Support for multiple signature files in scope.
Summary: A common pattern when programming with signatures is to combine multiple signatures together (signature linking). We achieve this by making it not-an-error to have multiple, distinct interface files for the same module name, as long as they have the same backing implementation. When a user imports a module name, they get ALL matching signatures dumped into their scope. On the way, I refactored the module finder code, which now distinguishes between exact finds (when you had a 'Module') and regular finds (when you had a 'ModuleName'). I also refactored the package finder code to use a Monoid instance on LookupResult to collect together various results. ToDo: At the moment, if a signature is declared in the local package, it completely overrides any remote signatures. Eventually, we'll want to also pull in the remote signatures (or even override the local signature, if the full implementation is available.) There are bunch of ToDos in the code for what to do once this is done. ToDo: At the moment, whenever a module name lookup occurs in GHCi and we would have seen a signature, we instead continue and return the Module for the backing implementation. This is correct for most cases, but there might be some situations where we want something a little more fine-grained (e.g. :browse should only list identifiers which are available through the in-scope signatures, and not ALL of them.) Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: simonpj, hvr, austin Subscribers: carter, thomie Differential Revision: https://phabricator.haskell.org/D790 GHC Trac Issues: #9252
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