May 2002 - Haskell-Cafe

Using "type"
by Samuel E. Moelius III 04 May '02

04 May '02

I apologize if this question has been asked before... It seems to me the following code should be legal: type Thing m = m () type Const a b = a f :: Thing m -> Thing m f x = x test :: Thing (Const Int) -> Thing (Const Int) test = f However, GHC gives the following error: Couldn't match `Thing m' against `Thing (Const Int)' Expected type: Thing m Inferred type: Thing (Const Int) Expected type: Thing (Const Int) -> Thing (Const Int) Inferred type: Thing m -> Thing m But if you change the definition of Const newtype Const a b = MakeConst a the code compiles fine. Is this the desired behavior? Is it the case that when you have quantification of a type variable of kind * -> * (in this case, m), that variable may only become bound to a type constructor? Sam Moelius

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Re: tricky instance declarations
by Ashley Yakeley 03 May '02

03 May '02

At 2002-05-03 03:53, Warrick Gray wrote: >instance Monad (IO (Either ConnError a)) where You do of course mean instance Monad (/\a => IO (Either ConnError a)) where ... ...but you just can't do type-lambdas in Haskell. -- Ashley Yakeley, Seattle WA

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User-Defined Types
by Matthias 01 May '02

01 May '02

Hello, I am a student of computer science at the university of Aachen in Germany and presently do a course about functional programming in haskell. I just wanted to introduce myself, because it is likely to happen that my posting apear frequently in the next couple of month. So my first question is about User-Defined Types. Stating the Gentle Introduction to Haskell 98 (2.2) , i already know that: "Type Constructors and date Constructors are in seperte namespaces. [example]" But according to the chapter (2.2.1) "Recursive Types" i see the polymorphic definition of a tree is: data Tree a = Leaf a | Branch (Tree a) (Tree a) where (Tree a) is obviously the data-constructor out of "the other namespace". I guess that the () is warping Tree a from the "data-constructor-namespace" into the "type-constructor-namespace". Since the gentle-intro implicitly uses (), which i am not used to - i would be happy about some hints about (). That might not be to difficult, i hope. Also i would not regret some hints about "->" which is used in function-type-definitions. I would prefer writing plus :: a,a -> a instead of the ->-variant in order to stress the distinction between arguments and returned type. What is meant that -> is "associating to the right"? Is there a name for '->'? Sincerely Matthias Pfeifer

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Re: Read File
by Jorge Adriano 01 May '02

01 May '02

----------------- [moving to haskell-cafe] > I want to read some data from a text file. I accomplished it but I want to > read data from file as a Integer list for example text file has [1,2,3,5] ----------------- Wait you actually have "[1,2,3,5]" --------------------- > and when I read this data from file I handeled it as string. What can I do > to get it as integer list. Hint: map read ["1","2","3"] ::[Int] ------------------- this would be helpfull if you had "1 2 3 5". It's easier then Hint: read "[1,2,3,5]"::[Int] J.A.

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Re: Read File
by Jorge Adriano 01 May '02

01 May '02

[moving to haskell-cafe] > I want to read some data from a text file. I accomplished it but I want to > read data from file as a Integer list for example text file has [1,2,3,5] > and when I read this data from file I handeled it as string. What can I do > to get it as integer list. Hint: map read ["1","2","3"] ::[Int] J.A.

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