May 2002 - Haskell-Cafe

Q: Resolving ambiguous type variable
by tty＠SAFe-mail.net 17 May '02

17 May '02

Hello, I am writing a function which returns an exception/error string should an unexpected parameter is passed in. Borrowing from the Maybe module I have the following: ------------------------------------------------------------ data Result a = Ex String | Value a deriving Show -- Testing the Result a type first_h :: [a] -> Result a first_h [] = Ex "No list" first_h (x:xs) = Value x -- Trying out the first_h main :: IO() main = print (first_h []) ------------------------------------------------------------ Which the compiler complains: Ambiguous type variable(s) `a' in the constraint `Show a' arising from use of `print' at atype.hs:8 In the definition of `main': print (first_h []) This is understandable since it is unable to unify the empty list with a concrete list of type 'a' i.e. there are an infinite types which would match. My question is how can I indicate to the compiler that it does not matter which type the empty list is since the return result is of a concreate type. Thanks Tee

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by tony 17 May '02

17 May '02

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Simple Question
by Jerry, JiJie 16 May '02

16 May '02

Hi, enlightenment much appreciated for my newbie question -- to achieve: append' [ [1, 2], [3, 4], [5] ] 6 -> [ [1, 2], [3, 4], [5, 6] ] append' [ ['1', '2'], ['3'] ] '4' -> [ ['1', '2'], ['3', '4'] ] append' [ [True], [True] ] False -> [ [True], [True, False] ] so I (naively) write: 18) append' :: [[a]] -> a -> [[a]] 19) append' [] y = [[y]] 20) append' x:xs y = [(init x:xs)] ++ [(tail xs)++[y]] and ghc(5.02.3) says: $ ghc p3a.hs p3a.hs:20: Parse error in pattern thanx in advance! -- Regards, Jerry

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by raeline 16 May '02

16 May '02

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��ȫ��ͼ�
by raeline 15 May '02

15 May '02

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RE: Replacing the Prelude
by Simon Peyton-Jones 14 May '02

14 May '02

Ashley writes | > I was hoping to do something similar for 'do' notation by redefining | > (>>), (>>=) etc., but unfortunately GHC is quite insistent | that 'do' notation quite specifically refers to GHC.Base.Monad Dylan replies | I'm surprised that ghc uses the fromInteger and fromRational | that are in scope; I thought there was general agreement that | it should use the Prelude.from{Integer,Rational} in scope. Ashley is referring to a GHC extension. Normally, GHC uses Prelude.fromInteger etc, regardless of what is in scope. But if you say -fno-implicit-prelude, GHC will instead use whatver fromInteger is in scope. (This is documented in the manual.) Ashley's question, as I understand is whether something similar could be done for monads. Answer: I think so, without much bother. I'm beavering away on a Haskell workshop paper at the moment, but ping me in a fortnight to do it. Simon

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Re: Replacing the Prelude
by Dylan Thurston 14 May '02

14 May '02

On Sun, May 12, 2002 at 09:31:38PM -0700, Ashley Yakeley wrote: > I have recently been experimenting writing code that replaces large > chunks of the Prelude, compiling with -fno-implicit-prelude. I notice > that I can happily redefine numeric literals simply by creating functions > called 'fromInteger' and 'fromRational': GHC will use whatever is in > scope for those names. > > I was hoping to do something similar for 'do' notation by redefining > (>>), (>>=) etc., but unfortunately GHC is quite insistent that 'do' > notation quite specifically refers to GHC.Base.Monad (i.e. Prelude.Monad, > as the Report seems to require). I don't suppose there's any way of > fooling it, is there? I was rather hoping 'do' notation would work like a > macro in rewriting its block, and not worry about types at all. > > I accept that this might be a slightly bizarre request. There are a > number of things I don't like about the way the Prelude.Monad class and > 'do' notation are set up, and it would be nice to be able to experiment > with alternatives. A while ago, there were extensive discussions about replacing the Prelude on this list. (Search for "Prelude shenanigans".) I started to write up a design document for how to enable replacing the Prelude. This boiled down to taking most of the "syntactic sugar" defined in the report seriously, ignoring the types (as you say). I'm surprised that ghc uses the fromInteger and fromRational that are in scope; I thought there was general agreement that it should use the Prelude.from{Integer,Rational} in scope. As I recall, there were several relevant bits of syntactic sugar: - Numeric types, including 'fromInteger', 'fromRational', and 'negate'. This all works fine, except that the defaulting mechanism is completely broken, causing a number of headaches. - Monads. The translation given in the report is clean, and it seems like it can be used without problems. - Bools. There was a slight problem here: the expansion of 'if ... then ... else ...' uses a case construct referring to the constructors of the Bool type, which prevents any useful redefinition of Bool. I would propose using a new function, 'Prelude.ifThenElse', if there is one in scope. - Enumerations. Clean syntactic sugar. - List comprehensions. The report gives one translation, but I think I might prefer a translation into monads. - Lists and tuples more generally. At some point the translations start getting too hairy; I think I decided that lists and tuples were too deeply intertwined into the language to change cleanly. I'll dig up my old notes and write more, and then maybe write a complete design document and get someone to implement it. --Dylan Thurston

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composeList
by Emre Tezel 12 May '02

12 May '02

Hi all, I recently bought Simon Thompson's Haskell book. I have been doing the exercises while I read on. There are couple questions that I can not solve. Any help would be greatly appreciated. I got stuck on this question in Chapter 10. Define a function composeList which composes a list of functions into a single function. What is the type of composeList? I naively tried the following but the hugs compiler complained about the inferred type not being generic enough. composeList :: [(a -> b)] -> (c -> b) composeList [] = id composeList (x:xs) = x . (composeList xs) or composeList :: [(a -> b)] -> (c -> b) composeList (x:xs) = x . (composeList xs) ERROR "Chapter10.hs":6 - Inferred type is not general enough *** Expression : composeList *** Expected type : [a -> b] -> c -> b *** Inferred type : [a -> a] -> b -> a Anybody can solve this? Emre

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Re: waiting for your reply
by Bob Johnson 12 May '02

12 May '02

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preprocessing printf/regex strings (like ocaml)
by Oliver George 12 May '02

12 May '02

Hi Haskellers, I'm interested to know why a string translating preprocessor doesn't exist for haskell. It seems to me that it would alleviate printf and regex like problems in an convenient, elegant and type-safe manner. An example of this I came across recently was the ocaml printf statement: # Printf.printf "roar";; roar- : unit = () # Printf.printf "numbers %d %d" 11 23;; numbers 11 23- : unit = () # Printf.printf "a number %d" "word";; This expression has type string but is here used with type int You can see logically how this might work in haskell, compile time evaluation would translate the string into the appropriate native expressions before the statements type is evaluated. I see a similar possibility for regular expression matching, wouldn't it be nice in a haskell script to write: perl like things... msg' = replace "s/love/lust/" msg or, nice regex stuff... main = case match "^(\d+)" of Nothing -> 0 Just (i) -> i the python string notation (str % tuple) would fit really well too... putStrLn "hello %s, you got %d right" % ("oliver", 5) Am I the only one who sees this as being a really valuable extension to haskell? or does it exist and i've just never noticed? cheers, Oliver.

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