Hi Tillmann,
is a shallow embedded DSL == an internal DSL and a deeply embedded DSL == an
external DSL or the other way around?
--Joerg
On Dec 3, 2012, at 11:40 PM, Tillmann Rendel wrote:
> Hi,
>
> Joerg Fritsch wrote:
>> I am working on a DSL that eventuyally would allow me to say:
>>
>> import language.cwmwl
>>
>> main = runCWMWL $ do
>>
>> eval ("isFib::", 1000, ?BOOL)
>>
>>
>> I have just started to work on the interpreter-function runCWMWL and I
>> wonder whether it is possible to escape to real Haskell somehow (and
>> how?) either inside ot outside the do-block.
>
> You can already use Haskell in your DSL. A simple example:
>
> main = runCWMWL $ do
> eval ("isFib::", 500 + 500, ?BOOL)
>
> The (+) operator is taken from Haskell, and it is available in your DSL
> program. This use of Haskell is completely for free: You don't have to do
> anything special with your DSL implementation to support it. I consider this
> the main benefit of internal vs. external DSLs.
>
>
> A more complex example:
>
> main = runCWMWL $ do
> foo <- eval ("isFib::", 1000, ?BOOL)
> if foo
> then return 27
> else return 42
>
> Here, you are using the Haskell if-then-else expression to decide which DSL
> program to run. Note that this example also uses (>>=) and return, so it only
> works because your DSL is monadic. Beyond writing the Monad instance, you
> don't have to do anything special to support this. In particular, you might
> not need an additional embed function if you've already implemented return
> from the Monad type class. I consider this the main benefit of the Monad type
> class.
>
> Tillmann
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