[Haskell] Re: Implicit Parameters

Simon Marlow simonmarhaskell at gmail.com
Wed Mar 1 06:53:42 EST 2006

Ashley Yakeley wrote:
> Simon Marlow wrote:
>> Simon & I have discussed doing some form of thread-local state, which 
>> covers many uses of implicit parameters and is much preferable IMO. 
>> Thread-local state doesn't change your types, and it doesn't require 
>> passing any extra parameters at runtime.  It works perfectly well for 
>> the OS example you give, for example.
> Interesting. What would that look like in code?

No concrete plans yet.  There have been proposals for thread-local 
variables in the past on this list and haskell-cafe, and other languages 
have similar features (eg. Scheme's support for dynamic scoping).  Doing 
something along these lines is likely to be quite straightforward to 
implement, won't require any changes to the type system, and gives you a 
useful form of implicit parameters without any of the drawbacks.

The main difference from implicit parameters would be that thread-local 
variables would be restricted to the IO monad.


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