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

[Henning Thielemann]

Bjorn Buckwalter schrieb:
> On Mon, Aug 18, 2008 at 2:02 PM, Henning Thielemann
>> <lemming at henning-thielemann.de> wrote:
 >>
>> Instead of
>>  muEarth :: GravitationalParameter a
>>  muEarth = ???
>>
>>  escapeVelocity :: a
>>  escapeVelocity = ... muEarth ...
>>
>> you would write
>>
>>  data AstroData a = AstroData
>>    { muEarth     :: GravitationalParameter a
>>    , leapSeconds :: LeapSecondTable
>>    }
>>
>>  escapeVelocity :: Reader (AstroData a) a
>>  escapeVelocity =
>>     do mu <- asks muEarth
>>        return (... mu ...)
>>
>> Even better you would introduce a newtype for Reader (AstroData a). This way
>> you can add any monadic functionality later (Writer et.al.).
> 
> Right, and I'd evaluate it using e.g.:
> 
>  runReader escapeVelocity myAstroData
> 
> But with implicit params I suppose I'd define (untested) e.g.:
> 
>  escapeVelocity :: (?astro :: AstroData a) => a
>  escapeVelocity = ... mu ... where mu = muEarth ?astro
> 
> To evaluate this I'd use:
> 
>  let ?astro = myAstroData in escapeVelocity
> 
> Which is comparable to the Reader version (with the
> advantage/disadvantage of the body of 'escapeVelocity' not being
> monadic).

In my opinion the implicit parameters don't make things simpler, only 
less portable, that's why I prefer the Reader monad.

 > In retrospect I think I misunderstood what you were saying in you
 > first email. I thought you were arguing that the monadic style would
 > have an advantage over implicit params in the Planck problem. But you
 > probably only meant to reemphasize the advantage (of either solution)
 > over hard-coding constants...

indeed