performance of monads

John Hughes
Thu, 24 Jan 2002 15:09:38 +0100 (MET)

	I've felt need to use State monads in two distinct situations:

	And I've seen two distict aproaches. Using a State monad like one
	provided with GHC (1), or a state monad like the one defined in the
	paper "Moands for the Working Haskell Programmer" (2).

	In (1), I can see how I can take advantage of the monad and make
	updates in place using referencies. But then you'll have to either: -
	pass the referencies as a parameter to the functions where you use
	them (needless too say that this defeats the (my) main purpose of
	using a state monad which is in case A, keep track of data with
	minimal signature change )

You can do this conveniently using implicit parameters bound to your
references: that way you don't have to add parameters to all your function
definitions just to pass in a reference. (The types do change, though).
I would probably use one implicit parameter bound to a record of references.


	Another state issue, sometimes I have some values that I want to keep 
	constant through the whole algorithm.
	(some very simple NNs for instance)
	- learning rate
	- activation function 
	- maximum number of steps
	- minimum error required

	So I'll just declare them as 'constants'. But what if I decide I want
	the user to be able to chose? Now I got two options: - pass them
	around as values to all the functions - And signatures get HUGE - just
	pass them to a higher level function that will encapsulate the
	functions that use them... which is ugly and complicates everything
	because you can't test the lower level functions in the interpreter.

	Am I missing something here or is this really the best you can do?

Implicit parameters solve this too.


I've written a short article on global variables in Haskell, which explains
this particular application.

John Hughes