[Haskell-cafe] Re: DSL question -- was: New slogan for haskell.org

[Bjorn Buckwalter]

On Dec 26, 2007 10:28 PM, Steve Lihn <stevelihn at gmail.com> wrote:
> >        arising from use of `/' at DSLTest.hs:11:14-28
>
> Thanks for the example. I am particularly amazed GHC is complaining at
> '/', not '+'. The type mismatch occurs (is reported) at much lower
> level. It would be nice if there is a way to bump it up a couple
> levels...

I suppose that is how the type inferencer works. To start with all it
really "knows" is the types of 'phi' and 'mu'. Since it knows the type
of 'mu' it can assume that 'v_GEO' has the same type, and from the
definition of 'v_GEO' (and 'phi') it can infer a type for 'r_GEO'. But
when checking that assumption against the definition of 'r_GEO' which
depends on only 'phi' and 'mu' it realizes that things are amiss. (I'm
just guessing here, the order, or way, in which the compiler does
stuff may be totally different.)

To be fair to the compiler it really has no way of knowing which
definition is wrong without explicit type signatures. As I noted in my
previous reply adding type signatures to all definitions allows the
compiler to locate the error:

  In the second argument of `(+)', namely `mu'
      In the expression: v_GEO + mu

I added this as an example to the library wiki[1]. The version of the
code with type signatures is there and the whole page can be
copy-pasted as literate haskell if you want to try it:

[1] http://code.google.com/p/dimensional/wiki/ErrorMessagesAndDebugging


> On Dec 26, 2007 12:56 PM, Bjorn Buckwalter <bjorn.buckwalter at gmail.com> wrote:
> > Steve Lihn <stevelihn <at> gmail.com> writes:
> >
> > > I do come aross a question while reading the DSL page on Wikipedia.
> > >
> > > http://en.wikipedia.org/wiki/Domain-specific_programming_language
> > >
> > > In the Disadvantage section (near the end), there is an item -- hard
> > > or impossible to debug. Can anybody explain why or what it means? And
> > > how does it apply to Haskell?
> >
> >
> > I think I can give a good example of how this can apply to EMBEDDED DSLs. My
> > library Dimensional (http://dimensional.googlecode.com) defines what someone
> > referred to as a EDSL for working with physical units. The library leverages the
> > Haskell type checker to provide static checking of physical dimensions. Without
> >  doing this I don't know how I could make such checking static.
> >
> > The downside of this is that while you will be informed at compile time if you
> > physical calculations are incorrect the error message itself is rarely useful.
> > Here is an example with lines numbered:
> >
> >  1 import Numeric.Units.Dimensional.Prelude
> >  2 import qualified Prelude
> >  3
> >  4 -- The angular velocity of Earth's rotation (Soop p. 7).
> >  5 phi = 360.985647 *~ (degree / day)
> >  6
> >  7 -- The gravitational parameter of Earth.
> >  8 mu = 3.986004400008003e14 *~ (meter ^ pos3 / second ^ pos2)
> >  9
> >  10 -- The ideal geostationary radius and velocity.
> >  11 r_GEO = cbrt (mu / phi ^ pos2)
> >  12 v_GEO = phi * r_GEO
> >  13
> >  14 -- Something obviously wrong.
> >  15 dont_try_this_at_home = v_GEO + mu
> >
> > Obviously we shouldn't be adding a velocity to a gravitational parameter on line
> > 15 and the compiler will catch this. However, this is the error message from
> > GHCi (6.6.1):
> >
> > DSLTest.hs:1:0:
> >    Couldn't match expected type `Numeric.NumType.Neg n'
> >           against inferred type `Numeric.NumType.Zero'
> >    When using functional dependencies to combine
> >      Numeric.NumType.Sub a Numeric.NumType.Zero a,
> >        arising from the instance declaration at Defined in Numeric.NumType
> >      Numeric.NumType.Sub Numeric.NumType.Zero
> >                          Numeric.NumType.Zero
> >                          (Numeric.NumType.Neg n),
> >        arising from use of `/' at DSLTest.hs:11:14-28
> >
> > I think you will agree that this isn't very helpful in pin-pointing the problem.
> > The compiler is pointing at the definition of 'r_GEO' which is twice removed
> > from the actual offender. Stuff like this can make EDSLs difficult to debug.
> >
> > (In this particular example adding type signatures to all definitions will allow
> > the compiler to pin-point the error to line 15, although the error message
> > remains cryptic.)
> >
> > Hope that helps,
> > Bjorn
> >
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>