On Fri, Jun 29, 2012 at 09:57:42AM +0100, Matt Ford wrote:
Hi,
I've been reading the following blog post
https://cdsmith.wordpress.com/2012/04/18/why-do-monads-matter/
And I think I like it. But there's a part that I don't get.
"For a set A, we will define the set Pref(A) to be the set of functions from application settings to the set A. Now watch closely: a function in context from A to B is just an ordinary function from A to Pref(B). In other words, you give it a value from the set A, and it gives you back another function that maps from application settings to the set B."
This is in the "functioning with dependency" section and is talking about a procedure that uses outside info from preferences or application settings.
If I set my prefs as follows
configvar = 3
and define a function as follows
add x = configvar + 6
So add’s signature is
add: int -> int
What does prefs(int) look like? Is that even the right thing to ask?
prefs(int) looks like Config -> Int (in your example perhaps we define type Config = Int), so add should have type Int -> (Config -> Int) The thing that is confusing the issue here is that you just made add implicitly use the 'configvar' which is in scope, so it does not need to take it as a parameter. But imagine that you want to be able to do multiple runs with different configurations, without recompiling your program -- then you will need to have any function that needs the configuation take it as an input. Like this: add x config = config + 6
By substituting the B for Prefs(B) and returning now only functions from Pref(B) don't we lose the rest of the mapping for add i.e., " + 6"?
I don't think I understand this question. -Brent