[Haskell] Implicit Parameters

27 Feb 2006

      Ben Rudiak-Gould wrote:
...
I'd advise against using implicit parameters, because (as you've seen) 
it's hard to reason about when they'll get passed to functions.
And Johannes Waldmann wrote:
...
Implicit parameters are *evil*. They seem to simplify programs
but they make reasoning about them much harder.
Feh. Implicit parameters are often exactly what you want. You just have 
to make sure to provide type signatures (-Wall -Werror can help here).

In fact it would be useful to allow implicit parameters and other type 
context at the top level of a module:

   forall m. (Monad m,?getCPUTime :: m Integer) => module MyModule where
     timeFunction :: forall a. m a -> m (Integer,a)
     timeFunction ma = do
       t0 <- ?getCPUTime
       a <- ma
       t1 <- ?getCPUTime
       return (t1 - t0,a)

This is just syntactic sugar that gives this:

   timeFunction :: forall m a. (Monad m,?getCPUTime :: m Integer) =>
      m a -> m (Integer,a)

In a future Haskell Operating System, this is how system functions could 
be provided to application code. This would make secure sandboxes easy 
to set up, for instance.

-- 
Ashley Yakeley