Commit d2df718c authored by Edward Z. Yang's avatar Edward Z. Yang

Add more documentation on mergeSignatures.

Signed-off-by: default avatarEdward Z. Yang <ezyang@cs.stanford.edu>
parent 5db41558
......@@ -555,37 +555,99 @@ mergeSignatures
--
gen_subst (nsubst,oks,ifaces) (imod@(IndefModule iuid _), ireq_iface) = do
let insts = indefUnitIdInsts iuid
isFromSignaturePackage =
let inst_uid = fst (splitUnitIdInsts (IndefiniteUnitId iuid))
pkg = getInstalledPackageDetails dflags inst_uid
in null (exposedModules pkg)
-- 3(a). Rename the exports according to how the dependency
-- was instantiated. The resulting export list will be accurate
-- except for exports *from the signature itself* (which may
-- be subsequently updated by exports from other signatures in
-- the merge.
as1 <- tcRnModExports insts ireq_iface
let inst_uid = fst (splitUnitIdInsts (IndefiniteUnitId iuid))
pkg = getInstalledPackageDetails dflags inst_uid
-- Setup the import spec correctly, so that when we apply
-- IEModuleContents we pick up EVERYTHING
ispec = ImpSpec
ImpDeclSpec{
is_mod = mod_name,
is_as = mod_name,
is_qual = False,
is_dloc = loc
} ImpAll
rdr_env = mkGlobalRdrEnv (gresFromAvails (Just ispec) as1)
-- 3(b). Thin the interface if it comes from a signature package.
(thinned_iface, as2) <- case mb_exports of
Just (L loc _)
| null (exposedModules pkg) -> setSrcSpan loc $ do
-- Suppress missing errors; we'll pick em up
-- when we test exports on the final thing
(msgs, mb_r) <- tryTc $
-- Check if the package containing this signature is
-- a signature package (i.e., does not expose any
-- modules.) If so, we can thin it.
| isFromSignaturePackage
-> setSrcSpan loc $ do
-- Suppress missing errors; they might be used to refer
-- to entities from other signatures we are merging in.
-- If an identifier truly doesn't exist in any of the
-- signatures that are merged in, we will discover this
-- when we run exports_from_avail on the final merged
-- export list.
(msgs, mb_r) <- tryTc $ do
-- Suppose that we have written in a signature:
-- signature A ( module A ) where {- empty -}
-- If I am also inheriting a signature from a
-- signature package, does 'module A' scope over
-- all of its exports?
--
-- There are two possible interpretations:
--
-- 1. For non self-reexports, a module reexport
-- is interpreted only in terms of the local
-- signature module, and not any of the inherited
-- ones. The reason for this is because after
-- typechecking, module exports are completely
-- erased from the interface of a file, so we
-- have no way of "interpreting" a module reexport.
-- Thus, it's only useful for the local signature
-- module (where we have a useful GlobalRdrEnv.)
--
-- 2. On the other hand, a common idiom when
-- you want to "export everything, plus a reexport"
-- in modules is to say module A ( module A, reex ).
-- This applies to signature modules too; and in
-- particular, you probably still want the entities
-- from the inherited signatures to be preserved
-- too.
--
-- We think it's worth making a special case for
-- self reexports to make use case (2) work. To
-- do this, we take the exports of the inherited
-- signature @as1@, and bundle them into a
-- GlobalRdrEnv where we treat them as having come
-- from the import @import A@. Thus, we will
-- pick them up if they are referenced explicitly
-- (@foo@) or even if we do a module reexport
-- (@module A@).
let ispec = ImpSpec ImpDeclSpec{
-- NB: This needs to be mod name
-- of the local signature, not
-- the (original) module name of
-- the inherited signature,
-- because we need module
-- LocalSig (from the local
-- export list) to match it!
is_mod = mod_name,
is_as = mod_name,
is_qual = False,
is_dloc = loc
} ImpAll
rdr_env = mkGlobalRdrEnv (gresFromAvails (Just ispec) as1)
setGblEnv tcg_env {
tcg_rdr_env = rdr_env
} $ exports_from_avail mb_exports rdr_env
(tcg_imports tcg_env) (tcg_semantic_mod tcg_env)
-- NB: tcg_imports is also empty!
emptyImportAvails
(tcg_semantic_mod tcg_env)
case mb_r of
Just (_, as2) -> return (thinModIface as2 ireq_iface, as2)
Nothing -> addMessages msgs >> failM
-- We can't think signatures from non signature packages
_ -> return (ireq_iface, as1)
let oks' | null (exposedModules pkg)
-- 3(c). Only identifiers from signature packages are "ok" to
-- import (that is, they are safe from a PVP perspective.)
-- (NB: This code is actually dead right now.)
let oks' | isFromSignaturePackage
= extendOccSetList oks (exportOccs as2)
| otherwise
= oks
-- 3(d). Extend the name substitution (performing shaping)
mb_r <- extend_ns nsubst as2
case mb_r of
Left err -> failWithTc err
......
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