Frozen dependencies are printed on their own lines

A new pretty-print has been introduced `D.ParseUtils.commaNewLineListField`.
It is similar to `D.ParseUtils.commaListField` but has each item on its own
line.

The constraints section of the package environment is pretty printed with
`commaNewLineListField`.

Cabal/Distribution/ParseUtils.hs

@@ -33,7 +33,7 @@ module Distribution.ParseUtils (
         parseSepList, parseCommaList, parseOptCommaList,
         showFilePath, showToken, showTestedWith, showFreeText, parseFreeText,
         field, simpleField, listField, spaceListField, commaListField,
-        optsField, liftField, boolField, parseQuoted,
+        commaNewLineListField, optsField, liftField, boolField, parseQuoted,
 
         UnrecFieldParser, warnUnrec, ignoreUnrec,
   ) where
@@ -188,14 +188,22 @@ simpleField :: String -> (a -> Doc) -> ReadP a a
 simpleField name showF readF get set
   = liftField get set $ field name showF readF
 
-commaListField :: String -> (a -> Doc) -> ReadP [a] a
+commaListField' :: ([Doc] -> Doc) -> String -> (a -> Doc) -> ReadP [a] a
                  -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
-commaListField name showF readF get set =
+commaListField' separator name showF readF get set =
   liftField get set' $
-    field name (fsep . punctuate comma . map showF) (parseCommaList readF)
+    field name (separator . punctuate comma . map showF) (parseCommaList readF)
   where
     set' xs b = set (get b ++ xs) b
 
+commaListField :: String -> (a -> Doc) -> ReadP [a] a
+                 -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
+commaListField = commaListField' fsep
+
+commaNewLineListField :: String -> (a -> Doc) -> ReadP [a] a
+                 -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
+commaNewLineListField = commaListField' sep
+
 spaceListField :: String -> (a -> Doc) -> ReadP [a] a
                  -> (b -> [a]) -> ([a] -> b -> b) -> FieldDescr b
 spaceListField name showF readF get set =

cabal-install/Distribution/Client/Sandbox/PackageEnvironment.hs

@@ -46,7 +46,7 @@ import Distribution.Simple.Setup       ( Flag(..), ConfigFlags(..)
                                        , fromFlagOrDefault, toFlag, flagToMaybe )
 import Distribution.Simple.Utils       ( die, info, notice, warn, lowercase )
 import Distribution.ParseUtils         ( FieldDescr(..), ParseResult(..)
-                                       , commaListField
+                                       , commaListField, commaNewLineListField
                                        , liftField, lineNo, locatedErrorMsg
                                        , parseFilePathQ, readFields
                                        , showPWarning, simpleField
@@ -390,7 +390,7 @@ pkgEnvFieldDescrs = [
     pkgEnvInherit (\v pkgEnv -> pkgEnv { pkgEnvInherit = v })
 
     -- FIXME: Should we make these fields part of ~/.cabal/config ?
-  , commaListField "constraints"
+  , commaNewLineListField "constraints"
     Text.disp Text.parse
     (configExConstraints . savedConfigureExFlags . pkgEnvSavedConfig)
     (\v pkgEnv -> updateConfigureExFlags pkgEnv