[Haskell-cafe] What *is* a DSL?

S. Doaitse Swierstra doaitse at swierstra.net
Wed Oct 28 17:18:50 EDT 2009

On 22 okt 2009, at 15:56, Robert Atkey wrote:
>>> ....
>> Previously parsed input /can/ determine what the parser will accept  
>> in
>> the future (as pointed out by Peter Ljunglöf in his licentiate  
>> thesis).
>> Consider the following grammar for the context-sensitive language
>> {aⁿbⁿcⁿ| n ∈ ℕ}:
> Yes, sorry, I was sloppy in what I said there. Do you know of a
> characterisation of what languages having a possibly infinite amount  
> of
> nonterminals gives you. Is it all context-sensitive languages or a
> subset?

The answer is: all context-sensitive languages. This is a very old  
insight which has come back in various forms in computer science. The  
earliest conception in CS terms is the concept of an affix-grammar, in  
which the infinite number of nonterminals is generated by  
parameterising non-terminals by trees. They were invented by Kees  
koster and Lambert Meertens (who applied them to generate music: http://en.wikipedia.org/wiki/index.html?curid=5314967) 
  in the beginning of the sixties of the last century. There is a long  
follow up on this idea, of which the two most well-known versions are  
the so-called two-level grammars which were used in the Algol68 report  
and the attribute grammar formalism first described by Knuth. The full  
Algol68 language is defined in terms of a two-level grammar. Key  
publications/starting points if you want to learn more about these are:

  - the Algol68 report: http://burks.brighton.ac.uk/burks/language/other/a68rr/rrtoc.htm
  - the wikipedia paper on affix grammars: http://en.wikipedia.org/wiki/Affix_grammar
  - a nice book about the basics od two-level grammars is the  
Cleaveland & Uzgalis book, "Grammars for programming languages", which  
may be hard to get,
      but there is hope: http://www.amazon.com/Grammars-Programming-Languages-languages/dp/0444001875
  - http://www.agfl.cs.ru.nl/papers/agpl.ps
  - http://comjnl.oxfordjournals.org/cgi/content/abstract/32/1/36

  Doaitse Swierstra

>>> And a general definition for parsing single-digit numbers. This  
>>> works
>>> for any set of non-terminals, so it is a reusable component that  
>>> works
>>> for any grammar:
>> Things become more complicated if the reusable component is defined
>> using non-terminals which take rules (defined using an arbitrary
>> non-terminal type) as arguments. Exercise: Define a reusable  
>> variant of
>> the Kleene star, without using grammars of infinite depth.
> I see that you have an answer in the paper you linked to above.  
> Another
> possible answer is to consider open sets of rules in a grammar:
>> data OpenRuleSet inp exp =
>>   forall hidden. OpenRuleSet (forall a. (exp :+: hidden) a ->
>>                                   Rule (exp :+: hidden :+: inp) a)
>> data (f :+: g) a = Left2 (f a) | Right2 (g a)
> So OpenRuleSet inp exp, exports definitions of the nonterminals in
> 'exp', imports definitions of nonterminals in 'inp' (and has a
> collection of hidden nonterminals).
> It is then possible to combine them with a function of type:
>> combineG :: (inp1 :=> exp1 :+: inp) ->
>>            (inp2 :=> exp2 :+: inp) ->
>>            OpenRuleSet inp1 exp1 ->
>>            OpenRuleSet inp2 exp2 ->
>>            OpenRuleSet inp (exp1 :+: exp2)
> One can then give a reusable Kleene star by stating it as an open rule
> set:
>> star :: forall a nt. Rule nt a -> OpenRuleSet nt (Equal [a])
> where Equal is the usual equality GADT.
> Obviously, this would be a bit clunky to use in practice, but maybe  
> more
> specialised versions combineG could be given.
> Bob
> -- 
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> Scotland, with registration number SC005336.
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