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

Bjorn Buckwalter schrieb:

On Mon, Aug 18, 2008 at 2:02 PM, Henning Thielemann

...

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