June 2011 - Beginners

How to avoid evaluating the second (undefined) argument of a Boolean AND operation?
by Costello, Roger L. 22 Jun '11

22 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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not getting heap profiling data from test runs
by Sean Perry 22 Jun '11

22 Jun '11

I have done the following: $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.3 $ ghc -O2 -o myapp --make main.hs -prof -auto-all -rtsopts -fforce-recomp $ ./myapp foo.csv +RTS -hb $ cat ./myapp.hp JOB "myapp foo.csv +RTS -hb" DATE "Wed Jun 22 00:34 2011" SAMPLE_UNIT "seconds" VALUE_UNIT "bytes" BEGIN_SAMPLE 0.00 END_SAMPLE 0.00 BEGIN_SAMPLE 0.00 END_SAMPLE 0.00 I get the same result from other -h options. There are a total of 4 hs files compiled for this app. I see the same thing with my rpn calculator. Leaving off the -O2 does not help. Nor does -rtsopts=all. What am I missing? This is on OSX using the Haskel platform with a cabal installed Text.CSV (which I did --reinstall -p on to be certain).

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boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]
by anthony niro 21 Jun '11

21 Jun '11

Hello, My name is Anthony i have few question about Haskell. I was reading a tutorial everything goes well. but now i have few things that i dont understang. HERE: boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x] I dont understand this line except the "if" "then" "else". What is xs? what is the | ? and why doing this " | x <- xs, odd x]" why x <- xs????? what is that and what is odd x? thx everyone for answer me.

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Unit numeric type pattern
by Christopher Howard 21 Jun '11

21 Jun '11

What is a "unit numeric type pattern", as in the error: "Only unit numeric type pattern is valid". I can cause this error by mixing numbers into type definitions. And, therefore, I know how to avoid this error. But the error indicates that there /is/ such a thing as "valid" "unit numeric type pattern", and consequently I am quite curious as to what a such a thing is and what it would look like in code. -- frigidcode.com theologia.indicium.us

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Text.Xhtml.Table problems, with a very simple table
by Tom Murphy 20 Jun '11

20 Jun '11

Hi All, I'm trying to create a table like this: ---------- | a1 | | |----| b | | a2 | | ---------- My code is this: triTable a1 a2 b = table << ((a1 </> a2) <-> (b)) But it's mashing elements together. What am I doing wrong?? Thanks for your time, Tom

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Web frameworks
by Manfred Lotz 20 Jun '11

20 Jun '11

Hi there, There are some web frameworks out there, as snap, yesod, happstack, and perhaps even more. I'd like to get a recommendation which web framework might be easiest to get into, and to use for a haskell beginner. -- Thanks, Manfred

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Still wrestling with lazy IO
by Raphael Päbst 19 Jun '11

19 Jun '11

Hey everyone! Sorry for asking more or less the same question again, but I'm still stuck wrestling with lazy IO. What I'm trying to do is the following: sending a Bytestring to a server, via TCP Do stuff with the Bytestring that results in another Bytestring Send that new Bytestring back to the original client. At the moment this all happens inside an IO handle and doesn't work. The stuff on the server side only gets executed, once the handle is closed on the client's side and therefore the response is lost somewhere. It's a very simple program otherwise and so I don't want to get too much into iteratees, iterators and that stuff, since it feels like overkill. Is there a way to get this working in the way I'm currently trying? Or can I do this in a strict way? Thanks again Raf

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Make synonym for Main?
by Christopher Howard 18 Jun '11

18 Jun '11

Say you have a function in Main that is the same name as a function you've imported, but you don't want to rename it. But then you have to prefix it with "Main" each time you use it: Main.functionName Is there some way to rename Main or make a synonym for it, kind of like the "import qualified" trick, so you could do... M.functionName -- frigidcode.com theologia.indicium.us

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