Reading around this more i think the extreme case (of what im interested in) is type arithemetic, ie the set of type "inside" the container is not necessarily finite in general, but yes, i see how you construct works. So in think for my specific purposes the associated type route is the first place for me to investigate Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. On 7 Jan 2015, at 20:58, Akash Ganesan mailto:akaberto@gmail.com> wrote: If I understand the issue at hand, you want the container, which is basically a collection of types to be closed under our operation. Since we are dealing with set of different typed objects, which is to be closed under a given operation, why not create a new type that captures the essence of the set (ie., a new data type)? Something like the following. data S = SInt Int | SChar Char | STuple (Char, Char) deriving (Show, Eq) class MyFoo a where op :: a -> a And the instance goes something like this. instance MyFoo S where op (SInt x) = SInt $ toEnum x op (SChar x) = STuple (x,x) op (STuple (x,y)) = SInt $ fromEnum x On Tuesday, 6 January 2015 23:13:59 UTC+5:30, Nicholls, Mark wrote: This is reposted from the beginnners….I’ve not done Haskell for a while and was struggling to get anything to work, I’ve hopefully refound my feet. Its quite common in maths to have operations in a theory that are (set) closed, i just want to translate that notion to a typeclass I have a suggestion that does work class Foo m where op :: m a -> m (S a) That is closed, but now were working on types of kind • -> • (S can be a type family or a data type…lets say it’s a type family….probably an associated type (if I know what that means)) Is this the idiom/pattern i should follow? Or can the closure contraint be expressed directly in a typeclass any other way? From: Haskell-Cafe [mailto:haskell-ca...@haskell.orgjavascript:] On Behalf Of Brandon Allbery Sent: 06 January 2015 1:59 PM To: Rob Leslie Cc: Haskell Cafe Subject: Re: [Haskell-cafe] Haskeline and asynchronous messages On Tue, Jan 6, 2015 at 3:27 AM, Rob Leslie javascript:> wrote: Does anyone have a suggestion on using Haskeline in an environment where another thread may be writing messages to the terminal -- is it possible to somehow print incoming messages above the input line without disturbing the input line? Terminals don't really work that way; you need to be looking at something like curses or vty. -- brandon s allbery kf8nh sine nomine associates allb...@gmail.comjavascript: ball...@sinenomine.netjavascript: unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. 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