2009/1/27 Matthijs Kooijman
Hi all,
I've been reading the gentle introduction to Haskell a bit more closely today and there a few things which I can't quite understand (presumably because they are typo's). I've found two issues with the "Using monads" section [1]. Not sure if this is the right place to report this, but there's probably somewhere here who can fix it :-)
The first one is in the state monad data type. It reads:
data SM a = SM (S -> (a,S)) -- The monadic type
I can't seem to find out what "S" is exactly here. It seems to be a universally quantified type variable, but then it should be lowercase. Considering that it is referred to later on as "s", I suspect this is a type?
Often when working through a concept for the first time, it's simpler to fix particular pieces; in this case, S is some fixed state type. Then, once you get everything working, you notice that you don't make any assumptions about S; then you are free to generalize which lets you universally quantify. Perhaps the tutorial should make this more clear, but this is a process I've gone through with my own code many times; start with the specific and generalize as needed. In addition, type signatures are simpler with as few type variables as possible:
get :: SM S put :: S -> SM ()
as opposed to
get :: SM s s put :: s -> SM s ()
In the state monad case this isn't much of a difference, but I've often found that my code wants three or four additional type variables on a structure. In these cases I do wish for some kind of "parametrized module" syntax, where the module itself is quantified over certain type variables, and the code does not need to explicitly list them everywhere. -- ryan