Give better error messages when dependencies are not present (#60)

* Give better error messages when dependencies are not present

This way in the common case where a dependency is not present we will
give a better error than "attribute missing".

This will bloat the size of the generated files a bit since a lot of the
messages are repeated, but I couldn't see a nice way to do it otherwise
while keeping the `ToNixExpr` functions self-contained.

I was a little bit unsure about `ExeDependency` - I *think* it's trying
to look in `components.exes` although at the moment it just looks at the
ambient scope. I made this explicit.

lib/Cabal2Nix.hs

@@ -36,12 +36,11 @@ import Data.String (fromString, IsString)
 import Distribution.Types.PackageId
 --import Distribution.Types.Condition
 import Distribution.Types.UnqualComponentName
-import Data.List.NonEmpty (NonEmpty(..))
 import Nix.Expr
 import Data.Fix(Fix(..))
 import Data.Text (Text)
 
-import Cabal2Nix.Util (quoted)
+import Cabal2Nix.Util (quoted, selectOr, mkThrow)
 
 data Src
   = Path FilePath
@@ -54,6 +53,14 @@ hsPkgs = "hsPkgs"
 pkgconfPkgs = "pkgconfPkgs"
 flags  = "flags"
 
+buildDepError, sysDepError, pkgConfDepError, exeDepError, legacyExeDepError, buildToolDepError :: Text
+buildDepError = "buildDepError"
+sysDepError = "sysDepError"
+pkgConfDepError = "pkgConfDepError" 
+exeDepError = "exeDepError"
+legacyExeDepError = "legacyExeDepError"
+buildToolDepError = "buildToolDepError" 
+
 ($//?) :: NExpr -> Maybe NExpr -> NExpr
 lhs $//? (Just e) = lhs $// e
 lhs $//? Nothing  = lhs
@@ -90,7 +97,7 @@ cabal2nix fileDetails src = \case
         (_, Right desc) -> pure desc
 
 gpd2nix :: CabalDetailLevel -> Maybe Src -> Maybe NExpr -> GenericPackageDescription -> NExpr
-gpd2nix fileDetails src extra gpd = mkFunction args $ toNix' fileDetails gpd $//? (toNix <$> src) $//? extra
+gpd2nix fileDetails src extra gpd = mkLets errorFunctions $ mkFunction args $ toNix' fileDetails gpd $//? (toNix <$> src) $//? extra
   where args :: Params NExpr
         args = mkParamset [ ("system", Nothing)
                           , ("compiler", Nothing)
@@ -100,6 +107,58 @@ gpd2nix fileDetails src extra gpd = mkFunction args $ toNix' fileDetails gpd $//
                           , (pkgconfPkgs, Nothing)]
                           True
 
+errorFunctions :: [Binding NExpr]
+errorFunctions = 
+  [ buildDepError $= mkFunction "pkg" (mkThrow $ 
+      Fix $ NStr $ Indented 0 
+          [ Plain "The Haskell package set does not contain the package: " 
+          , Antiquoted "pkg"
+          , Plain " (build dependency).\n\n"
+          , Plain haskellUpdateSnippet
+          ])
+  , sysDepError $= mkFunction "pkg" (mkThrow $ 
+      Fix $ NStr $ Indented 0 
+          [ Plain "The Nixpkgs package set does not contain the package: " 
+          , Antiquoted "pkg"
+          , Plain " (system dependency).\n\n"
+          , Plain systemUpdateSnippet
+          ])
+  , pkgConfDepError $= mkFunction "pkg" (mkThrow $ 
+      Fix $ NStr $ Indented 0 
+          [ Plain "The pkg-conf packages does not contain the package: " 
+          , Antiquoted "pkg"
+          , Plain " (pkg-conf dependency).\n\n"
+          , Plain "You may need to augment the pkg-conf package mapping in haskell.nix so that it can be found."
+          ])
+  , exeDepError $= mkFunction "pkg" (mkThrow $ 
+      Fix $ NStr $ Indented 0 
+          [ Plain "The local executable components do not include the component: " 
+          , Antiquoted "pkg"
+          , Plain " (executable dependency)."
+          ])
+  , legacyExeDepError $= mkFunction "pkg" (mkThrow $ 
+      Fix $ NStr $ Indented 0 
+          [ Plain "The Haskell package set does not contain the package: " 
+          , Antiquoted "pkg"
+          , Plain " (executable dependency).\n\n"
+          , Plain haskellUpdateSnippet
+          ])
+  , buildToolDepError $= mkFunction "pkg" (mkThrow $ 
+      Fix $ NStr $ Indented 0 
+          [ Plain "Neither the Haskell package set or the Nixpkgs package set contain the package: " 
+          , Antiquoted "pkg"
+          , Plain " (build tool dependency).\n\n"
+          , Plain "If this is a system dependency:\n"
+          , Plain systemUpdateSnippet
+          , Plain "\n\n"
+          , Plain "If this is a Haskell dependency:\n"
+          , Plain haskellUpdateSnippet
+          ])
+  ]
+  where
+    systemUpdateSnippet = "You may need to augment the system package mapping in haskell.nix so that it can be found."
+    haskellUpdateSnippet = "If you are using Stackage, make sure that you are using a snapshot that contains the package. Otherwise you may need to update the Hackage snapshot you are using, usually by updating haskell.nix."
+
 class IsComponent a where
   getBuildInfo :: a -> BuildInfo
   getMainPath :: a -> Maybe FilePath
@@ -264,30 +323,43 @@ instance ToNixExpr' GenericPackageDescription where
             (bindTo "tests"       . mkNonRecSet <$> filter (not . null) [ uncurry component <$> condTestSuites   gpd ]) ++
             (bindTo "benchmarks"  . mkNonRecSet <$> filter (not . null) [ uncurry component <$> condBenchmarks   gpd ])
 
+-- WARNING: these use functions bound at he top level in the GPD expression, they won't work outside it
+
 instance ToNixExpr Dependency where
-  toNix = (@.) (mkSym hsPkgs) . fromString . show . pretty . depPkgName
+  toNix d = selectOr (mkSym hsPkgs) (mkSelector $ quoted pkg) (mkSym buildDepError @@ mkStr pkg)
+    where
+      pkg = fromString . show . pretty . depPkgName $ d
 
 instance ToNixExpr SysDependency where
-  toNix = (@.) (mkSym pkgs) . quoted . fromString . unSysDependency
+  toNix d = selectOr (mkSym pkgs) (mkSelector $ quoted pkg) (mkSym sysDepError @@ mkStr pkg)
+    where
+      pkg = fromString . unSysDependency $ d
 
 instance ToNixExpr PkgconfigDependency where
-  toNix (PkgconfigDependency name _versionRange)= (@.) (mkSym pkgconfPkgs) . quoted . fromString . unPkgconfigName $ name
+  toNix (PkgconfigDependency name _versionRange) = selectOr (mkSym pkgconfPkgs) (mkSelector $ quoted pkg) (mkSym pkgConfDepError @@ mkStr pkg)
+    where
+      pkg = fromString . unPkgconfigName $ name
 
 instance ToNixExpr ExeDependency where
-  toNix (ExeDependency pkgName' _unqualCompName _versionRange) = mkSym . fromString . show . pretty $ pkgName'
+  toNix (ExeDependency pkgName' _unqualCompName _versionRange) = selectOr (mkSym "exes") (mkSelector $ pkg) (mkSym exeDepError @@ mkStr pkg)
+    where
+      pkg = fromString . show . pretty $ pkgName'
 
 instance ToNixExpr BuildToolDependency where
   toNix (BuildToolDependency pkgName') =
       -- TODO once https://github.com/haskell-nix/hnix/issues/52
       -- is reolved use something like:
       -- [nix| hsPkgs.buildPackages.$((pkgName)) or pkgs.buildPackages.$((pkgName)) ]
-      Fix $ NSelect (mkSym hsPkgs) buildPackagesDotName
-        (Just . Fix $ NSelect (mkSym pkgs) buildPackagesDotName Nothing)
+      selectOr (mkSym hsPkgs) buildPackagesDotName
+        (selectOr (mkSym pkgs) buildPackagesDotName (mkSym buildToolDepError @@ mkStr pkg))
     where
-      buildPackagesDotName = StaticKey "buildPackages" :| [StaticKey (fromString . show . pretty $ pkgName')]
+      pkg = fromString . show . pretty $ pkgName'
+      buildPackagesDotName = mkSelector "buildPackages" <> mkSelector pkg
 
 instance ToNixExpr LegacyExeDependency where
-  toNix (LegacyExeDependency name _versionRange) = mkSym hsPkgs @. fromString name
+  toNix (LegacyExeDependency name _versionRange) = selectOr (mkSym hsPkgs) (mkSelector $ quoted pkg) (mkSym legacyExeDepError @@ mkStr pkg)
+    where
+      pkg = fromString name
 
 instance {-# OVERLAPPABLE #-} ToNixExpr String where
   toNix = mkStr . fromString

lib/Cabal2Nix/Util.hs

@@ -13,6 +13,7 @@ import Crypto.Hash.SHA256 (hash)
 import qualified Data.ByteString.Base16 as Base16
 
 import Data.List.NonEmpty (NonEmpty)
+import Data.Fix(Fix(..))
 import Nix.Expr
 
 listDirectories :: FilePath -> IO [FilePath]
@@ -23,6 +24,12 @@ listDirectories p =
 quoted :: (IsString a, Semigroup a) => a -> a
 quoted str = "\"" <> str <> "\""
 
+selectOr :: NExpr -> NAttrPath NExpr -> NExpr -> NExpr
+selectOr obj path alt = Fix (NSelect obj path (Just $ alt))
+
+mkThrow :: NExpr -> NExpr
+mkThrow msg = (mkSym "builtins" @. "throw") @@ msg
+
 sha256 :: String -> String
 sha256 = unpack . Base16.encode . hash . pack