- Beginners - Haskell.org

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 24 Jun '11

24 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 23 Jun '11

23 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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How to avoid evaluating the second (undefined) argument of a Boolean AND operation?
by Costello, Roger L. 23 Jun '11

23 Jun '11

Hi Folks, Here is my own version of the Bool datatype, and my own version of the Boolean AND: data MyBool = F | T myAND :: MyBool -> MyBool -> MyBool myAND F x = F myAND T x = x If the first argument is F then return F. I assumed that the second argument would not even bother being evaluated. I figured that I could provide an undefined value for the second argument: myAND F (1 / 0) However, that doesn't work. I get this error message: No instance for (Fractional MyBool) arising from a use of `/' Possible fix: add an instance declaration for (Fractional MyBool) In the second argument of `myAND', namely `(1 / 0)' In the expression: myAND F (1 / 0) In the definition of `t4': t4 = myAND F (1 / 0) Why does it evaluate the second argument when the answer is already known from the first argument? How can I design it so that if the answer is known from the first argument, then an undefined second argument doesn't produce an error? /Roger

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Homework help - calculator function
by SM Design 22 Jun '11

22 Jun '11

I have a homework which is very important to be done but I can't complete the task at all. The program i should write is: Make 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 name = ... where the site stands blank corresponding value. 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"]. Thank you an advance!

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yesod dependency hell
by Michael Litchard 22 Jun '11

22 Jun '11

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

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