June 2015 - Beginners

Reading binary data into numeric types
by Barry DeZonia 20 Jun '15

20 Jun '15

Hello, I need to write some code that will read binary data from a file into various types defined in Data.Int, Data.Word, and the floating types. I am looking to make functions with signatures like Handle->IO Word16 and Handle->IO Int32 and Handle->IO Float and others similarly defined. One complication I am running into is that the data is stored in big endian byte order. So my functions need to be flexible with byte ordering (and I need to determine the endian ordering of the current platform). Anyhow I'm pretty much a newbie and don't know how to do this. For a float in an imperitive language I might read 4 bytes, possibly reorder them based on platform endian value, and interpret the bytes as a float via a ptr. Would the process be similar in Haskell? Pointers appreciated.

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Adapting code from an imperative loop
by Matt Williams 19 Jun '15

19 Jun '15

Dear All, I have been wrestling with this for a while now. I have a list of data items, and want to be able to access them, in a Hash Map, via a short summary of their characteristics. In an imperative language this might look like: myMap = new map() for elem in myElems: key = makeKey(elem) myMap[key] = myMap[key] + elem I have been trying to do this in Haskell, but am stuck on how to hand the Map back to itself each time. I have a function :: elem -> [p,q] to make the key, but the Map.insert function has the following signature: insert :: (Hashable <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-Hashable.html…> k, Ord <https://hackage.haskell.org/packages/archive/base/4.2.0.2/doc/html/Data-Ord…> k) => k -> a -> HashMap <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-HashMap.html#…> k a -> HashMap <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-HashMap.html#…> k a My thought was that I needed to go through the list of the elems, and at each point add them to the Hash Map, handing the updated Map onto the next step - but this is what I cannot write. Any help much appreciated. Thanks, Matt

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'Simple' function
by Mike Houghton 19 Jun '15

19 Jun '15

Hi, I’ve been tryimg to write a function with signature asString :: IO String -> String Does someone please have the patience to explain to me what the compiler error messages really mean for these two attempts and exactly what I’m doing (!!!) If I *do not* give this function any type signature then it works i.e.. asString ioStr = do str <- ioStr return $ str and the compiler tells me its signature is asString :: forall (m :: * -> *) b. Monad m => m b -> m b which, at this stage of my Haskell progress, is just pure Voodoo. Why isn’t it’s signature asString :: IO String -> String ? Another naive attempt is asString ioStr = str where str <- ioStr and then compiler says parse error on input ‘<-’ Many Thanks Mike

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(no subject)
by steve barak 18 Jun '15

18 Jun '15

Unsubscribe me already!!! Steve Barak

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Re: [Haskell-beginners] [Haskell-cafe] phantom types and record syntax
by Chaddaï Fouché 18 Jun '15

18 Jun '15

Le jeu. 18 juin 2015 à 04:45, Dimitri DeFigueiredo <defigueiredo(a)ucdavis.edu> a écrit : > I am surprised that this program compiles in GHC: > > snip > > My trouble is that it seems the record syntax is *implicitly* converting > from one type to the other. It seems I would have to remove the phantom > type by adding a tag to avoid this: > This is faulty reasoning because it imply that a value changed type in your code, based on your understanding that the record syntax is "changing" a value... This is not the case ! Haskell is functional, immutability is the rule. The record syntax is just a shortcut to create a new value sharing some of its parts with the old value. It is thus perfectly normal that the type of this new value can be different of the type of the old value if no other constraint prevent this (and Phantom types are by essence unconstrained by the value, that is why they're "Phantom"). > > But this makes me unwilling to use phantom types for security as I would > be worried of unwittingly making the conversion. > > The value of Phantom types is generally only displayed if you make them abstract types, since the only way to constrain the phantom type is by imposing restrictive signatures for your handling functions (and then using those restricted functions)... This is why you usually won't export the constructors, only smart constructors that can only produce a precise type of value with the right phantom type. In these conditions, you can't "unwittingly make a conversion" and your API impose a safe pattern of use, helped by the type system to interdict unsafe combination without any runtime cost. > > Could somebody sprinkle some insight into why this is "converted > automatically" for phantom types? > Is there another way around this? > <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe> > As I said, your perspective is wrong : there is no "conversion" here, simply a new value with a new type. I really fail to see why you would want your program to fail to compile, your "sanitize" is exactly the kind of function Phantom types are for : supposing you can only create "Username UserSupplied" with your API, and a part of your API only accept "Username Safe", you'll need a function like sanitize to bridge those two parts. -- Jedaï

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phantom types and record syntax
by Dimitri DeFigueiredo 18 Jun '15

18 Jun '15

Hi All, I am a little suprised that this program compiles in GHC: ---- data ReqTime = ReqTime data AckTime = AckTime data Order a = Order { price ::Double, volume ::Int, timestamp ::Int } convertToReq :: Order AckTime -> Order ReqTime convertToReq o = o{price = 1} main = putStrLn "Hi!" ---- My trouble is that it seems the record syntax is *implicitly* converting from one type to the other. It seems I would have to remove the phantom type to avoid this: ---- data Order a = Order { price ::Double, volume ::Int, timestamp ::Int, myType :: a } convertToReq :: Order AckTime -> Order ReqTime convertToReq o = o{price = 1} -- fail!! ---- Could somebody sprinkle some insight into why this is "converted automatically" for phantom types? Can I avoid this behavior? Thanks! Dimitri

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Difference between types and values
by Matt Williams 16 Jun '15

16 Jun '15

Dear All, I am sure this is a common mistake, and I am happy to be pointed elsewhere for reading. I have spent the last couple of days on the Haskell irc channel, which was very helpful. However, one of the points of discussion left me confused. When we have a type, T, with constructors A and B (e.g. data T = A x y z | B x y) How do I understand the relationship between A, B and T? I had thought I could use the sub-class relationship, but that doesn't seem to be true. Any other pointers very welcome. Matt

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Syntax of Complex
by Shishir Srivastava 15 Jun '15

15 Jun '15

Hi, The Data.Complex package defines the new data type 'Complex' as ---------- data Complex a = !a :+ !a ------- Where ':+' is an infix operator. I don't however understand the usage of '!' in front of the type variable 'a'. What exactly is the purpose of '!' ? Any help would be appreciated. Thanks, Shishir Srivastava

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splicing
by derek riemer 15 Jun '15

15 Jun '15

Hi guys, As a newby to haskell, I was curious, what is the best way to splice an array or do things in the middle? For example, binary search requires i do in sudocode define binary search array target. Find middle element. If target is middle element then return target else if target < middle element then binary search array[0:target] else binary search array[target:end] How can I get this splicing with haskell? I can't just use head here. I can't do array!!n: where n is some number. Thanks, Derek

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splicing
by derek riemer 14 Jun '15

14 Jun '15

Hi guys, As a newby to haskell, I was curious, what is the best way to splice an array or do things in the middle? For example, binary search requires i do in sudocode define binary search array target. Find middle element. If target is middle element then return target else if target < middle element then binary search array[0:target] else binary search array[target:end] How can I get this splicing with haskell? I can't just use head here. I can't do array!!n: where n is some number. Thanks, Derek

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