[MLton-user] Extended Basis Library: Suggested change to the MONADP signature

[Geoffrey Alan Washburn]

    In any event, I just finally got around to trying to revise my code 
so that I can change the definition of MONADP's zero from

        val zero : errval -> 'a monad

to

        val zero : 'a monad

and it occurred to me that with the value restriction my version might 
actually be the better choice.  As it stands it is only possible to 
define zero in terms of values.  Which is mostly okay, except that the 
usual eta-expansion trick cannot be applied unless your monad is 
implemented as a functional type. 

Therefore I am thinking that it might be good to change the MONAP_CORE 
signature to be

signature MONADP_CORE = sig
   include MONAD_CORE
   type errval
   val zero : errval -> 'a monad
   val <|> : 'a monad BinOp.t
end

and in those cases that where you have no interesting error values one 
could just use MONADP_CORE with type errval = unit.  Of course, this 
does mean that in such a case you have to write the aesthetically less 
pleasing zero () all over your code. 

-- 
[Geoff Washburn|geoffw at cis.upenn.edu|http://www.cis.upenn.edu/~geoffw/]

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mlton.org/pipermail/mlton-user/attachments/20070401/cc0a9791/attachment.html