- Beginners - Haskell.org

fold over elem?
by Logesh Pillay 21 Sep '08

21 Sep '08

I want to check if all the numbers in a list A are elements of a bigger unordered list B. Is there some way to fold /A into elem /B?

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Request for Another State Monad Example
by Mike Sullivan 19 Sep '08

19 Sep '08

Hi All, As I'm sure all Haskell beginners do, I'm having a bit of a struggle wrapping my head around all of the uses for monads. My current frustration is trying to figure out how to use the state monad to attach some persistent state between different calls to a function. I have two questions that I would appreciate it if somebody could help me with. The ubiquitous state monad example for Haskell tutorials seems to be a random number generator, with a function like the following (from http://www.haskell.org/all_about_monads/html/statemonad.html#example) getAny :: (Random a) => State StdGen a getAny = do g <- get (x,g') <- return $ random g put g' return x My first question is very basic, but here it goes: I see it everywhere, but what does the "=>" signify? Specifically, in this example what does "(Random a) =>" do in the type signature? My second question is more of a request, I suppose. I think it would be useful to get another example that does not have the added complications of dealing with the Random package, and saves more than one piece of data as state. How would one go about (for example) creating a Fibonacci sequence generator that saves the last state, such that on each call it returns the next number in the Fibonacci sequence? Thank you, Mike

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Looking for a data structure
by Levi Stephen 17 Sep '08

17 Sep '08

Hi, I'm looking for how best to represent selections from a set of items. For example, consider a soccer/football type game. You would have a Player data type. The game would store a player pool of all players in that game. Each club will have a number of players from that player pool, each match will involve that club selecting a number of players from its list of players. What is the best way to structure this data and hopefully at the type level enforce this subset type relationship? Thanks, Levi

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Random nub questions!
by Casey Rodarmor 15 Sep '08

15 Sep '08

Hi everybody, Haskell is great! I have some questions: Why is this: (+ 1) a curried invocation of the the plus operator, but this: (+ 1 2) is an error? And in the same vein, why is this a negative number: (- 1) instead of a curried invocation of the subtraction operator? Why can't I do this: 1 `(+)` 2 even if it is a little pathological? And this might be a little implementation dependent, although I'll hazard it anyways: Why can't I assign to things when running GHCi? Given my experience with C like languages, the first thing I wanted to do with GHCi was to start assigning to variables. I know that Haskell is purely functional, but it seems like it should be able to allow assignment from the interpreter by raising errors on rebinding. (Just like in erl, the erlang interpreter.) All the best, Casey

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Named (custom) recursions vs clever comonadic recursions
by Federico Brubacher 14 Sep '08

14 Sep '08

Hey all, Today I had a small exchange with another reddit user about CT and why is important to Haskell programming... What I don't get yet (as the subject says) is a real-world example on where you might apply cathegory theory to a recursion. Here is the small thread : fbru02: Sorry for maybe the OT posting but I have a question of my own: While learning Haskell and being a curious procrastinator I started learning Category theory but although I get , I don't get it yet. What I want to ask is practical uses of cathegory theory ? I know that it is used for high availability systems for theoretical proof that a System has the correct side effects. Also I know there are in reality recursions that might need this stuff as they are not as simple as what one would write in a toy project. What are this recursion schemes ?? What other uses there are for CT? nbloomf: Disclaimer- I am only beginning to understand this stuff myself. I am also a mathematician, not a computer scientist or a professional programmer. Two papers that give a good overview of what categories can do are "Functional Programming with Bananas, Lenses, etc." and "Recursion Schemes from Comonads". My understanding of the role of categories in FP (at least partly) is as follows. Arbitrary recursion is very useful to writers of programs. However, like arbitrary GOTOs, it can make it hard to see what's "really" happening in a program. A long time ago people figured out that fold/reduce was a common recursive pattern, but it was generally only used on lists and couldn't express all the recursion people wanted to do. More recently it was discovered that if your language is strongly typed, you can think of the types as objects in a category. Then some type constructors become endofunctors, and the traditional fold can be expressed as a particular map, parameterized by the functor, having a nice universal property. Moreover, as the "Recursion schemes from comonads" paper discusses, by parameterizing on the comonad the fold operator can become several different recursion schemes. This means a much smaller amount of code can express a larger number of ideas- even ideas that haven't been had yet. Moreover, it makes optimizations (such as fold fusion) much more powerful. So the real power of category theory is as a metalanguage- these ideas might never have become "obvious" without the language of functors, algebras, initial objects, and such. fbru02 : thank you for great response. Have you yourself used this parametraizing of the comonad? In which context or solving which problem and why couldn't you solving easily making a normal named recursion :D Thanks a lot ! :D Thanks ! -- Federico Brubacher www.fbrubacher.com Colonial Duty Free Shop www.colonial.com.uy

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Non-exhaustive case leads to runtime error?
by Rafal Kolanski 10 Sep '08

10 Sep '08

Greetings, I just ran into the following situation in an IO do block: case msel of Just sel -> do toggleFretArea selectionRef sel widgetQueueDraw canvas Nothing -> return () except I omitted the last line. I expected the compiler to complain, but it silently compiled it, and I got a runtime exception when the program ran instead. Is this behaviour intentional? Can I get ghc to warn me about it anyway (something like -Wall)? Two more guiding questions: 1. What is a good idiom for "In this case do blah otherwise don't do anything"? 2. Is there a shorter way of writing a no-op in a do block than "return ()"? Thanks in advance! Rafal Kolanski.

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(no subject)
by Anish Muttreja 09 Sep '08

09 Sep '08

-- As an adolescent I aspired to lasting fame, I craved factual certainty, and I thirsted for a meaningful vision of human life - so I became a scientist. This is like becoming an archbishop so you can meet girls. -Matt Cartmill, anthropology professor and author (1943- )

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[Haskell-cafe] select :: [(Float, a)] -> a -- Weighted stochastic selection - help?
by Tim Millea 08 Sep '08

08 Sep '08

I am already using the following function pickOne xs g = (xs !! r, g') where (r, g') = Random.randomR (0, length xs - 1) g but I need to weight the items in the list, e.g. [(0.1, 'a') , (0.9, 'b')] for stochastic selection in a purely functional way, i.e. no monads. Any ideas welcome. Tim.

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Explanation of double astrix
by Paul Johnston 03 Sep '08

03 Sep '08

Hi Was playing around with ghci and lambda expressions and: *Main> map (\x -> 2 * x) [1 ..3] [2,4,6] Then thinking back to Fortran (yes I'm not young anymore!) *Main> map (\x -> 2 ** x) [1 ..3] [2.0,4.0,8.0] Curious as to what is going on. *Main> :t (\x -> 2 ** x) (\x -> 2 ** x) :: (Floating t) => t -> t *Main> :t (\x -> 2 * x) (\x -> 2 * x) :: (Num t) => t -> t Somehow the type has gone from Num to Floating I am using the excellent (IMHO) tutorial by Hal Daume and the book by Graham Hutton but can find no clues. Cheers Paul

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Nullable attribute of a language grammar howto
by Larry Evans 03 Sep '08

03 Sep '08

The following is my first attempt at specifying a language grammar. The method of specification is to define an algebra for the grammar expression. This algebra is GramExpr with constants from enums ArithInp and ArithVar. The functions correspond to the choice and concatenation operators in compiler texts. Specification of grammar worked OK, as shows by the successful compile of all the prints for each expression as well as the successful print of the list of GramProd's. However, when I tried to calculate the Nullable attributes of the expressions, I ran into problems. What I wanted to do was execute a homomorphism from the grammar expressions to another algebra, the GramNull algebra. This GramNull algebra will eventually produce a set of recursive equations corresponding to the grammar productions. The solution of these equations will calculate whether a non-terminal (a member of the enum ArithVar) can derive the empty string. Would someone please let me know what I'm doing wrong or suggest a better way of calculating the Nullable attribute of the non-terminals. Eventually, I'd like to calculate the First and Follow attributes in a similar way. The source file is: <--- gram_alg.hs --- {- Purpose: Algebraic specification of language grammar. -} module Main where data ArithInp = Ident | Plus | Mult | ParenLeft | ParenRight deriving(Enum,Show) data ArithVar = Term | Factor | Expr deriving(Enum,Show) data GramExpr inp_type var_type --Grammar Expresson, i.e. the rhs of a production. = NullExpr --the empty string, i.e. the epsilon in Compiler texts. | InpExpr inp_type --a terminal, i.e. an inp_type, is a Grammar Expression | VarExpr var_type --a non-terminal, i.e. a var_type, is a Grammar Expression | (:|) (GramExpr inp_type var_type) (GramExpr inp_type var_type) {- :| is the choice grammar operator. E.g. input, i, matches x | y if i either matches x or y. -} | (:>>) (GramExpr inp_type var_type) (GramExpr inp_type var_type) {- :>> is the concatenation grammar operator. E.g. input, i, matches x :>> y if i matches x followed by y. -} deriving(Show) data GramProd inp_type var_type = (:==) var_type (GramExpr inp_type var_type) deriving(Show) data GramNull inp_type var_type = OneNull --can derive empty string, | ZeroNull --can't derive empty string. | VarNull var_type --unknow whether var_type can derive empty string | AltNull (GramNull inp_type var_type) (GramNull inp_type var_type) | CatNull (GramNull inp_type var_type) (GramNull inp_type var_type) deriving(Show) expr2null :: (GramExpr inp_type var_type) -> (GramNull inp_type var_type) {- expr2null GramExpr returns a GramNull expression which indicates whether the GramExpr can derive the empty string. -} expr2null NullExpr = OneNull expr2null (InpExpr inp_valu) = ZeroNull::(GramNull inp_type var_type) expr2null (VarExpr var_valu) = (VarNull var_valu)::(GramNull inp_type var_type) main = do { print Ident ; print Expr ; print (InpExpr Ident::GramExpr ArithInp ArithVar) ; print (VarExpr Expr::GramExpr ArithInp ArithVar) ; print ((InpExpr Ident :| VarExpr Factor)::GramExpr ArithInp ArithVar) ; print ((InpExpr Ident :>>VarExpr Factor)::GramExpr ArithInp ArithVar) ; print ((Factor :== InpExpr Ident)::GramProd ArithInp ArithVar) ; print ([ Factor :== ( InpExpr Ident :| ( InpExpr ParenLeft :>> VarExpr Expr :>> InpExpr ParenRight ) ) , Term :== ( VarExpr Factor :>> InpExpr Mult :>> VarExpr Factor ) , Expr :== ( VarExpr Term :>> InpExpr Plus :>> VarExpr Term ) ] ::[GramProd ArithInp ArithVar]) {- The above arg to print is the arithmetic expression grammar. -} ; print ((expr2null (InpExpr Mult))::(GramNull ArithInp ArithVar)) ; print ((expr2null (VarExpr Factor))::(GramNull ArithInp ArithVar)) ;} >--- gram_alg.hs --- The compile output is: <--- gram_alg.compile --- Compilation started at Mon Sep 1 11:02:19 make -k runghc -XMultiParamTypeClasses -XPatternSignatures gram_alg.hs gram_alg.hs:62:40: Couldn't match expected type `var_type1' against inferred type `var_type' `var_type1' is a rigid type variable bound by the polymorphic type `forall inp_type var_type1. GramNull inp_type var_type1' at gram_alg.hs:62:31 `var_type' is a rigid type variable bound by the type signature for `expr2null' at gram_alg.hs:53:32 In the first argument of `VarNull', namely `var_valu' In the expression: (VarNull var_valu) :: GramNull inp_type var_type In the definition of `expr2null': expr2null (VarExpr var_valu) = (VarNull var_valu) :: GramNull inp_type var_type make: *** [all] Error 1 >--- gram_alg.compile --- TIA. -regards, Larry

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