[Haskell-cafe] Thoughts on program annotations.
jason.dusek at gmail.com
Fri Mar 4 07:32:53 CET 2011
I am working on a Bash config generation system. I've decided
to factor out the Bash AST and pretty printer, here in a
One thing I'd like to support is generic annotations, so that
at a future time I can add (and render) comments, mark
subscripts with what privilege (package install, sudo) they
require or otherwise characterize the script outside its
lexical structure. I ended up making my statement type a
Functor with a Foldable instance.
Given that every statement has an annotation, it seemed better
to me to use mutually recursive datatypes, using one datatype
to capture "annotatedness", like this:
-- From https://github.com/solidsnack/bash/blob/c718de36d349efc9ac073a2c7082742c45606769/hs/Language/Bash/Syntax.hs
data Annotated t = Annotated t (Statement t)
data Statement t = SimpleCommand Expression [Expression]
| IfThen (Annotated t) (Annotated t)
I wonder what folks think of this approach? It does mean I end
up with all leaf-level annotations being potentially without
annotations; this allows for relatively generic definitions,
on the one hand; but forces type annotations at the use-site
in many cases. It also means I have mutually recursive Functor
and Foldable instances.
Another option for annotations would be a sort of tree of
zippers pointing in to the statement tree; this seems horrible
at first glance since it leaves open the question of how the
annotations are associated with their nodes in the first
place. However, it does have the nice feature of simplifying
the type of statements and also is just much more modular
What does the list think?
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