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Closure conversion as part of vectorisation

TODO describe the treatment of higher-order functions and closure conversion here. The relevant paper is The approach is described in more detail in

Closure-converted types as indexed-types

After some brainstorming, Roman and I (= Manuel) came to the conclusion that we can save ourselves a lot of bookkeeping if we can represent closure-converted types by indexed types (i.e., keeping track of which original types correspond to which converted types). The idea is to use a class

class CC a where
  data CConv a        -- closure converted 'a'
  to :: a -> CConv a
  fr :: CConv a -> a

The most interesting instance is that for functions, which reads

data Clo a b = forall e. Clo (c -> a -> b) e

class (CC a, CC b) => CC (a -> b) where
  data CConv (a -> b) = CCArrow (Clo a b)
  to f = Clo (\_ -> f) ()
  fr (Clo f e) = f e

From the Skype discussion, 16 Mar 07

For each function f::ty, create a closure-converted function fc::ty', where ty' is the closure-converted verion of ty. Optimisation: make fc=f, if the code for fc will be identical to that of f.

For each data type T, create a closure-converted data type Tc, whose constructors use Clo instead of ->. Optimisation: if Tc is identical to T, don't create a new data type.

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