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

Nicholls, Mark nicholls.mark at vimn.com
Tue Jan 6 17:43:47 UTC 2015

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-cafe-bounces at haskell.org] 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 <rob at mars.org<mailto:rob at mars.org>> 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
allbery.b at gmail.com<mailto:allbery.b at gmail.com>                                  ballbery at sinenomine.net<mailto:ballbery at sinenomine.net>
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net

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