Generate dot graph to visualize import graph

[basile-henry]

This PR adds a small option to dhall resolve that outputs the dependency graph at the import level in dot format.

It is a similar feature to stack's dependency visualization but at the module/import level as opposed to the package level.

If the feature is of any interest, I'd love to improve it and make it more user friendly! 😄

Here is what it currently looks like on ./Prelude/package.dhall (in .png as GitHub doesn't seem to support .svg uploads):

dhall resolve --dot <<< "./Prelude/package.dhall" |
  dot -Tpng > dhall-prelude.png

There is currently a small issue with diamond dependencies:

./c.dhall is pointed at twice. I have some idea on how to fix that, and I'll try to implement that, but if you have some advice on how I should go about it, I'm all ears!
I'm also open to hearing all your suggestions to make the graph look better! 😄

[Gabriella439]

Looks great to me! Just one tiny suggestion

[basile-henry]

@Gabriel439 Is it possible the caching mechanism for import doesn't work as it should?

With:

diff --git a/dhall/src/Dhall/Import.hs b/dhall/src/Dhall/Import.hs
index 15760b5..75d0372 100644
--- a/dhall/src/Dhall/Import.hs
+++ b/dhall/src/Dhall/Import.hs
@@ -198,6 +198,8 @@ import qualified Text.Megaparsec
 import qualified Text.Parser.Combinators
 import qualified Text.Parser.Token
 
+import Debug.Trace
+
 -- | An import failed because of a cycle in the import graph
 newtype Cycle = Cycle
     { cyclicImport :: Import  -- ^ The offending cyclic import
@@ -732,7 +734,7 @@ loadWith expr₀ = case expr₀ of
     expr <- if here `elem` canonicalizeAll imports
         then throwMissingImport (Imported imports (Cycle import_))
         else do
-            case Map.lookup here _cache of
+            case Map.lookup here $ traceShowId _cache of
                 Just expr -> return expr
                 Nothing   -> do
                     -- Here we have to match and unwrap the @MissingImports@

➜ tail -n +1 -- *.dhall
==> a.dhall <==
./c.dhall

==> b.dhall <==
./c.dhall

==> c.dhall <==
./d.dhall

==> d.dhall <==
42

==> diamond.dhall <==
{ a = ./a.dhall
, b = ./b.dhall
}

➜ cabal new-run -- dhall <<< "./diamond.dhall"
Up to date
fromList []
fromList []
fromList []
fromList []
fromList [(Import {importHashed = ImportHashed {hash = Nothing, importType = Local Here (File {directory = Directory {components = []}, file = "a.dhall"})}, importMode = Code},NaturalLit 42)]                     
fromList [(Import {importHashed = ImportHashed {hash = Nothing, importType = Local Here (File {directory = Directory {components = []}, file = "a.dhall"})}, importMode = Code},NaturalLit 42)]                     
fromList [(Import {importHashed = ImportHashed {hash = Nothing, importType = Local Here (File {directory = Directory {components = []}, file = "a.dhall"})}, importMode = Code},NaturalLit 42)]                     
{ a = 42, b = 42 }

I would expect to see other files than just a.dhall in the cache. The way the imports are being traversed (depth first) isn't making it easy, but this looks like a bug to me.

What do you think?

[Gabriella439]

@basile-henry: Yes, it is! I think you may have found the long-sought cause of Dhall's inefficient import resolution. I have the fix up here: #702

[f-f]

Oh wow, nice :)

@Gabriel439 could this be the cause of #642? (I recall there was another issue like "we do a suspiciously big amount of operations when resolving imports", but cannot find it anymore)

[Gabriella439]

@f-f: You might be thinking of #580

[f-f]

No, I think it was older than that, and I think we closed it when fixing some other bug related to imports. So it looks like we don't have any issue open for inefficient imports, which is good :)

[basile-henry]

Alright, diamond dependencies handled! 😄

The code might be a bit messy as I am using the cache to store the node id I need. Any suggestions on how to improve that and keep the code maintainable?

[basile-henry]

Bonus graph dhall-to-cabal:

Do you think I should use relative imports in the graph? Would that make it more readable? 😄

[basile-henry]

I can do relative imports but the result is not good. The relative paths is different depending on where it's relative to which is an issue when multiple arrows point to the same import. I'll leave the PR as is for now!