[Haskell-cafe] do

Brandon S. Allbery KF8NH allbery at ece.cmu.edu
Mon Oct 15 09:48:49 EDT 2007

On Oct 15, 2007, at 7:02 , jerzy.karczmarczuk at info.unicaen.fr wrote:

> IO is different, you *cannot* make it non-monadic.

Not really true; it's just much more painful.  You just e.g.  
explicitly do what the ghc library's implementation of IO does:   
construct a chain of functions with an opaque (and optionally(?)  
existential to enforce the opacity) "RealWorld" type which is passed  
as state from one invocation to the next, with the "top level"  
application partially applied.  Or one of the operationally  
equivalent tricks used by other Haskell runtime implementations, cf.  
"IO Inside".

It's not so much hard as it is far too much "busywork" for a  
programmer to want to deal with when programming... hence hiding the  
scaffolding in the type, which is perhaps the most general  
description of what a Haskell monad actually is.

brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

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