June 2011 - Beginners

Need help understanding (1) typeclass instances that have multiple parameter types, and (2) overlapping instances
by Costello, Roger L. 26 Jun '11

26 Jun '11

Hi Folks, I have questions about: - typeclass instances that have multiple parameter types - overlapping instances Let's take an example of a typeclass with two instances: class MyShow a where toString :: a -> String instance MyShow Int where toString = show instance MyShow (Int, Int) where toString (a, b) = toString a ++ ", " ++ toString b The first instance has one parameter type (Int) and the second instance has two parameter types (Int, Int). When the class and instances are compiled, this error is generated: Illegal instance declaration for `MyShow (Int, Int)' (All instance types must be of the form (T a1 ... an) where a1 ... an are type *variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `MyShow (Int, Int)' I interpret that error message to mean that Haskell has this rule: Instances with one parameter type can specify non-variable types (such as Int), but instances with multiple parameter types can only use variable types (e.g., a, b, c). Is that the rule? What is the rationale for that rule? Next, I placed this at the top of my file: {-# LANGUAGE FlexibleInstances #-} and the error message went away. Apparently the pragma is telling the compiler: Even though the Haskell code does not strictly conform to the Haskell specification, please let it compile. Is that what the pragma is telling the compiler? There must be a good reason why Haskell prohibits non-variable types in multi-parameter instances. So there must be a downside to adding that pragma. What is the downside? Now let's move on to my questions about overlapping instances. I created a third instance. It is for a pair of values of any type: instance (MyShow a, MyShow b) => MyShow (a, b) where toString (a, b) = ">>" ++ toString a ++ " " ++ toString b ++ "<<" It compiles without error. But if I try to use the second instance (the one that was defined for two Int's): toString ((34 :: Int), (44 :: Int)) then I get an overlapping instances error: Overlapping instances for MyShow (Int, Int) arising from a use of `toString' Matching instances: instance MyShow (Int, Int) -- Defined at Overlap.hs:11:9-25 instance (MyShow a, MyShow b) => MyShow (a, b) In the expression: toString ((34 :: Int), (44 :: Int)) I see how this: toString ((34 :: Int), (44 :: Int)) can match with the second and third instances, and thus the compiler has ambiguity on which instance to use. Is there a way to express, Hey compiler, please use this instance: instance MyShow (Int, Int) That is, when confronted with overlapping instances, what should I do? /Roger

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Fwd: Help
by Stoyan Peev 24 Jun '11

24 Jun '11

Yes, There are two way for me in this situation. The first one is to write function in another file for string list, that calls the parsing function, for every string in the list. But since now i have been trying in a couple of days and i can't do it successfully. The second options is just to rewrite this function for list of strings instead of just a string, but i couldn't do that either. So my question is can anybody help me finish it in one of the two ways. Thanks again! On Fri, Jun 24, 2011 at 11:11 PM, David Place <d(a)vidplace.com> wrote: > > On Jun 24, 2011, at 3:50 PM, Stoyan Peev wrote: > >> I'll be glad to finish the task myself, but i am going to need some help. > > Do you have a specific question? > > ____________________ > David Place > Owner, Panpipes Ho! LLC > http://panpipesho.com > d(a)vidplace.com > > > >

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Help
by Stoyan Peev 24 Jun '11

24 Jun '11

Hello all, I am experiencing some issues to do my course task in university. I have to write a calculator- function in Haskell. The function argument is a list of strings and also form such list, as each string of the argument made definite action: - If the string has the form of an arithmetic _expression_ - calculate this _expression_. The string result becomes part of the list-result. If the _expression_ contains a variable which is not assigned value, the result is displayed "undefined". - If the string has the form- Name = value calculated from the last _expression_ is assigned to the variable with the corresponding name in the list, and in the result list is formed a string with type - If there is not a calculated _expression_ to be assigned to form a string "no value". - If the string is non-blank, but there is a species different from the above two case, form the string "error". - If the string is empty, incl. when it contains only spaces, in the result there is not form a string. Expressions consist of integers without sign variables, operations + (Addition), - (subtraction), * (multiplication) and / (divide) and parentheses. Where no brackets, the operations are performed from left to right, but * and / precede the + and -. Implementation of any operation gives integer; in the division rejected the fractional part, if any. Variables have names of one letter - from the Latin small letter. In the beginning, end or between the elements of each row can have spaces - they are irrelevant to its correctness. Example: the list-argument ["3 +7 / 2" "2 + x", "= s", "2 * s +4", "", "2 + +4 / 5] function should provide a result-list ["6", "undefined", "s = 6", "16", "error"]. I say another person have the same task, but he didn't do anything. I started doing this task myself but i get stuck in the middle. Then i started searching for something that could help me and find out you :) The code i have written so far uses the library file "Parsing.lhs" but what i have written is taking those actions that i already described, only for a string. I cannot modify it to work for list of string, and complete the whole task. I'll be glad to finish the task myself, but i am going to need some help. Here is the code i have already written: import Parsing expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return (t+e) +++ do symbol "-" e <- expr return (t-e) +++ return t term :: Parser Int term = do f <- factor do symbol "*" t <- term return (f * t) +++ do symbol "/" t <- term return (f-t) +++ return f factor :: Parser Int factor = do symbol "(" e <- expr symbol ")" return e +++ natural eval :: String -> Int eval xs = case (parse expr xs) of [(n,[])] -> n [(_,out)] -> error ("undefined") [] -> error "error" Thanks all in advance :) -- Best Wishes Stoyan Peev

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Re: [Haskell-beginners] ~ type operator
by Guy 24 Jun '11

24 Jun '11

On 24/06/2011 12:10, Daniel Fischer wrote: > On Friday 24 June 2011, 10:26:42, Guy wrote: >> What does the ~ type operator mean? I've sometimes seen types such as (a >> ~ b) in error messages, but can't understand what GHC is trying to tell >> me. > > Type equality, (a ~ b) means that a and b are the same type (rather, that > the compiler can prove they're the same). So where is it useful to say a ~ b, rather than using a twice?

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Getting Stuff out of IO Handles
by Raphael Päbst 24 Jun '11

24 Jun '11

Hey everyone! This is probably a stupid question, but is there a way to get stuff out of a IO handle? If I do something with an IO handle, reading in data from a file for example and then do something with the data, is there a way to get the results out of the handle, comparable to the return in a do block? Or do all operations on the data have to happen inside the handle? Thanks Raf

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About repeat function
by divyanshu ranjan 24 Jun '11

24 Jun '11

As i was going through Learn You a Haskell for Great Good , i came across following repeat function implementation : repeat' :: x - > [x] repeat' x = x : repeat' x My question is why it produces list in first place. 1:2:3 is not a list but 1:2:3:[] is. How comes haskell know that the things which we are adding will eventually added to the beginning of empty list given things are infinite so you can specify the end which is [] . Then why doing 1:3:4 is not acceptable ? Thanks Divyanshu Ranjan

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'case of' question
by Manfred Lotz 23 Jun '11

23 Jun '11

Hi there, Let's say I have something like this (it is an artificial example just for illustration): case x of 1 -> doThis 5 -> doThis 76 -> doThis 112 -> doThat 21 -> doThat _ -> doOtherwise I would like to have something like this (which is illegal) case x of 1 | 5 | 76 -> doThis 112 | 21 -> doThat _ -> doOtherwise Is something like this possible to do? The only idea I got is: processX x | x `elem` [ 1, 5, 76 ] = doThis | x `elem` [ 112, 21 ] = doThat | otherwise = doOtherwise -- Manfred

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cabal and dependencies
by Manfred Lotz 23 Jun '11

23 Jun '11

Hi there, I had something like this before but now I checked explicitly: I did 'ghc-pkg check' which showed nothing. Fine! Then I did 'cabal install gitit' and after successfully installing gitit I ran 'ghc-pkg check' again and got: There are problems in package yesod-form-0.1.0.1: dependency "xss-sanitize-0.2.6-bfb18d04010fadbceea5ba5136c33373" doesn't exist There are problems in package warp-0.4.0.1: dependency "sendfile-0.7.3-1d8424f5c18454ab33888680f53a7340" doesn't exist There are problems in package happstack-state-6.0.0: dependency "hslogger-1.1.4-d65ba05b9e69912e34b53b6153cc40ff" doesn't exist There are problems in package happstack-jmacro-6.0.0: dependency "happstack-server-6.1.3-8a8613c407db656b76de3be81e531fb5" doesn't exist The following packages are broken, either because they have a problem listed above, or because they depend on a broken package. yesod-form-0.1.0.1 warp-0.4.0.1 happstack-state-6.0.0 happstack-jmacro-6.0.0 yesod-0.8.2.1 yesod-auth-0.4.0.2 For example, hslogger was reinstalled for some reason and got a new id. Now it is hslogger-1.1.4-dff59692d8bdac1df0c425c217a20b0c.conf Why is this happening? Could I prevent it somehow? -- Manfred

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How to compile stub?
by Zhi-Qiang Lei 23 Jun '11

23 Jun '11

Hi, My attempt which compiles some files with stub failed. Can you tell me how to compile them? Thanks. PrettyJSON.hs: renderJValue :: JValue -> Doc renderJValue (JBool True) = text "true" renderJValue (JBool False) = text "false" renderJValue JNull = text "null" renderJValue (JNumber num) = double num renderJValue (JString str) = string str PrettyStub.hs: import SimpleJSON data Doc = ToBeDefined deriving (Show) string :: String -> Doc string str = undefined text :: String -> Doc text str = undefined double :: Double -> Doc double num = undefined Result: bogon% ghc -c PrettyJSON.hs PrettyStub.hs SimpleJSON.o Warning: the following files would be used as linker inputs, but linking is not being done: SimpleJSON.o PrettyJSON.hs:1:16: Not in scope: type constructor or class `JValue' PrettyJSON.hs:1:26: Not in scope: type constructor or class `Doc' PrettyJSON.hs:2:14: Not in scope: data constructor `JBool' PrettyJSON.hs:2:29: Not in scope: `text' PrettyJSON.hs:3:14: Not in scope: data constructor `JBool' PrettyJSON.hs:3:29: Not in scope: `text' PrettyJSON.hs:4:13: Not in scope: data constructor `JNull' PrettyJSON.hs:4:29: Not in scope: `text' PrettyJSON.hs:5:14: Not in scope: data constructor `JNumber' PrettyJSON.hs:5:29: Not in scope: `double' PrettyJSON.hs:6:14: Not in scope: data constructor `JString' PrettyJSON.hs:6:29: Not in scope: `string' Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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confusion over yesod-forms-0.2.0
by Michael Litchard 22 Jun '11

22 Jun '11

I installed the new yesod-forms-0.2.0 I tried out the sample program hello-forms.hs. The new changes work just fine. No problems. I don't understand some of the notation, but that didn't stop me from using yesod-form prior to the changes. I copied the style of hello-form exactly, I think. But I got a compile error. I don't think the error indicates the actual problem, and I suspect I have left out something. Below is a link to hello-form, the code I am trying to compile and the error I get. I would appreciate any insight offered. hello-forms.hs http://hpaste.org/48177 Handler/SSLvpn.hs http://hpaste.org/48178 error Handler/SSLvpn.hs:43:14: No instance for (RenderMessage Aframe FormMessage) arising from a use of `selectField' Possible fix: add an instance declaration for (RenderMessage Aframe FormMessage) In the first argument of `areq', namely `(selectField tests)' In the second argument of `(<*>)', namely `areq (selectField tests) "Pick a test" Nothing' In the second argument of `($)', namely `pure (,) <*> areq (multiSelectField servers) "Pick some servers" Nothing <*> areq (selectField tests) "Pick a test" Nothing'

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