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Diffstat (limited to 'docs/backpack/backpack-impl.tex')
| -rw-r--r-- | docs/backpack/backpack-impl.tex | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/docs/backpack/backpack-impl.tex b/docs/backpack/backpack-impl.tex index 43a897a32e..10f199bac4 100644 --- a/docs/backpack/backpack-impl.tex +++ b/docs/backpack/backpack-impl.tex @@ -2436,6 +2436,33 @@ while it might be aesthetically displeasing to have the signature impose extra restrictions on linking identities, we can carry this out without violating the linking restriction. +\subsection{Orphans} + +Controlling instance visibility via signature problems poses an implementation +challenge similar to that of orphan instances. To describe this problem, +we first have to describe how instance resolution works for orphans and +non-orphans in GHC today. + +Type information for already compiled code in other packages is cached +on disk using interface files. For efficiency reasons, it's desirable to +avoid loading interface file until absolutely necessary: if we don't +use any of the identifiers for a file, it should not be necessary to +load the interface. Among other things, type class instances are stored +in interface files. + +Signatures and hs-boot files notwithstanding, non-orphan instance +resolution is achieved through a (somewhat) astonishing coincidence: at +the point when a type class is resolved, we are guaranteed to have +loaded the interfaces for all of the names involved in the type class +instantiation. This means that if there is a type class, we will have +seen it; conversely, it means that non-orphan instances are a closed +world: if we load all these interfaces and see no non-orphan instance, +we know there never be a non-orphan instance. + +Things are a bit worse for orphans: these instances are an open world, +and so the only way to tell if an orphan instance is in scope is by consulting +the transitive closure of module imports. + \section{Bits and bobs} \subsection{Abstract type synonyms} |