- Beginners - Haskell.org

A Pascal-like Language Compiler
by Pranesh Srinivasan 23 Oct '08

23 Oct '08

Hi all, I have been learning haskell for the last month or so. I have just started becoming familiar with monads and looking at Parsec. As part of the Language Translators Course this semester, in the university I study, we are to implement a Pascal-like toy language to C translator[1]. The aim is to take a simple subset of Pascal and translate it to an intermediate 3 operand format, before translating it to simple C. The time I have for this project is about a month. I am contemplating doing this in Haskell, since I have read in several places that Haskell (and other functional languages) is very good for writing compilers. In fact, I have started writing code for lexing the basic tokens. I do have doubts about my ability to finish it. The curiosity to do something of a moderate size in Haskell, and the extremely friendly folks on #haskell, tipped me on this decision. I looked around for similar things on the internet, in vain. I have several doubts at this stage some of which include, * how the parser will determine the scope of the variables, i.e, how can a symbol table be implemented (in the state of the parser ?) * how easy or difficult will basic code optimisation (like unused variables, and loops with no side effects) and type checking be? * how would one go about implementing basic error recovery in the parser (make it skip the current line, or block) ? I would love it if I get a few pointers from people who have been there, done that. Or am I being too ambitious too early? :) [1] One of the suggested programming exercises at the end of the "Dragon" Compilers book, by Aho, et al. -- Pranesh S

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how to define a user datatype consisting of instances of String?
by Benjamin L.Russell 23 Oct '08

23 Oct '08

How is it possible to define a user datatype consisting of instances of String? Suppose I am preparing a party for wine connoisseurs, and want to define the following data types: data Wine = Red | White data Red = "Merlot" data White = "Sauvignon Blanc" data Entree = "pork" | "chicken" | "tuna" data SideDish = "garlic bread" | "mozzarella sticks" | "caviar" Then, I want to write a Haskell function that takes two lists; e.g., ["pork", "chicken", "tuna"] and ["garlic bread", "mozzarella sticks", "caviar"] and that constructs a three-tuple of the following type: (Entree, SideDish, Wine) such that which Wine is chosen depends on the particular combination of Entree and SideDish, with respect to the following rules: ("pork", "garlic bread") -> ("pork", "garlic bread", "Merlot") ("pork", "mozzarella sticks") -> ("pork", "mozzarella sticks", "Merlot") ("pork", "caviar") -> ("pork", "Beluga", "Sauvignon Blanc") ("chicken", "garlic bread") -> ("chicken", "garlic bread", "Merlot") ("chicken", "mozzarella sticks") -> ("chicken", "mozzarella sticks", "Sauvignon Blanc") ("chicken", "caviar") -> ("chicken", "caviar", "Merlot") ("tuna", "garlic bread") -> ("tuna", "garlic bread", "Sauvignon Blanc") ("tuna", "mozzarella sticks") -> ("tuna", "mozzarella sticks", "Merlot") ("tuna", "caviar") -> ("tuna", "caviar", "Merlot") So far, I have written the following Haskell code: module Wine where data Wine = Red | White data Red = "Merlot" data White = "Sauvignon Blanc" data Entree = "pork" | "chicken" | "tuna" data SideDish = "garlic bread" | "mozzarella sticks" | "caviar" wine :: [Entree] -> [SideDish] -> [(Entree, SideDish, Wine)] wine entree sidedish | entree == "pork" = | sidedish == "garlic bread" = ("pork", "garlic bread", "Merlot") | sidedish == "mozzarella sticks" = ("pork", "mozzarella sticks", "Merlot") | sidedish == "caviar" = ("pork", "caviar", "Sauvignon Blanc") | entree == "chicken" = | sidedish == "garlic bread" = ("chicken", "garlic bread", "Merlot") | sidedish == "mozzarella sticks" = ("chicken", "mozzarella sticks", "Sauvignon Blanc") | sidedish == "caviar"= ("chicken", "caviar", "Merlot") | entree == "tuna" = | sidedish == "garlic bread" = ("tuna", "garlic bread", "Sauvignon Blanc") | sidedish == "mozzarella sticks" = ("tuna", "mozzarella sticks", "Merlot") | sidedish == "caviar"= ("tuna", "caviar", "Merlot") However, when I load this code into GHCi, I get the following error message: [1 of 1] Compiling Wine ( wine.hs, interpreted ) wine.hs:18:11: parse error on input `"' Failed, modules loaded: none. Prelude> I discovered the following seemingly relevant HaskellWiki page, but am not sure about how to apply the advice in it to this problem, because the advice does not seem sufficiently specific or detailed: List instance - HaskellWiki http://www.haskell.org/haskellwiki/List_instance What would be a good way to overcome this error? -- Benjamin L. Russell

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what type is 'Val 9' when 'Val Int' a ctor for 'Expr e'?
by Larry Evans 23 Oct '08

23 Oct '08

The attachment compiles OK; yet, I don't understand how the compiler knows what type val_9 has. The compile/run is: <---cut here--- Compilation started at Tue Oct 21 13:17:41 make runghc uniplate.try.hs val_9=Val 9 add_8_9=Add (Val 8) (Val 9) children_add_8_9=[Val 8,Val 9] Compilation finished at Tue Oct 21 13:17:42 >---cut here--- I would have expected that I would need to code this: let val_9 = (Val 9)::(Expr Eo) but apparently not. Could somebody explain? TIA. -Larry

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Re: [Haskell-beginners] do not understand Semantics of Case Expressions in report
by John Dorsey 21 Oct '08

21 Oct '08

Michael, (Sorry for duplicates; I sent this from the wrong address initially.) > (m) case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> e' } > = case e' of { > y -> > case v of { > K { f1 = p1 } -> > case v of { K { f2 = p2 , ... } -> e ; _ -> y }; > _ -> y }} > where f1, f2, ... are fields of constructor K; y is a new variable [...] > I'm totally confused with this identity. Especialy with " = case e'". > Can anyone explain a little what does this rule stand, please? > It is better to see some example. You got me curious, so I looked over this. The table you reference lists identities that should hold for all implementations of case expressions. (m) is about how case matches against constructors with named fields, and it shows that a case expression that matches against multiple fields (f1, f2, etc.) can be broken into nested case expressions which each match a single field (eg. f1). It looks like "case e' of { y -> ..." was introduced solely to give e' a name that can be used in two places, since the nested form may fail to match at each level. It seems to me that without this, you could get code explosion from duplicating the expression e' below; with this you get sharing. Here it is with a interim step added: -- Match v against a constructor with multiple fields case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> e' } -- Same, but introduce y to represent e' without loss of sharing case e' of { y -> case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> y }} -- Same, but match first field of K independent of the rest case e' of { y -> case v of { K { f1 = p1 } -> case v of { K { f2 = p2 , ... } -> e ; _ -> y }; _ -> y }} I hope this helps. Regards, John

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Weird compilation error while doing Euler problems?
by Ian Duncan 21 Oct '08

21 Oct '08

'm trying to compile my file that has my Euler problems in it to output the solution to problem four, but I'm getting a compile error. Here's my .hs file: ------------------------- module Euler1 where import Data.List import Data.Ord main = mapM_ putStrLn problem4 problem1 = foldl1' (+) $ nub $ (takeWhile (< 1000) [3,6..] ++ takeWhile (< 1000) [5,10..]) problem2 = sum $ takeWhile (<= 4000000) [x | x <- fibs, even x] where fibs = unfoldr (\(a,b) -> Just (a,(b,a+b))) (0,1) problem3 z = maximumBy compare (filter (\x -> z `mod` x == 0) (takeWhile (<= ceiling (sqrt (fromIntegral z))) primes)) problem4 = nub [ show $ y * z | y <- [100..999], z <- [100..999], show (y*z) == reverse (show $ y*z)] prime p = p `elem` primes primes = small ++ large where 1:p:candidates = roll $ mkWheel small small = [2,3,5,7] large = p : filter isPrime candidates isPrime n = all (not . divides n) $ takeWhile (\p -> p*p <= n) large divides n p = n `mod` p == 0 mkWheel ds = foldl nextSize w0 ds nextSize (Wheel n rs) p = Wheel (p*n) [r' | k <- [0..(p-1)], r <- rs, let r' = n*k+r, r' `mod` p /= 0] w0 = Wheel 1 [1] roll (Wheel n rs) = [n*k+r | k <- [0..], r <- rs] data Wheel = Wheel Integer [Integer] ------------------------ Here's my output when I try to compile: ian$ ghc ~/Documents/eulerProblem1.hs -o test Undefined symbols: "___stginit_ZCMain", referenced from: ___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o) "_ZCMain_main_closure", referenced from: _ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o) ld: symbol(s) not found collect2: ld returned 1 exit status What's going on? I'm running GHC 6.8.3, if that helps.

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do not understand Semantics of Case Expressions in report
by Michael Mekhanoshin 20 Oct '08

20 Oct '08

at http://www.haskell.org/onlinereport/exps.html on figure Figure 4 (Semantics of Case Expressions, Part 2) one can see: (m) case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ - > e' } = case e' of { y -> case v of { K { f1 = p1 } -> case v of { K { f2 = p2 , ... } -> e ; _ -> y }; _ -> y }} where f1, f2, ... are fields of constructor K; y is a new variable (In Figures 3.1--3.2: e, e' and ei are expressions; g and gi are boolean-valued expressions; p and pi are patterns; v, x, and xi are variables; K and K' are algebraic datatype (data) constructors (including tuple constructors); and N is a newtype constructor.) I'm totally confused with this identity. Especialy with " = case e'". Can anyone explain a little what does this rule stand, please? It is better to see some example. Respect, Michael

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Re: [Haskell-beginners] requested module name differs from name found in interface file
by Larry Evans 20 Oct '08

20 Oct '08

On 10/20/08 11:38, Justin Bailey wrote: > On Mon, Oct 20, 2008 at 9:25 AM, Larry Evans <cppljevans(a)suddenlink.net> wrote: > [snip] > >> Do you mean install it manually as shown in the aforementioned >> How_to_install_a_Cabal_package? >> > > No, I mean go to http://hackage.haskell.org and download the .tar.gz > file directly (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras) > Unzip/untar that package. I haven't looked at it, but there is > probably a "src" directory. Put your example file in there and load it > in ghci. Your source file should then use the source available in that > directory tree rather than the category-extras install which seems > broken. This works because the GHC will look at the import > "Control.Functor.Fix" and use that to look for > "Control/Functor/Fix.hs" from the current directory. > Tried that but: <--- cut here --- GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :load "/home/evansl/download/haskell/libs/category-extras-0.53.5/src/catamorphism.example.hs" Control/Category/Dual.hs:17:7: Could not find module `Control.Category': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude> <--- cut here --- If it's any help, the directory list for src/Control/Category is: <--- cut here --- /home/evansl/download/haskell/libs/category-extras-0.53.5/src/Control/Category: total used in directory 52 available 148798604 drwxr-xr-x 3 evansl evansl 4096 Jun 30 08:08 . drwxr-xr-x 9 evansl evansl 4096 Jun 30 08:08 .. -rw-r--r-- 1 evansl evansl 2118 Jun 30 08:08 Associative.hs -rw-r--r-- 1 evansl evansl 1668 Jun 30 08:08 Braided.hs drwxr-xr-x 2 evansl evansl 4096 Jun 30 08:08 Cartesian -rw-r--r-- 1 evansl evansl 4973 Jun 30 08:08 Cartesian.hs -rw-r--r-- 1 evansl evansl 1020 Jun 30 08:08 Discrete.hs -rw-r--r-- 1 evansl evansl 1340 Jun 30 08:08 Distributive.hs -rw-r--r-- 1 evansl evansl 668 Jun 30 08:08 Dual.hs -rw-r--r-- 1 evansl evansl 578 Jun 30 08:08 Hask.hs -rw-r--r-- 1 evansl evansl 3254 Jun 30 08:08 Monoidal.hs -rw-r--r-- 1 evansl evansl 1746 Jun 30 08:08 Object.hs >--- cut here --- > You might have more luck finding out what's going on by emailing > Haskell-Cafe directly, too. > Yes, thanks. > Justin > >

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requested module name differs from name found in interface file
by Larry Evans 19 Oct '08

19 Oct '08

With a file containing: > module Main where > > import Array > import Control.Functor.Fix I get: > make > ghc -i/root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 -c > catamorphism.example.hs > > catamorphism.example.hs:19:0: > Bad interface file: > /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2/Control/Functor/Fix.hi > Something is amiss; requested module main:Control.Functor.Fix > differs from name found in the interface file > category-extras-0.53.5:Control.Functor.Fix > make: *** [all] Error 1 I used cabal to install category-extras: > /home/evansl/download/haskell/libs # cabal install category-extras > Resolving dependencies... > Downloading category-extras-0.53.5... > Configuring category-extras-0.53.5... > Preprocessing library category-extras-0.53.5... ... > /usr/bin/ar: creating dist/build/libHScategory-extras-0.53.5.a > Installing library in /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 > Registering category-extras-0.53.5... > Reading package info from "dist/installed-pkg-config" ... done. > Saving old package config file... done. > Writing new package config file... done. > /home/evansl/download/haskell/libs # What should I do to import the Functor.Fix as shown here: http://comonad.com/reader/2008/recursion-schemes/ TIA. -Larry

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Mathematical Blundering
by Jeffrey Drake 17 Oct '08

17 Oct '08

I have defined myself a set of functions to test: fact 1 = 1 fact n = n * (fact $ n - 1) sine x = x - (x^3/(fact 3)) + (x^5/(fact 5)) - (x^7/(fact 7)) Where my code is 'sine' and the prelude's is sin: *Main> sine 1 0.841468253968254 *Main> sin 1 0.8414709848078965 *Main> sine 2 0.9079365079365079 *Main> sin 2 0.9092974268256817 *Main> sine 3 9.107142857142847e-2 *Main> sin 3 0.1411200080598672 *Main> sine 4 -1.3841269841269837 *Main> sin 4 -0.7568024953079282 After 2 they seem to diverge rather rapidly, and I am not sure why. Any ideas? I would have thought that 4 terms would have been enough. - Jeff.

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IO Problem
by Jamie McCloskey 17 Oct '08

17 Oct '08

Hi everyone, I have just started learning Haskell, it is a very interesting language. As a first project, I am building a brainfuck interpreter, but have hit a stumbling block: IO. My program source is located at http://hpaste.org/11219 My program handles the state of a brainfuck program by passing State(lists of ints) objects around. When I added my getIn fuction, however, it needed to return an IO State object. This broke the run function, as it returned a State. No problem, I thought, and changed run to return IO State. This broke my exec function, which calls run on each element in a list. As exec passes on the state returned by the run function to itself, it needs to take an IO State. This causes problems in the loop function, which calls exec on a subprogram. Because exec now takes IO State, loop also needs to. However, loop is called by run, so now run needs to take IO State. Now, all functions called by run need to take IO State. All this creates a whole load of functions with IO, even though it should not be necessary. What I would like to know, is how I can avoid this, possibly by modifying exec to just take a State. Whew! What a long-winded explanation! Any ideas? --Jamie P.S. Please ignore my long-winded list iteration functions -- I'm working on a cleaner version.

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