabal-install (0.8.2) cannot be compiled with ghc-7.0.1 set of boot libraries. It requires cabal 1.8.* wich fails to compile. Does anyone worked this out ?

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Why does cabal unnecessarily reinstall dependencies?
by Martijn van Steenbergen 23 Nov '10

23 Nov '10

Hello cafe, When I want to locally install my own package through "cabal install" it tries to reinstall dependencies convertible-1.0.9.1, HDBC-2.2.6.1 and HDBC-mysql-0.6.3 even though they are already installed (and work fine). Why does it do this? cabal-install version 0.8.2 using version 1.8.0.2 of the Cabal library Mac OS Leopard Thanks, Martijn.

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VNC server in Haskell
by C K Kashyap 23 Nov '10

23 Nov '10

Hi, I've implemented a VNC server (very incomplete) - What it can do currently is display a blue screen (100x100 screen). It's very trivial to try out - https://github.com/ckkashyap/Chitra I look forward to some feedback/comments on this. -- Regards, Kashyap

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Wrong Package-URL in regex-tdfa.cabal
by José Romildo Malaquias 23 Nov '10

23 Nov '10

Hello. The cabal file for the regex-tddfa-1.1.6 package seems to have an invalid URL for the Package-URL field: Package-URL: http://darcs.haskell.org/packages/regex-unstable/regex-tdfa/ It should be fixed. Regards. Romildo

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Build problems with HDBC-mysql on the Mac
by Martijn van Steenbergen 22 Nov '10

22 Nov '10

Dear cafe, Following the instructions at http://matt.freels.name/2010/hdbc-mysql-os-x.html I successfully installed HDBC-mysql on my Mac! However when doing a "cabal install" in a local package which depends on HDBC-mysql, cabal proceeds to reinstall HDBC-mysql (is this a bug? see my other email from a few moments ago), this time unsuccessfully because it is using the unpatched cached version downloaded from hackage. This breaks my installation: ghc-pkg list colors the package red. Now comes the weird part. :-) I try to reinstall HDBC-mysql with my unpacked, patched version. It fixes my installation of HDBC-again, even though it exits prematurely with some errors! I've included the output at the end of this email. I wonder: Where do these errors suddenly come from? The first time the installation went without trouble. How come it fixes my installation even though it reports errors? Thank you in advance, Martijn. Output of "cabal install --extra-lib-dirs=/usr/local/lib:" in a local version of HDBC-mysql, patched according to http://matt.freels.name/2010/hdbc-mysql-os-x.html: Resolving dependencies... [1 of 1] Compiling Main ( Setup.lhs, dist/setup/Main.o ) Setup.lhs:8:0: Warning: In the use of `findProgramOnPath' (imported from Distribution.Simple.Program): Deprecated: "use findProgramLocation instead" Linking ./dist/setup/setup ... Configuring HDBC-mysql-0.6.3... Preprocessing library HDBC-mysql-0.6.3... Connection.hsc:23:19: error: mysql.h: No such file or directory Connection.hsc: In function ‘main’: Connection.hsc:166: error: ‘MYSQL_FIELD’ undeclared (first use in this function) Connection.hsc:166: error: (Each undeclared identifier is reported only once Connection.hsc:166: error: for each function it appears in.) Connection.hsc:170: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:170: error: expected ‘;’ before ‘)’ token Connection.hsc:170: error: expected statement before ‘)’ token Connection.hsc:171: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:171: error: expected ‘;’ before ‘)’ token Connection.hsc:171: error: expected statement before ‘)’ token Connection.hsc:172: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:172: error: expected ‘;’ before ‘)’ token Connection.hsc:172: error: expected statement before ‘)’ token Connection.hsc:173: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:173: error: expected ‘;’ before ‘)’ token Connection.hsc:173: error: expected statement before ‘)’ token Connection.hsc:174: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:174: error: expected ‘;’ before ‘)’ token Connection.hsc:174: error: expected statement before ‘)’ token Connection.hsc:175: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:175: error: expected ‘;’ before ‘)’ token Connection.hsc:175: error: expected statement before ‘)’ token Connection.hsc:203: error: ‘MYSQL_BIND’ undeclared (first use in this function) Connection.hsc:210: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:210: error: expected ‘;’ before ‘)’ token Connection.hsc:210: error: expected statement before ‘)’ token Connection.hsc:211: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:211: error: expected ‘;’ before ‘)’ token Connection.hsc:211: error: expected statement before ‘)’ token Connection.hsc:212: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:212: error: expected ‘;’ before ‘)’ token Connection.hsc:212: error: expected statement before ‘)’ token Connection.hsc:213: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:213: error: expected ‘;’ before ‘)’ token Connection.hsc:213: error: expected statement before ‘)’ token Connection.hsc:214: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:214: error: expected ‘;’ before ‘)’ token Connection.hsc:214: error: expected statement before ‘)’ token Connection.hsc:215: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:215: error: expected ‘;’ before ‘)’ token Connection.hsc:215: error: expected statement before ‘)’ token Connection.hsc:216: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:216: error: expected ‘;’ before ‘)’ token Connection.hsc:216: error: expected statement before ‘)’ token Connection.hsc:228: error: ‘MYSQL_TIME’ undeclared (first use in this function) Connection.hsc:232: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:232: error: expected ‘;’ before ‘)’ token Connection.hsc:232: error: expected statement before ‘)’ token Connection.hsc:233: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:233: error: expected ‘;’ before ‘)’ token Connection.hsc:233: error: expected statement before ‘)’ token Connection.hsc:234: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:234: error: expected ‘;’ before ‘)’ token Connection.hsc:234: error: expected statement before ‘)’ token Connection.hsc:235: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:235: error: expected ‘;’ before ‘)’ token Connection.hsc:235: error: expected statement before ‘)’ token Connection.hsc:236: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:236: error: expected ‘;’ before ‘)’ token Connection.hsc:236: error: expected statement before ‘)’ token Connection.hsc:237: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:237: error: expected ‘;’ before ‘)’ token Connection.hsc:237: error: expected statement before ‘)’ token Connection.hsc:242: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:242: error: expected ‘;’ before ‘)’ token Connection.hsc:242: error: expected statement before ‘)’ token Connection.hsc:243: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:243: error: expected ‘;’ before ‘)’ token Connection.hsc:243: error: expected statement before ‘)’ token Connection.hsc:244: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:244: error: expected ‘;’ before ‘)’ token Connection.hsc:244: error: expected statement before ‘)’ token Connection.hsc:245: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:245: error: expected ‘;’ before ‘)’ token Connection.hsc:245: error: expected statement before ‘)’ token Connection.hsc:246: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:246: error: expected ‘;’ before ‘)’ token Connection.hsc:246: error: expected statement before ‘)’ token Connection.hsc:247: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:247: error: expected ‘;’ before ‘)’ token Connection.hsc:247: error: expected statement before ‘)’ token Connection.hsc:368: error: ‘MYSQL_TYPE_NULL’ undeclared (first use in this function) Connection.hsc:381: error: ‘MYSQL_TYPE_VAR_STRING’ undeclared (first use in this function) Connection.hsc:385: error: ‘MYSQL_TYPE_LONGLONG’ undeclared (first use in this function) Connection.hsc:389: error: ‘MYSQL_TYPE_TINY’ undeclared (first use in this function) Connection.hsc:397: error: ‘MYSQL_TYPE_DOUBLE’ undeclared (first use in this function) Connection.hsc:401: error: ‘MYSQL_TYPE_LONG’ undeclared (first use in this function) Connection.hsc:422: error: ‘MYSQL_TYPE_DATETIME’ undeclared (first use in this function) Connection.hsc:438: error: ‘MYSQL_TYPE_TIME’ undeclared (first use in this function) Connection.hsc:492: error: ‘UNSIGNED_FLAG’ undeclared (first use in this function) Connection.hsc:510: error: ‘MYSQL_TYPE_STRING’ undeclared (first use in this function) Connection.hsc:512: error: ‘MYSQL_TYPE_SHORT’ undeclared (first use in this function) Connection.hsc:513: error: ‘MYSQL_TYPE_INT24’ undeclared (first use in this function) Connection.hsc:514: error: ‘MYSQL_TYPE_YEAR’ undeclared (first use in this function) Connection.hsc:515: error: ‘MYSQL_TYPE_ENUM’ undeclared (first use in this function) Connection.hsc:516: error: ‘MYSQL_TYPE_DECIMAL’ undeclared (first use in this function) Connection.hsc:518: error: ‘MYSQL_TYPE_NEWDECIMAL’ undeclared (first use in this function) Connection.hsc:520: error: ‘MYSQL_TYPE_FLOAT’ undeclared (first use in this function) Connection.hsc:521: error: ‘MYSQL_TYPE_DATE’ undeclared (first use in this function) Connection.hsc:522: error: ‘MYSQL_TYPE_TIMESTAMP’ undeclared (first use in this function) Connection.hsc:523: error: ‘MYSQL_TYPE_NEWDATE’ undeclared (first use in this function) Connection.hsc:524: error: ‘MYSQL_TYPE_BLOB’ undeclared (first use in this function) Connection.hsc:544: error: ‘MYSQL_DATA_TRUNCATED’ undeclared (first use in this function) Connection.hsc:545: error: ‘MYSQL_NO_DATA’ undeclared (first use in this function) Connection.hsc:607: error: ‘NOT_NULL_FLAG’ undeclared (first use in this function) Connection.hsc:629: error: ‘MYSQL_TYPE_VARCHAR’ undeclared (first use in this function) Connection.hsc:630: error: ‘MYSQL_TYPE_BIT’ undeclared (first use in this function) Connection.hsc:633: error: ‘MYSQL_TYPE_SET’ undeclared (first use in this function) Connection.hsc:634: error: ‘MYSQL_TYPE_TINY_BLOB’ undeclared (first use in this function) Connection.hsc:635: error: ‘MYSQL_TYPE_MEDIUM_BLOB’ undeclared (first use in this function) Connection.hsc:636: error: ‘MYSQL_TYPE_LONG_BLOB’ undeclared (first use in this function) Connection.hsc:640: error: ‘MYSQL_TYPE_GEOMETRY’ undeclared (first use in this function) compiling dist/build/Database/HDBC/MySQL/Connection_hsc_make.c failed command was: /usr/bin/gcc -c -m32 -D__GLASGOW_HASKELL__=612 -I-L/usr/local/Cellar/readline/6.1/lib -I-arch -Ii386 -I-L/usr/local/Cellar/mysql/5.1.51/lib/mysql -I-lmysqlclient -I-lz -I-lm -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/time-1.1.4/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/old-time-1.0.0.5/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/bytestring-0.9.1.7/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/base-4.2.0.2/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/include/ dist/build/Database/HDBC/MySQL/Connection_hsc_make.c -o dist/build/Database/HDBC/MySQL/Connection_hsc_make.o cabal: Error: some packages failed to install: HDBC-mysql-0.6.3 failed during the building phase. The exception was: ExitFailure 1

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Derived type definition
by kg 22 Nov '10

22 Nov '10

Hi, I've tried to simplify as much as possible my problem. Finally, I think I can resume it like that: Suppose these following data types : data Rec a r = Rec a r data RecNil = RecNil data Wrapper a = Wrapper a Then, we can build the following type: type TTest = Rec Int (Rec String RecNil) or this type: type TTestWrapped = Rec (Wrapper Int) (Rec (Wrapper String) RecNil) Is it possible to build TTestWrapped from TTest ? Thx in advance, Antoine.

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