[Haskell-cafe] typeclass woes...how to constain a typeclass to be "closed" under an operation....

[Nicholls, Mark]

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 <akaberto at gmail.com<mailto:akaberto at 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... at haskell.org<javascript:>] 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 <r... at mars.org<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... at gmail.com<javascript:>                                  ball... at sinenomine.net<javascript:>
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net


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