Séminaire Cambium, Inria Paris, 2 rue Simone Iff
                          Salle Lions 2, bâtiment C
                           Lundi 7 novembre, 10h30

                                Daniel Mercier
                                   AdaCore

         Towards a solution to the expression problem for compilers:
                          strongly typed nano-passes

We present a new solution to the problem of (strongly) typing compiler
passes when the source and destination language share multiple
constructors. We are interested in approaches that limit the amount of
boilerplate that needs to be (re)written for each language, while
still maintaining a very strict typing discipline. Our solution uses
well-known tools, but combines them in a novel way:

- the idea of nanopasses, in which each pass encodes a very limited
  transformation

- the use of ppx syntax extensions to express only the differences
  between a language and the next

- the traversal of the AST using top-down and bottom-up recursion
  schemes

- OCaml polymorphic variants, which are instrumental to precisely type
  our (non-recursive) transformation functions.

We present our implementation of this approach in the context of the
Ada frontend for the Infer analyzer, which as of today contains 26
passes. We will also discuss the various pitfalls we have encountered
along the way, as well as possible improvements (in particular
deforesting the combination of multiple passes).

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