On Sat, 17 Jul 2010 22:45:57 -0700, Ivan Lazar Miljenovic
<ivan.miljeno...@gmail.com> wrote:
Carter Schonwald <carter.schonw...@gmail.com> writes:
Hello All, I'm not sure if this either a bug in how ghc does path/module
lookup or it simply is invalid haskell:
consider modules A, A.B and A.B.C
where A imports A.B, and A.B imports A.B.C
with the following file system layout
A.hs
A/B.hs
A/B/C.hs
minimal file examples:
module A where
import A.B
testA = "will it really really work?
------------
module A.B where
import A.B.C
testB = "will it work
-----------------
module A.B.C where
testC = "will this work?"
----------
if i run ghci A.hs everything's fine
but if in directory B i rune ghci B.hs, i get
A/B.hs:2:8:
Could not find module `A.B.C':
Use -v to see a list of the files searched for.
-----------
it seems to me that if the default search path for ghc(i) includes the
current directory (which according to docs it does), this shouldn't be
happening. (or is there some why this is good Behavior?)
I think ghci is just not smart enough to know that it should change to
the parent directory and run it from there. As such, it's trying to
find "A.B.C" from the context of the current directory, and the file is
not in A/A/B/C.hs so it can't find it.
So it's just a limitation of ghci (I think).
I'm afraid I disagree and would view this as expected behavior.
"import A.B.C" translates internally to something like
load_file_using_paths("A/B/C.hs").
When you are running this from the top level directory (e.g. "top"), ghci includes the current path
"top" so the lookup is for "top/A/B/C.hs", which clearly exists.
When you are in directory B, ghci includes the current path "top/A/B" so the lookup is
for "top/A/B/A/B/C.hs"... which does not exist, thus your error.
Your example would require ghci to try load_file_using_paths("B/C.hs") (and then
load_file_using_paths("C.hs") to be complete), which discards the directory heirarchy specified by the module
nomenclature. This is not adviseable because it introduces ambiguities. For example, if you also had a C.hs in A and
another C.hs in A/B, which C.hs should it load when you say "import A.B.C"? Or "import C"? If
ghc/ghci discarded paths, then the results would be either (1) a different C.hs depending on your current directory,
(2) the bottom-most C.hs, (3) the C.hs in the current directory, (4) random?. Worse, any of the above results in a
trojan-horse style security hole. Also, what if there was a C.hs in the directory above you (top/..)? A 1:1 mapping
between module heirarchy specification and directory paths is the only dependable mechanism.
The better solution is to specifically set the paths you expect to form the roots of the
(non-default) module heirarchy if you plan to work from within subdirectories of your
source tree. If you invoked ghci as "$ ghci -i /path/to/top" then it would
work regardless of your current directory. I believe that this is the proper solution to
http://hackage.haskell.org/trac/ghc/ticket/3140 as well.
-KQ
--
-KQ
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