How to switch lexers midflight?

Hi all,

I'm trying to parse Haskell, it has nice block structure where layout in
active. For example here are three top level blocks:

x = 5

fun a b =
  a + b

data X = A | B | C

I have a lexer that can split source into blocks correctly by looking at
indentation. Then I have a rule:

%start topblock
%start topdecl
%token TOPBLOCK

topblock : TOPBLOCK
           (EXPANDFULL $1 topdecl)
         ;

topdecl : .... ;

Now, the lexer used inside EXPANDFULL should be a proper Haskell tokenizer.
How do I change tokenizer midflight?

Thanks

-- 
Gracjan

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