[Haskell-cafe] Reader monad, implicit parameters, or something else altogether?

Bjorn Buckwalter bjorn.buckwalter at gmail.com
Mon Aug 18 12:32:35 EDT 2008

On Mon, Aug 18, 2008 at 11:16 AM, Henning Thielemann
<lemming at henning-thielemann.de> wrote:
> On Mon, 18 Aug 2008, Bjorn Buckwalter wrote:
>> I would like to know if there is any consensus on what is the best way
>> to make such a data structure accessible in pure functions. Passing it
>> explicitly would be a mess. It seems that two options are to use
>> either a Reader monad or implicit parameters. Using a Reader monad is
>> straight forward enough though it requires writing/converting code
>> in/to monadic style and adds some clutter to the formulae. It seems
>> implicit parameters could be cleaner but I've seen them referred to as
>> everything from evil to "just what you need" and rendering the Reader
>> monad obsolete...
> I expect that you will get the same range of opinions as you got from your
> search. As far as I know implicit parameters break referential transparency.
>  http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue2/FunWithLinearImplicitParameters
>  So I prefer Reader monad. The burden of converting to monadic style pays
> off when you need to use the same code with different values for the
> "constants". (E.g. find out for which value of the Planck constant the
> universe collapses and for which it oscillates etc. :-)

Love the example but could you elaborate a little on how monadic style
helps with this? (This is probably a matter of it not being obvious to
me what approach you would take to solving the problem.)

By the way, can anyone comment on implicit parameters interacting with
monadic code? If I were to use implicit parameters and later realize
that I need other monadic functionality, e.g. Writer, would I be in a
tough spot or would the implicit parameters continue to work
"seemlesly" in the monadic code?

Bjorn Buckwalter

More information about the Haskell-Cafe mailing list