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LineRange and StringDiff invariants static checks #34

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9a81d5f
`StringDiff` and `LineRange` invariants
ninioArtillero Apr 22, 2026
d5e616d
Remove unneeded case split
ninioArtillero Jul 22, 2026
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113 changes: 86 additions & 27 deletions src/Data/Algorithm/DiffOutput.hs
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Original file line number Diff line number Diff line change
@@ -1,3 +1,14 @@
-- With PLE (Proof by Logical Evaluation) reflected predicates like
-- '_coherentDiff' are automatically unfolded when verifying refinement types.
-- Without it, LiquidHaskell cannot prove obligations like @_coherentDiff (Both x x) = True@
-- because the reflected function's definition is not instantiated in the SMT context.
-- For example, the following check would fail:
--
-- {-@ good :: StringDiff @-}
-- good = Both ["hello"] ["hello"]
--
-- without PLE.
{-@ LIQUID "--ple" @-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Algorithm.DiffOutput
Expand All @@ -16,51 +27,76 @@ import Text.PrettyPrint hiding ((<>))
import Data.Char
import Data.List

-- Non-empty lists as refinements of regular lists.
{-@ type NonEmpty a = {xs : [a] | len xs >0}@-}

{-@ reflect _coherentDiff @-}
-- | This functions checks that both contents match whenever we have a 'Both' value.
-- Used in LiquidHaskell logic as a predicate.
_coherentDiff :: Eq c => Diff c -> Bool
_coherentDiff (Both x y) = x == y
_coherentDiff (First _) = True
_coherentDiff (Second _) = True

-- This refinement type synonym encodes the invariants on the 'Diff' type
-- that we expect throughout this module. Namely:
--
-- * They have non-empty contents
-- * 'Both' values have equal arguments
{-@ type StringDiff = {d : Diff (NonEmpty String) | _coherentDiff d} @-}

-- | Converts 'Diff's to 'DiffOperation's. 'First' and 'Second'
-- ocurrances are converted to 'Addition' and 'Deletion', respectively, while
-- consecutive ocurrances of them are replaced by a 'Change'.
{-@ diffToLineRanges :: [StringDiff] -> [DiffOperation LineRange] @-}
diffToLineRanges :: [Diff [String]] -> [DiffOperation LineRange]
diffToLineRanges = toLineRange 1 1
where
-- | In @toLineRange x y ds@, @x@ is the index of the current string in the
-- left input of the diff @ds@, and @y@ is the index of the corresponding
-- string in the right input of the diff @ds@.
{-@ toLineRange :: Int -> Int -> diffs : [Diff[String]] -> [DiffOperation LineRange] / [len diffs, 0] @-}
{-@ toLineRange :: {l : Int | l >= 1}
-> {r : Int | r >= 1}
-> diffs : [StringDiff]
-> [DiffOperation LineRange] / [len diffs, 0] @-}
toLineRange :: Int -> Int -> [Diff [String]] -> [DiffOperation LineRange]
toLineRange _ _ []=[]
-- If the lines are the same, we just move forward.
toLineRange leftLine rightLine (Both ls _:rs)=
let lins=length ls
in toLineRange (leftLine+lins) (rightLine+lins) rs
-- A 'Change' is introduced when an addition is followed by a deletion, or vice versa.
toLineRange leftLine rightLine (Second lsS:First lsF:rs)=
toLineRange leftLine rightLine (Second lsS : First lsF : rs) =
toChange leftLine rightLine lsF lsS rs
toLineRange leftLine rightLine (First lsF:Second lsS:rs)=
toLineRange leftLine rightLine (First lsF : Second lsS : rs) =
toChange leftLine rightLine lsF lsS rs
-- Introduce 'Addition's.
toLineRange leftLine rightLine (Second lsS:rs)=
let linesS=length lsS
diff=Addition (LineRange (rightLine,rightLine+linesS-1) lsS) (leftLine-1)
in diff : toLineRange leftLine (rightLine+linesS) rs
toLineRange leftLine rightLine (Second lsS : rs) =
let diff = Addition (mkLineRange rightLine lsS) (leftLine-1)
in diff : toLineRange leftLine (rightLine + length lsS) rs
-- Introduce 'Deletion's.
toLineRange leftLine rightLine (First lsF:rs)=
let linesF=length lsF
diff=Deletion (LineRange (leftLine,leftLine+linesF-1) lsF) (rightLine-1)
in diff: toLineRange(leftLine+linesF) rightLine rs
toLineRange leftLine rightLine (First lsF : rs)=
let diff = Deletion (mkLineRange leftLine lsF) (rightLine-1)
in diff : toLineRange (leftLine + length lsF) rightLine rs
-- | Build 'Change's from adjacent additions and deletions.
{-@ toChange :: Int -> Int -> [String] -> [String] -> diffs : [Diff [String]] -> [DiffOperation LineRange] / [len diffs, 1] @-}
{-@ toChange :: {l : Int | l >= 1}
-> {r : Int | r >= 1}
-> {lf : [String] | len lf > 0}
-> {ls : [String] | len ls > 0}
-> diffs : [StringDiff]
-> [DiffOperation LineRange]
/ [len diffs, 1] @-}
toChange :: Int -- ^ Current left line number.
-> Int -- ^ Current right line number.
-> [String] -- ^ Lines from the 'First' list (corresponding to deletions).
-> [String] -- ^ Lines from the 'Second' list (corresponding to additions).
-> [Diff [String]] -- ^ Remaining 'Diff's.
-> [DiffOperation LineRange]
toChange leftLine rightLine lsF lsS rs=
let linesS=length lsS
linesF=length lsF
in Change (LineRange (leftLine,leftLine+linesF-1) lsF) (LineRange (rightLine,rightLine+linesS-1) lsS)
: toLineRange (leftLine+linesF) (rightLine+linesS) rs
Change (mkLineRange leftLine lsF) (mkLineRange rightLine lsS)
: toLineRange (leftLine + length lsF) (rightLine + length lsS) rs

{-@ ppDiff :: [StringDiff] -> String @-}
-- | Pretty print the differences. The output is similar to the output of the @diff@ utility.
--
-- > > putStr (ppDiff (getGroupedDiff ["a","b","c","d","e"] ["a","c","d","f"]))
Expand Down Expand Up @@ -114,6 +150,7 @@ parsePrettyDiffs = reverse . doParse [] . lines
Just nd -> doParse (nd:acc) r
_ -> doParse acc r

{-@ parseDiff :: s:{[String] | len s > 0} -> {v:(Maybe (DiffOperation LineRange), [String]) | len (snd v) < len s} @-}
parseDiff :: [String] -> (Maybe (DiffOperation LineRange), [String])
parseDiff [] = (Nothing,[])
parseDiff (h:rs) = let
Expand All @@ -128,46 +165,56 @@ parsePrettyDiffs = reverse . doParse [] . lines
('c':hrs2) -> parseChange r1 hrs2 rs
_ -> (Nothing,rs)

{-@ parseDel :: (Nat, Nat) -> String -> rs:[String] -> {v:(Maybe (DiffOperation LineRange), [String]) | len (snd v) <= len rs} @-}
parseDel :: (LineNo, LineNo) -> String -> [String] -> (Maybe (DiffOperation LineRange), [String])
parseDel r1 hrs2 rs = let
-- NOTE: the wildcard should correspond to the end of line,
-- but is ignored for simplicity.
(r2,_) = parseRange hrs2
(ls,rs2) = span (isPrefixOf "<") rs
in (Just $ Deletion (LineRange r1 (map (drop 2) ls)) (fst r2), rs2)
contents = map (drop 2) ls
in case contents of
(_:_) -> (Just $ Deletion (mkLineRange (fst r1) contents) (fst r2), rs2)
_ -> (Nothing, rs2)
Comment on lines +174 to +176

@facundominguez facundominguez Jul 22, 2026 •
edited
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This is changing behavior of parseDel.

Before parseDel (a, b) "1,2" [] == (Just (Deletion (LineRange (a, a - 1) []) 1), []).

After parseDel (a, b) "1,2" [] == (Nothing, "").

The other parsing functions are modified in a similar way.


{-@ parseAdd :: (Nat, Nat) -> String -> rs:[String] -> {v:(Maybe (DiffOperation LineRange), [String]) | len (snd v) <= len rs} @-}
parseAdd :: (LineNo, LineNo) -> String -> [String] -> (Maybe (DiffOperation LineRange), [String])
parseAdd r1 hrs2 rs = let
-- NOTE: the wildcard should correspond to the end of line,
-- but is ignored for simplicity.
(r2,_) = parseRange hrs2
(ls,rs2) = span (isPrefixOf ">") rs
in (Just $ Addition (LineRange r2 (map (drop 2) ls)) (fst r1), rs2)
contents = map (drop 2) ls
in case contents of
(_:_) -> (Just $ Addition (mkLineRange (fst r2) contents) (fst r1), rs2)
_ -> (Nothing, rs2)

{-@ parseChange :: (Nat, Nat) -> String -> rs:[String] -> {v:(Maybe (DiffOperation LineRange), [String]) | len (snd v) <= len rs} @-}
parseChange :: (LineNo, LineNo) -> String -> [String] -> (Maybe (DiffOperation LineRange), [String])
parseChange r1 hrs2 rs = let
-- NOTE: the wildcard should correspond to the end of line,
-- but is ignored for simplicity.
(r2,_) = parseRange hrs2
(ls1,rs2) = span (isPrefixOf "<") rs
in case rs2 of
-- The left and right diff of a 'Change' are separated by a "---" line.
("---":rs3) -> let
(ls2,rs4) = span (isPrefixOf ">") rs3
in (Just $ Change (LineRange r1 (map (drop 2) ls1)) (LineRange r2 (map (drop 2) ls2)), rs4)
contents1 = map (drop 2) ls1
contents2 = map (drop 2) ls2
in case (contents1, contents2) of
(_:_, _:_) -> (Just $ Change (mkLineRange (fst r1) contents1) (mkLineRange (fst r2) contents2), rs4)
_ -> (Nothing, rs4)
_ -> (Nothing,rs2)

parseRange :: String -> ((LineNo, LineNo),String)
{-@ parseRange :: String -> {v : ((Nat, Nat), String) | fst (fst v) <= snd (fst v)} @-}
parseRange :: String -> ((LineNo, LineNo), String)
parseRange l = let
(fstLine,rs) = span isDigit l
a = max 0 (read fstLine)
(sndLine,rs3) = case rs of
-- The comma is used to separate
-- the start and end line numbers in a range,
-- but is omitted if they are the same.
-- i.e. the range is a single line.
(',':rs2) -> span isDigit rs2
_ -> (fstLine,rs)
in ((read fstLine,read sndLine),rs3)
b = max a (read sndLine)
in ((a, b), rs3)

@facundominguez facundominguez Jul 22, 2026

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Looks like this is changing behavior of parseRange.

Before parseRange "3,2abc" == ((3, 2), "abc").

After parseRange "3,2abc" == ((3, 3), "abc").


-- | Line number alias. Always non-negative.
type LineNo = Int
Expand All @@ -180,11 +227,23 @@ type LineNo = Int
-- > snd lrNumbers - fst lrNumbers + 1 == length lrContents
--
-- which imply @lrContents@ cannot be empty.
{-@
data LineRange = LineRange { lrNumbers :: {range : (Nat, Nat) | fst range <= snd range}
, lrContents :: {contents : [String] | snd lrNumbers - fst lrNumbers = len contents - 1}
}
@-}
data LineRange = LineRange { lrNumbers :: (LineNo, LineNo)
, lrContents :: [String]
}
deriving (Show, Read, Eq, Ord)

-- | Smart constructor for 'LineRange' that computes the end line from the
-- start line and the content length, guaranteeing that its content length and
-- range match.
{-@ mkLineRange :: start:Nat -> contents:{[String] | len contents > 0} -> LineRange @-}
mkLineRange :: Int -> [String] -> LineRange
mkLineRange start contents = LineRange (start, start + length contents - 1) contents

-- | Diff operation representing changes to apply.
data DiffOperation a
= Deletion a LineNo -- ^ Element deleted on the left input, line number
Expand Down
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