August 2011 - Beginners

linker error, protips welcome
by Michael Litchard 02 Aug '11

02 Aug '11

ghci is trying to load both network-2.3.0.2 and network-2.3.0.4 I don't have 2.3.02 installed I checked package.conf, and it's not there. What can I check to see what's going on? hcifoo> :main Loading package bytestring-0.9.1.10 ... linking ... done. Loading package transformers-0.2.2.0 ... linking ... done. Loading package mtl-2.0.1.0 ... linking ... done. Loading package parsec-3.1.1 ... linking ... done. Loading package unix-2.4.2.0 ... linking ... done. Loading package network-2.3.0.2 ... linking ... done. Loading package array-0.3.0.2 ... linking ... done. Loading package old-locale-1.0.0.2 ... linking ... done. Loading package old-time-1.0.0.6 ... linking ... done. Loading package HTTP-4000.1.1 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package deepseq-1.1.0.2 ... linking ... done. Loading package text-0.11.0.6 ... linking ... done. Loading package curl-1.3.7 ... linking ... done. Loading package HUnit-1.2.2.3 ... linking ... done. Loading package filepath-1.2.0.0 ... linking ... done. Loading package directory-1.1.0.0 ... linking ... done. Loading package process-1.0.1.5 ... linking ... done. Loading package time-1.2.0.3 ... linking ... done. Loading package random-1.0.0.3 ... linking ... done. Loading package haskell98-1.1.0.1 ... linking ... done. Loading package network-2.3.0.4 ... GHCi runtime linker: fatal error: I found a duplicate definition for symbol my_inet_ntoa whilst processing object file /home/mlitchard/.cabal/lib/network-2.3.0.4/ghc-7.0.3/HSnetwork-2.3.0.4.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry.

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Suggestions for creating a "bind" function that can be applied to different things?
by Costello, Roger L. 02 Aug '11

02 Aug '11

Hi Folks, When I say: "Let x be of datatype Int." I think the proper terminology is "bind". That is, I bind the name "x" to the Int datatype. Is that the correct terminology? Assuming it is, I would like to bind a name to many different things. I seek your suggestions on how to do this. First let dt1, dt2, dt3, dt4 be "DataTypes": dt1, dt2, dt3, dt4 :: DataType And let "Name" be synonymous with String: type Name = String And let title, author, date, cost be "Elements": title, author, date, cost, book :: Element I would like a "bind" function that binds a Name to a DataType to create an Element: bind :: Name -> DataType -> Element Here are a few examples of using bind to create Elements: title = bind "Title" dt4 author = bind "Author" dt4 date = bind "Date" dt4 I would also like to bind a Name to a DataType to create an "Attribute": bind :: Name -> DataType -> Attribute Here's an example of using bind to create an Attribute: currency = bind "Currency" dt3 And I'd like to bind an Attribute to an Element: bind :: Attribute -> Element -> Element Here's an example of binding the currency Attribute to a cost Element: cost = bind currency (bind "Cost" dt1) Finally, I'd like to create an Element by binding a Name to a list of Elements: bind :: Name -> [Element] -> Element Here's an example of this: book = bind "Book" [title, author, date, cost] As you see, I'd like to "bind" to lots of different things. But I can't create multiple functions with the same name, "bind". And I'd rather not create a function for each thing that I want to bind to, e.g., elementBind, attributeBind, attributeToElementBind, multipleElementsBind. There must be an elegant solution. What is your suggestion? /Roger

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A question on types
by C K Kashyap 01 Aug '11

01 Aug '11

Hi, To clarify my understanding, I created this table | | weak | strong | |---------+------+---------| | dynamic | perl | Ruby | | static | C | Haskell | |---------+------+---------| Could someone please ratify this? Regards, Kashyap

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Problem with catching error
by Alexey G 01 Aug '11

01 Aug '11

Hello. I have some problem with catching error. My function try to read value from string and if it's fails - return default (d). With this: >import Control.Exception >readIOWith :: Read a => a -> String -> IO a >readIOWith d x = catch tryRead errHandler > where errHandler :: SomeException -> IO a > errHandler _ = return d > tryRead = readIO x I have error: Could not deduce (a ~ a1) from the context (Read a) bound by the type signature for readIOWith :: Read a => a -> String -> IO a at BWClub/Common/Helpers/Packets.hs:(19,1)-(22,33) `a' is a rigid type variable bound by the type signature for readIOWith :: Read a => a -> String -> IO a at BWClub/Common/Helpers/Packets.hs:19:1 `a1' is a rigid type variable bound by the type signature for errHandler :: SomeException -> IO a1 at BWClub/Common/Helpers/Packets.hs:21:11 In the first argument of `return', namely `d' In the expression: return d In an equation for `errHandler': errHandler _ = return d But this code works: >readIOWith d x = catch tryRead (\e -> print (e :: SomeException) >> return d) > where tryRead = readIO x Sorry for my english.

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Haskell-style types in C or C++
by Christopher Howard 01 Aug '11

01 Aug '11

One of the things that I love about Haskell is the syntax for creating user defined types. E.g.: Data QueryResult = NoResult | DatabaseError String | Results [String] I can prototype an entire program around these self-made types and it is a lot of fun. Out of intellect curiosity, though: what would be the equivalent construction in C or C++? (Say, for the type defined above). -- frigidcode.com theologia.indicium.us

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