Collect reports in a difference list

Using a difference list we get O(1) appending vs. O(n) with the built-in
list type. Overall for n reports this results in O(n) operations instead
of O(n^2) operations.

diagnose.cabal

@@ -1,6 +1,6 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.34.6.
+-- This file has been generated from package.yaml by hpack version 0.34.4.
 --
 -- see: https://github.com/sol/hpack
 
@@ -67,6 +67,7 @@ library
   build-depends:
       base >=4.7 && <5
     , data-default >=0.7 && <1
+    , dlist ==1.0.*
     , hashable >=1.3 && <2
     , prettyprinter >=1.7.0 && <2
     , prettyprinter-ansi-terminal >=1.1.0 && <2
@@ -111,6 +112,7 @@ test-suite diagnose-megaparsec-tests
       base >=4.7 && <5
     , data-default >=0.7 && <1
     , diagnose
+    , dlist ==1.0.*
     , hashable >=1.3 && <2
     , prettyprinter >=1.7.0 && <2
     , prettyprinter-ansi-terminal >=1.1.0 && <2
@@ -150,6 +152,7 @@ test-suite diagnose-parsec-tests
       base >=4.7 && <5
     , data-default >=0.7 && <1
     , diagnose
+    , dlist ==1.0.*
     , hashable >=1.3 && <2
     , prettyprinter >=1.7.0 && <2
     , prettyprinter-ansi-terminal >=1.1.0 && <2
@@ -188,6 +191,7 @@ test-suite diagnose-rendering-tests
       base >=4.7 && <5
     , data-default >=0.7 && <1
     , diagnose
+    , dlist ==1.0.*
     , hashable >=1.3 && <2
     , prettyprinter >=1.7.0 && <2
     , prettyprinter-ansi-terminal >=1.1.0 && <2

package.yaml

@@ -8,11 +8,12 @@ category: "Error Reporting"
 
 dependencies:
 - base >= 4.7 && < 5
+- data-default >= 0.7 && < 1
+- dlist >= 1.0 && < 1.1
+- hashable >= 1.3 && < 2
 - prettyprinter >= 1.7.0 && < 2
 - prettyprinter-ansi-terminal >= 1.1.0 && < 2
 - unordered-containers >= 0.2 && < 0.3
-- hashable >= 1.3 && < 2
-- data-default >= 0.7 && < 1
 - wcwidth >= 0.0.1 && <1
 - text >= 1.0.0.0 && <= 2.0
 # ^^^ This is unfortunately required, but as 'prettyprinter' already depends on it, it will already have been fetched

src/Error/Diagnose/Diagnostic/Internal.hs

@@ -20,8 +20,11 @@ import Control.Monad.IO.Class (MonadIO, liftIO)
 import Data.Aeson (ToJSON(..), encode, object, (.=))
 import Data.ByteString.Lazy (ByteString)
 #endif
+
+import Data.DList (DList)
+import qualified Data.DList as DL
 import Data.Default (Default, def)
-import Data.Foldable (fold)
+import Data.Foldable (fold, toList)
 import Data.HashMap.Lazy (HashMap)
 import qualified Data.HashMap.Lazy as HashMap
 import Data.List (intersperse)
@@ -38,7 +41,7 @@ import System.IO (Handle)
 --   to create a new empty diagnostic, and 'addFile' and 'addReport' to alter its internal state.
 data Diagnostic msg
   = Diagnostic
-      [Report msg]
+      (DList (Report msg))
       -- ^ All the reports contained in a diagnostic.
       --
       --   Reports are output one by one, without connections in between.
@@ -85,7 +88,7 @@ prettyDiagnostic ::
   Diagnostic msg ->
   Doc Annotation
 prettyDiagnostic withUnicode tabSize (Diagnostic reports file) =
-  fold . intersperse hardline $ prettyReport file withUnicode tabSize <$> reports
+  fold . intersperse hardline $ prettyReport file withUnicode tabSize <$> toList reports
 {-# INLINE prettyDiagnostic #-}
 
 -- | Prints a 'Diagnostic' onto a specific 'Handle'.
@@ -127,7 +130,7 @@ addReport ::
   Report msg ->
   Diagnostic msg
 addReport (Diagnostic reports files) report =
-  Diagnostic (reports <> [report]) files
+  Diagnostic (reports `DL.snoc` report) files
 {-# INLINE addReport #-}
 
 #ifdef USE_AESON