Re: [Haskell-cafe] distinguish functions from non-functions in a class/instances
oleg-7 wrote: In fact, that distinction is possible. The following article How to write an instance for not-a-function http://okmij.org/ftp/Haskell/typecast.html#is-function-type specifically describes a method of writing an instance which is selected only when the type in question is NOT a function. The method is quite general and has been extensively used (for example, to implement deep monadic join). That's really incredible, and yet I don't quite understand how IsFunction works. Here is my very short but powerful solution (nary is renamed to wrap). http://www.nabble.com/file/p14220591/wrap.hs wrap.hs -- View this message in context: http://www.nabble.com/distinguish-functions-from-non-functions-in-a-class-instances-tf4952209.html#a14220591 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] distinguish functions from non-functions in a class/instances
Ryan Ingram wrote: No, that doesn't work; it's close, but not quite. liftM doesn't have the right type signature. liftM :: Monad m = (a - r) - (m a1 - m r) What would work is if you could define a function liftLast :: Monad m = (a0 - a1 - ... - aN - r) - (a0 - a1 - ... - aN - m r) then nary' f = runIdentity . nary (liftLast f) -- ryan I don't see a way to implement liftLast or nary for functions like (a - b - ... - r) where r is not of the form (m s). Of course one can use the Identity Monad for m, but in either case you have to modify functions like (Int - Int) to something like (Int - m Int) for a fixed type m (e.g. Identity). -- philipp n. -- View this message in context: http://www.nabble.com/distinguish-functions-from-non-functions-in-a-class-instances-tf4952209.html#a14208302 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] distinguish functions from non-functions in a class/instances
Hello, i'm trying to wrap functions (a - b - ... - z) of any arity to functions of type ([String] - y), where list of strings replaces the typed arguments. one attempt looks like this (here written with type families, you can replace it by functional dependencies or what ever): type family Res x type instance Res x = x type instance Res (x-y) = Res y class Nary x where nary :: x - [String] - Res x instance Nary x where nary x [] = x instance Nary (x-y) where nary f (x:xs) = nary (f $ read x) xs i hope you can get the idea. the problem is, that you cannot distinguish type (x-y) from z, so these instances are overlapping. the odd thing is. you can get this to work, if you have a terminating type as result type (for example (IO x)). then you can work with all types (IO x), (a - IO x), (a - b - IO x), ... but i don't want this delimiter IO! any ideas? greetings Philipp N. -- View this message in context: http://www.nabble.com/distinguish-functions-from-non-functions-in-a-class-instances-tf4952209.html#a14180315 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] distinguish functions from non-functions in a class/instances
On Dec 5, 2007, at 16:00 , Philipp N. wrote: the odd thing is. you can get this to work, if you have a terminating type as result type (for example (IO x)). then you can work with all types (IO x), (a - IO x), (a - b - IO x), ... but i don't want this delimiter IO! any ideas? Use ST instead? (just tossing ideas in the wind...) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] distinguish functions from non-functions in a class/instances
No, that doesn't work; it's close, but not quite. liftM doesn't have the right type signature. liftM :: Monad m = (a - r) - (m a1 - m r) What would work is if you could define a function liftLast :: Monad m = (a0 - a1 - ... - aN - r) - (a0 - a1 - ... - aN - m r) then nary' f = runIdentity . nary (liftLast f) -- ryan On 12/5/07, Dan Weston [EMAIL PROTECTED] wrote: Wouldn't any isomorphism do (like the Identity monad)? How about nary' f = runIdentity . nary (liftM f) . return Brandon S. Allbery KF8NH wrote: On Dec 5, 2007, at 16:00 , Philipp N. wrote: the odd thing is. you can get this to work, if you have a terminating type as result type (for example (IO x)). then you can work with all types (IO x), (a - IO x), (a - b - IO x), ... but i don't want this delimiter IO! any ideas? Use ST instead? (just tossing ideas in the wind...) ___ 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] distinguish functions from non-functions in a class/instances
Wouldn't any isomorphism do (like the Identity monad)? How about nary' f = runIdentity . nary (liftM f) . return Brandon S. Allbery KF8NH wrote: On Dec 5, 2007, at 16:00 , Philipp N. wrote: the odd thing is. you can get this to work, if you have a terminating type as result type (for example (IO x)). then you can work with all types (IO x), (a - IO x), (a - b - IO x), ... but i don't want this delimiter IO! any ideas? Use ST instead? (just tossing ideas in the wind...) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
