different flavours of Monad Template Library

Wolfgang Jeltsch g9ks157k at acme.softbase.org
Mon Jan 12 10:17:11 EST 2009


it’s already problematic, that State, StateT etc. are under Control.Monad.  
While they are monads, they are also applicative functors, for example.  And 
some “monad transformers” are also applicative functor transformers and 
probably all of them are functor transformers.  There are situations where 
you want to use these types only as applicative functors, for example when 
composing context-free parsers.

Since they are all functors at least, it might be sensible to put them under 
Control.Functor.  Or it might be a good idea to put them under Data in the 
module hierarchy.  That way, a compatibility package could provide the old 
interface under Control.Monad and new stuff would be situated under 
Control.Functor or Data.

During this restructuring, we could also rename State to StateTrans since 
values of State are not states but state transformers.  There are similar 
problems with Cont since values of Cont type are not continuations but 
functions from continuations to results.  It’s problematic when you deal with 
states and state transformers or continuations and Cont values.  How do you 
name your local variables?  I’d like to say “state” for a state but it would 
also be sensible to say “state” for a value of type State, i.e., a state 

Comments?  Ideas?

Best wishes,

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