[Haskell-cafe] Re: Haskell vs OCaml

[Jacques Carette]

Duncan Coutts wrote:

>On Wed, 2005-05-04 at 18:29 -0400, Jacques Carette wrote:
>  
>
>>There is also Template Haskell vs MetaOCaml.
>>
>>For the life of me, I still cannot fathom why Template Haskell is untyped, while MetaOCaml is fully typed.  Which is 
>>the main reason I write meta-program in MetaOCaml and 'other' programs in Haskell.
>>    
>>
>The reason for this is that Template Haskell is more powerful and
>correspondingly harder to type so it is currently untyped.
>  
>
``more powerful'' meaning that it can do some level of introspection, 
right?  I know introspection can be very powerful (I designed Maple's 
modern reflection and reification  facilities...), but I was under the 
impression that type systems that can handle 'too much' introspection 
were unsound [a result of Walid Taha].

>The main reason it is more porwerful is that TH allows you to pattern
>match on the abstract syntax of a quoted expression and perform
>arbitrary transformations on that. It would be hard to preserve the
>well-typedness of the AST and still allow arbitrary transformations.
>  
>
Can't this be regarded as a 'convenience' rather than as actual extra 
power?  Using an abstract interpretation formalism (see papers of Taha 
and co-authors), it is possible to 'lift' the type of code values into 
staging-time terms, and pattern-match on that instead.  Then 
well-typedness of transformations is much easier to show.  In this way 
one can even implement some transformations that are equivalent to what 
are currently 'hints' to GHC as well-typed code [I have some code that 
does this, but there are only hints of this in a preprint of mine -- see 
the conclusion section of "*Gaussian Elimination: a case study in 
efficient genericity with MetaOCaml"*  
<http://www.cas.mcmaster.ca/%7Ecarette/publications/ge.pdf>available 
from http://www.cas.mcmaster.ca/~carette/publications.html for some 
hints on how this is done

>A colleague of mine is currently developing a type system for Template
>Haskell that should catch most errors without restricting too much the
>programs you can write.
>  
>
I am eagerly awaiting this!  Writing Monadic meta-programs in MetaOCaml 
is feasible, but the lack of native support for Monads makes it 
significantly harder.  But after spending 12 years writing programs in a 
fully dynamically typed language (Maple), I am much happier writing *all 
*parts of my programs in a strongly typed language!

Jacques
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