July 2011 - Beginners

Problem with defining functions and guarded rules
by This is still my display name 25 Jul '11

25 Jul '11

Hi, I just started learning Haskell. But I have been having a problem with defining functions. For example,diff :: Float-> Float diff = if x>= y then x-y else y-x<interactive>:1:29: parse error on input `='It is rather weird as I have checked quite a few websites and this was supposed to work.The same occurs when I try for guarded rules, except that it is now for |.diff :: Float -> Float -> Floatdiff x y | x >= y = x - y| otherwise = y - x<interactive>:1:42: parse error on input `|'Please do help!Thanks a lot.

2 1

Re: [Haskell-beginners] [Haskell-cafe] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by KC 24 Jul '11

24 Jul '11

I like the following but again "+" denotes addition and not a general binary operation. > I personally often define the alias: > > (<+>) = mappend A lot of math books use "+" or "x" enclosed in a circle to indicate that the usual meaning of "+" nor "x" is intended for the meaning of the binary operation. I can't figure out if this would compile, the inside "()" representing a circle. ((+)) = mappend It would be easier for beginners to "grok". >I don't think so... but while we're at it, what's with that weird name >"Monoid" anyway, let alone "Functor", "Monad", etc.? ;-) Ivan: I had thought those were words expressing valid mathematical concepts. In order to find similarities between apparently different operations & data one wants to reason abstractly; similar to mathematics. -- -- Regards, KC

3 2

Using intersecting ranges to narrow a range yields an "Out of Memory" error
by Costello, Roger L. 24 Jul '11

24 Jul '11

Hi Folks, I am creating an Int range by starting with the full range of Int: let r1 = [minBound::Int .. maxBound::Int] Then I restrict the upper bound of the range, e.g., let r2 = r1 `intersect` [minBound::Int .. 10] Then I restrict the lower bound of the range, e.g., let r3 = r2 `intersect` [0 .. maxBound::Int] No error thus far. However, when I try to display r3 I get an "Out of Memory" error. Why is that? I created a function that restricts a range using the above approach: restrict :: [Int] -> Restriction-> [Int] restrict r (MinInclusive v) = r `intersect` [v .. maxBound::Int] restrict r (MaxInclusive v) = r `intersect` [minBound::Int .. v] The arguments to "restrict" is a range, r, and an indication of whether the upper bound is to be restricted (MaxInclusive v) or the lower bound is to be restricted (MinInclusive v). Is there a better way to do this? What can I do to my restrict function to avoid getting an "Out of Memory" error? /Roger

2 1

For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by KC 24 Jul '11

24 Jul '11

It would be easier for beginners to "grok". -- -- Regards, KC

2 1

Re: [Haskell-beginners] [web-devel] yesod dependency hell
by Michael Litchard 24 Jul '11

24 Jul '11

My thinking is that this problem comes up anytime someone wants to play around with experimental code. As evidenced by the yesodwiki.cabal file, I think there's a more straightforward solution. I'm just not seeing it. On Wed, Jun 22, 2011 at 1:46 PM, Thomas Hartman <thomashartman1(a)googlemail.com> wrote: > one idea: move .ghc and .cabal to temp directory. > > start over with a clean slate. > > On Wed, Jun 22, 2011 at 1:30 PM, Michael Litchard <michael(a)schmong.org> wrote: >> Configuring Aframe-0.0.1... >> Warning: This package indirectly depends on multiple versions of the same >> package. This is highly likely to cause a compile failure. >> package yesod-auth-0.4.0.2 requires yesod-form-0.1.0.1 >> package yesod-0.8.2.1 requires yesod-form-0.1.0.1 >> package Aframe-0.0.1 requires yesod-form-0.2.0 >> >> I specified in Aframe.cabal yesod-form 0.2.0. The above warning is >> what I was looking to avoid. What is the solution to this problem? I >> could not find an obvious solution in yesodwiki.cabal, even though I >> see it's doing something similar to what I want. How did you deal with >> the above warning message? >> >> On Wed, Jun 22, 2011 at 12:32 PM, Michael Snoyman <michael(a)snoyman.com> wrote: >>> Have a look at the yesodwiki[1]; basically, you can't use the yesod >>> package itself, only the subpackages. This is by design so that a >>> single Yesod release will keep API stability, while users can still >>> experiment with newer versions of underlying packages. >>> >>> Michael >>> >>> [1] https://github.com/snoyberg/yesodwiki/blob/master/yesodwiki.cabal >>> >>> On Wed, Jun 22, 2011 at 10:29 PM, Michael Litchard <michael(a)schmong.org> wrote: >>>> I need to use yesod-form 0.2.0. The problem is that the yesod package, >>>> and the yesod-auth package expect yesod-form to be 0.1.0* >>>> I tried changing the .cabal files but that breaks in ways that makes >>>> me think the fix is not so simple. How do I go about being able to use >>>> yesod-form 0.2.0 >>>> >>>> _______________________________________________ >>>> web-devel mailing list >>>> web-devel(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/web-devel >>>> >>> >> >> _______________________________________________ >> web-devel mailing list >> web-devel(a)haskell.org >> http://www.haskell.org/mailman/listinfo/web-devel >> >

3 4

I wrote a state monad example - help me make it more Haskellic
by Rohit Garg 23 Jul '11

23 Jul '11

Hi, I have written my first program using state monads and I would like some feedback from the community regarding how Haskellic it is. It's a simple simulator for a toy microprocessor. The contents of registers are the state that needs to be preserved across instruction evaluation. I have tried to implement the code dealing with monads using both >>=, >> and the do block. Also, I'd like to make the RegisterVal type Word32, but when I do that, then ================ initial_rf = [0,0,0,0] ================ this line fails to typecheck. If there are any bugs or non-idiomatic use of haskell, please let me know. Thanks and regards, Rohit ================= import Data.Word import Control.Monad.State type RegisterVal = Int type RegisterFile = [RegisterVal] type RegisterID = Int --destination comes first data Instruction = Add RegisterID RegisterID RegisterID | Sub RegisterID RegisterID RegisterID | Mov RegisterID RegisterID | Movc RegisterID RegisterVal deriving (Show) get_operand :: RegisterFile -> RegisterID -> RegisterVal get_operand registers regid = registers !! regid store_operand :: RegisterFile -> RegisterID -> RegisterVal -> RegisterFile store_operand registers regid value = (take regid registers) ++ [value] ++ (drop (regid+1) registers) eval_inst :: Instruction -> RegisterFile -> RegisterFile eval_inst (Add dstid src1id src2id) rf = rf_final where rf_final = store_operand rf dstid res --store res = op1 + op2 --evaluate (op1, op2) = (get_operand rf src1id , get_operand rf src2id) --load eval_inst (Sub dstid src1id src2id) rf = rf_final where rf_final = store_operand rf dstid res --store res = op1 - op2 --evaluate (op1, op2) = (get_operand rf src1id , get_operand rf src2id) --load eval_inst (Mov dstid srcid) rf = rf_final where rf_final = store_operand rf dstid res --store res = op1 --evaluate op1 = get_operand rf srcid --load eval_inst (Movc dstid imm) rf = rf_final where rf_final = store_operand rf dstid res --store res = imm --evaluate initial_rf :: RegisterFile initial_rf = [0,0,0,0] insts = [Movc 0 231, Movc 1 (-42), Add 2 1 0, Sub 3 1 0] execute_inst2 :: Instruction -> State RegisterFile RegisterFile {- execute_inst2 inst = get >>= \rf1 -> let rf2 = eval_inst inst rf1 in put rf2 >> return rf2 -} execute_inst2 inst = do rf1 <- get let rf2 = eval_inst inst rf1 put rf2 return rf2 execute_program2 :: [Instruction] -> State RegisterFile RegisterFile execute_program2 [] = do rf <- get put rf return rf execute_program2 (x:xs) = do rf1 <- execute_inst2 x rf2 <- execute_program2 xs put rf2 return rf2 {- execute_program2 [] = get >>= \rf -> put rf >> return rf execute_program2 (x:xs) = (execute_inst2 x) >>= \a -> (execute_program2 xs) >>= \b -> return b -} main :: IO () main = putStrLn $ show $ execState (execute_program2 insts) initial_rf -- Rohit Garg http://rpg-314.blogspot.com/

3 4

Re: [Haskell-beginners] [Haskell-cafe] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by Arlen Cuss 23 Jul '11

23 Jul '11

On 24.07.2011 08:20, Ivan Lazar Miljenovic wrote: > On 24 July 2011 04:41, KC <kc1956(a)gmail.com> wrote: >> It would be easier for beginners to "grok". I quite like mempty/mappend; they (f)map onto the [] instance as a mnemonic. Similarly, MonadPlus (as a concept) maps to mzero/mplus quite well (and it's distinguishing the sets of empty/append and zero/plus that helps me remember which goes with what). mid and mbinop seem a bit more arbitrary, and don't 'look' as nice to me (though that's probably just because they're not as familiar as what we already have).

1 0

Data.Typeable
by Benjamin Edwards 23 Jul '11

23 Jul '11

Hi All, Does anyone have a good resource as to where I can learn more about this module and its uses? Ben

2 1

The Fun of Programming, Chapter 6, How to Write a Financial Contract
by Costello, Roger L. 23 Jul '11

23 Jul '11

Hi Folks, Chapter 6 is exciting. I learned a lot about how to write programs in a functional way. The chapter is written by Simon Peyton Jones. He is an excellent author. I created slides that summarizes the chapter and the lessons that I learned: http://www.xfront.com/The-Fun-of-Programming/Chapter6/How-to-write-a-financ… /Roger

1 0

Repeat function application x times?
by Christopher Howard 23 Jul '11

23 Jul '11

Is there a convenient stock function in prelude/base that applies a function to a single value x number of times and returns the result? (I.e., does not return a list, but the final value.) Something like applyN :: Integral b => (a -> a) -> b -> a -> a Honestly I looked through the base hierarchy and at hoogle but I am missing it. -- frigidcode.com theologia.indicium.us

4 3