Deforestation of literal lists

[Carsten Schultz]

Hi!

Just a thought...

Would it be a sensible thing to not desugar

    [x, y, z]

into

    (:) x ((:) y ((:) z [])),

but into

    build (\ c n -> c x (c y (c z n)))?

Alternatively, how about the following rules?

{-# RULES 
  "singleton" [~1] forall x . (:) x [] = build (\c n -> c x n)
 #-}

{-# RULES 
  "cons/build" [~1] forall (x::a) (f:: forall b . (a->b->b) -> b -> b) . 
      (:) x (build f) = build (\c n -> c x (f c n))
 #-}

This should be useful for example in conjuction with sequence_ or //
applied to short lists.  Has this been discussed before?

Greetings,

Carsten

-- 
Carsten Schultz (2:38, 33:47), FB Mathematik, FU Berlin
http://carsten.codimi.de/
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fingerprint on my home page.
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