Re: [Haskell-cafe] Trouble with types
Vladimir Reshetnikov wrote: Hi Daniel, Could you please explain what does mean 'monomorphic' in this context? I thought that all type variables in Haskell are implicitly universally quantified, so (a - a) is the same type as (forall a. a - a) At the top level (i.e. definition level), yes. However, each use site this polymorphism may be restricted down (in particular, to the point of monomorphism). In a syntax more like Core, the definition of the identity function is, id :: forall a. a - a id @a (x :: a) = x Where the @ is syntax for reifying types or for capital-lambda application (whichever interpretation you prefer). In this syntax it's clear to see that |id| (of type forall a. a - a) is quite different than any particular |id @a| (of type a-a for the particular @a). So in your example, there's a big difference between these definitions, (f,g) = (id,id) (f', g') @a = (id @a, id @a) The latter one is polymorphic, but it has interesting type sharing going on which precludes giving universally quantified types to f' and g' (the universality is for the pair (f',g') and so the types of f' and g' must covary). Whereas the former has both fields of the tuple being polymorphic (independently). Both interpretations of the original code are legitimate in theory, but the former is much easier to work with and reason about. It also allows for tupling definitions as a way of defining local name scope blocks, without side effects on types. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Trouble with types
Hi, I tried this code: --- f, g :: a - a (f, g) = (id, id) --- Hugs: OK GHC: Couldn't match expected type `forall a. a - a' against inferred type `a - a' In the expression: id In the expression: (id, id) In a pattern binding: (f, g) = (id, id) What does mean this error message? And what of them (Hugs, GHC) is correct? Thanks Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
Am Montag 01 Juni 2009 14:44:37 schrieb Vladimir Reshetnikov: Hi, I tried this code: --- f, g :: a - a (f, g) = (id, id) --- Hugs: OK GHC: Couldn't match expected type `forall a. a - a' against inferred type `a - a' In the expression: id In the expression: (id, id) In a pattern binding: (f, g) = (id, id) What does mean this error message? And what of them (Hugs, GHC) is correct? http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs-and-infelicities.html Section 12.1.1.4, Declarations and bindings GHC's typechecker makes all pattern bindings monomorphic by default; this behaviour can be disabled with -XNoMonoPatBinds. See Section 7.1, “Language options”. Hugs is correct, it's a known infelicity in GHC which can be disabled. Thanks Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
Hi Daniel, Could you please explain what does mean 'monomorphic' in this context? I thought that all type variables in Haskell are implicitly universally quantified, so (a - a) is the same type as (forall a. a - a) Thank you, Vladimir On 6/1/09, Daniel Fischer daniel.is.fisc...@web.de wrote: Am Montag 01 Juni 2009 14:44:37 schrieb Vladimir Reshetnikov: Hi, I tried this code: --- f, g :: a - a (f, g) = (id, id) --- Hugs: OK GHC: Couldn't match expected type `forall a. a - a' against inferred type `a - a' In the expression: id In the expression: (id, id) In a pattern binding: (f, g) = (id, id) What does mean this error message? And what of them (Hugs, GHC) is correct? http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs-and-infelicities.html Section 12.1.1.4, Declarations and bindings GHC's typechecker makes all pattern bindings monomorphic by default; this behaviour can be disabled with -XNoMonoPatBinds. See Section 7.1, “Language options”. Hugs is correct, it's a known infelicity in GHC which can be disabled. Thanks Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
Hi Konstantin, Here is yet another possible approach: Perhaps you really meant what you wrote in your definition of firstFunction - namely, that it needs to be polymorphic, so that it can convert to _any_ type that is an instance of SecondClass. In that case, you might want to add another method to SecondClass: class SecondClass a where secondConstructor :: Double - a secondFunction :: a - Double instance SecondClass SecondData where secondConstructor = SecondData secondFunction (SecondData d) = d Now you can say: instance FirstClass FirstData where firstFunction (FirstData d) = secondConstructor d In short, it all depends on what exactly you are trying to do. Hope this helps, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
Hello Konstantin, Tuesday, December 25, 2007, 8:11:34 AM, you wrote: class FirstClass a where firstFunction :: (SecondClass b) = a - b this looks like one more attempt to use OOP thinking in Haskell. look at http://haskell.org/haskellwiki/OOP_vs_type_classes and especially papers referenced there -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Trouble with types
Colleagues! The essence of trouble may be given by code: [haskell] module TypeTrouble where class FirstClass a where firstFunction :: (SecondClass b) = a - b class SecondClass a where secondFunction :: a - Double data FirstData = FirstData Double data SecondData = SecondData Double instance SecondClass SecondData where secondFunction (SecondData d) = d instance FirstClass FirstData where firstFunction (FirstData d) = SecondData d [/haskell] I need, the firstFunction of FirstClass types to return a value of a SecondClass type. For example SecondData for FirstData, but for some FirstClass ThirdData, some SecondClass FourthData, etc. GHC 6.8.1 produces an error: [quote] typetrouble.hs:17:31: Couldn't match expected type `b' against inferred type `SecondData' `b' is a rigid type variable bound by the type signature for `firstFunction' at typetrouble.hs:4:31 In the expression: SecondData d In the definition of `firstFunction': firstFunction (FirstData d) = SecondData d In the definition for method `firstFunction' [/quote] How can I make this idea work? -- With best regards, Konstantin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
On Tue, Dec 25, 2007 at 08:11:34AM +0300, Konstantin Vladimirov wrote: [haskell] module TypeTrouble where class FirstClass a where firstFunction :: (SecondClass b) = a - b class SecondClass a where secondFunction :: a - Double [/haskell] I need, the firstFunction of FirstClass types to return a value of a SecondClass type. For example SecondData for FirstData, but for some FirstClass ThirdData, some SecondClass FourthData, etc. The problem, as is often the case, is that that which is unwritten does not resolve in the way you expect and require it to. FirstClass' true signature is class FirstClass a where firstFunction :: a - forall b. SecondClass b = b That is to say, any implementation of firstFunction must work for ANY instance of SecondClass. But you want SOME, not ANY. And SOME (normally notated exists tvar. tspec) is not supported in any known dialect of Haskell. It's possible to get fairly close with GHC Haskell's fundeps / type families: -- fundep version class SecondClass b = FirstClass a b where firstFunction :: a - b -- type family version class SecondClass (Cod a) = FirstClass a where type Cod a :: * firstFunction :: a - Cod a It is almost certainly possible to accomplish your goals within standard Haskell, but the best approach is not obvious at the current level of detail. Stefan signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
On Dec 25, 2007 12:11 AM, Konstantin Vladimirov [EMAIL PROTECTED] wrote: class FirstClass a where firstFunction :: (SecondClass b) = a - b snip! instance FirstClass FirstData where firstFunction (FirstData d) = SecondData d The problem is that the type of firstFunction as producing a result of type 'b', where 'b' is *any* type inahbiting the typeclass SecondClass. Your definition of firstFunction can produce only *one* of the inhabitants of SecondClass. You'd think this would be okay, seeing as there is only one member of the typeclass, but because anyone can come along and add a new typeclass instance, you're definition of firstFunction needs to account for that. If you're type-classes only ever have one instance, maybe it's easier to not use them, and go with: convert :: FirstData - SecondData convert (FirstData d) = SecondData d But I'm not sure if that suggestion is at all helpful. -Antoine ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Trouble with types
I'm new to Haskell, as you can probably tell...
I have the following data types:
data Real a = Energy a = Energy a deriving (Eq, Ord, Show)
data Real a = HeatC a = HeatC a deriving (Eq, Ord, Show)
data Object h c = Object {energy :: (Energy h), heatc :: (HeatC c)} deriving (Eq, Show)
data Real a = Temp a = Temp a deriving (Eq, Ord, Show)
I'd like to make a function to calculate the temperature of an object.
Physically this is the total heat times the heat capacity.
I tried this with the following:
temp :: (Real a) = Object (Energy a) (HeatC a) - Temp a
temp Object (Energy e) (HeatC c) = Temp e*c
But this fails in hugs with:
ERROR temp.hs:22 - Constructor Object must have exactly 2 arguments in pattern
What's the syntax to use here? Should I do this in some other way?
Should I use newtype here instead of data?
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Re: [Haskell-cafe] Trouble with types
On Fri, Jul 23, 2004 at 07:18:28PM +0300, Kari Pahula wrote: temp :: (Real a) = Object (Energy a) (HeatC a) - Temp a temp Object (Energy e) (HeatC c) = Temp e*c Probably this, but I didn't check: temp (Object (Energy e) (HeatC c)) = Temp e*c Best regards, Tom -- .signature: Too many levels of symbolic links ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
On 2004-07-23 at 19:18+0300 Kari Pahula wrote: temp :: (Real a) = Object (Energy a) (HeatC a) - Temp a temp Object (Energy e) (HeatC c) = Temp e*c But this fails in hugs with: ERROR temp.hs:22 - Constructor Object must have exactly 2 arguments in pattern You've given temp three arguments: Object, (Energy e) and (HeatC c). You meant: temp (Object (Energy e) (HeatC c)) = Temp e*c HTH -- Jón Fairbairn [EMAIL PROTECTED] ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
On Fri, Jul 23, 2004 at 06:22:57PM +0200, Tomasz Zielonka wrote: temp (Object (Energy e) (HeatC c)) = Temp e*c temp (Object (Energy e) (HeatC c)) = Temp (e*c) Best regards, Tom -- .signature: Too many levels of symbolic links ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Trouble with types
Kari,
Others have not mentioned the change required in the type signature yet.
temp :: (Real a) = Object (Energy a) (HeatC a) - Temp a
temp Object (Energy e) (HeatC c) = Temp e*c
temp :: forall a . (Real a) = Object a a - Temp a
temp (Object (Energy e) (HeatC c)) = Temp (e * c)
HTH,
Stefan
data (Real a) = Energy a = Energy a deriving (Eq, Ord, Show)
data (Real a) = HeatC a = HeatC a deriving (Eq, Ord, Show)
data (Real a) = Temp a = Temp a deriving (Eq, Ord, Show)
data Object h c = Object { energy :: Energy h,
heatc :: HeatC c
} deriving (Eq, Show)
main = let obj = Object { energy = Energy 2.5
, heatc = HeatC 12.0
}
in print (temp obj) -- prints Temp 30.0
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