Improve parser error handling with Megaparsec best practices

Remove redundant try wrappers:
- symbol is auto-backtracking since Megaparsec 4.4.0
- Removed 10 redundant try wrappers from keyword and delimiter parsing
- Fixed duplicate "constr" entry bug

Add strategic labels for better error messages:
- Use <?> operator for two-level labeling (delimiters + keywords)
- Replace explicit fail with <?> in type parser (more idiomatic)
- Labels provide clearer context in parse errors

Refactor to use between combinator:
- Cleaner, more declarative code structure
- Standard Megaparsec pattern for delimited parsing
- Eliminates intermediate bindings

Expand test coverage:
- Add 6 new error message tests covering common error scenarios
- Extract testParseError helper to reduce duplication
- Organize error tests into dedicated test group
- Verify labels appear correctly in error messages

All 2,973 tests pass (781 UPLC + 1,864 PLC + 328 PlutusIR)

Author: Unisay

plutus-core/plutus-core/src/PlutusCore/Parser/Type.hs

@@ -135,29 +135,29 @@ i.e. parse into @Tree Text@ and do the kind checking afterwards, but given that
 to do the kind checking of builtins regardless (even for UPLC), we don't win much by deferring
 doing it. -}
 defaultUni :: Parser (SomeTypeIn (Kinded DefaultUni))
-defaultUni = do
-  choice $
-    map
-      try
-      [ trailingWhitespace (inParens defaultUniApplication)
-      , someType @_ @Integer <$ symbol "integer"
-      , someType @_ @ByteString <$ symbol "bytestring"
-      , someType @_ @Text <$ symbol "string"
-      , someType @_ @() <$ symbol "unit"
-      , someType @_ @Bool <$ symbol "bool"
-      , someType @_ @[] <$ symbol "list"
-      , someType @_ @Strict.Vector <$ symbol "array"
-      , someType @_ @(,) <$ symbol "pair"
-      , someType @_ @Data <$ symbol "data"
-      , someType @_ @BLS12_381.G1.Element <$ symbol "bls12_381_G1_element"
-      , someType @_ @BLS12_381.G2.Element <$ symbol "bls12_381_G2_element"
-      , someType @_ @BLS12_381.Pairing.MlResult <$ symbol "bls12_381_mlresult"
-      , someType @_ @Value <$ symbol "value"
-      , -- We include an explicit failure case here to produce clearer error messages.
-        -- Without this, using `choice` with `symbol` results in error messages that cover the longest possible SrcSpan,
-        -- which in this context would be 20 characters spanning the entire "bls12_381_G2_element" token.
-        fail "Unknown type, expected one of: bool, integer, bytestring, string, unit, list, array, pair, data, value, bls12_381_G1_element, bls12_381_G2_element, bls12_381_mlresult, or a type application in parens"
-      ]
+defaultUni =
+  ( choice $
+      map
+        try
+        [ trailingWhitespace (inParens defaultUniApplication)
+        , someType @_ @Integer <$ symbol "integer"
+        , someType @_ @ByteString <$ symbol "bytestring"
+        , someType @_ @Text <$ symbol "string"
+        , someType @_ @() <$ symbol "unit"
+        , someType @_ @Bool <$ symbol "bool"
+        , someType @_ @[] <$ symbol "list"
+        , someType @_ @Strict.Vector <$ symbol "array"
+        , someType @_ @(,) <$ symbol "pair"
+        , someType @_ @Data <$ symbol "data"
+        , someType @_ @BLS12_381.G1.Element <$ symbol "bls12_381_G1_element"
+        , someType @_ @BLS12_381.G2.Element <$ symbol "bls12_381_G2_element"
+        , someType @_ @BLS12_381.Pairing.MlResult <$ symbol "bls12_381_mlresult"
+        , someType @_ @Value <$ symbol "value"
+        ]
+  )
+    <?> "type name (integer, bytestring, string, unit, bool, list, array, pair,\
+        \ data, value, bls12_381_G1_element, bls12_381_G2_element,\
+        \ bls12_381_mlresult, or type application)"
 
 tyName :: Parser TyName
 tyName = TyName <$> name

plutus-core/untyped-plutus-core/src/UntypedPlutusCore/Parser.hs

@@ -100,31 +100,31 @@ term =
       ]
   where
     tryAppTerm :: Parser PTerm
-    tryAppTerm = do
-      withSpan $ \sp -> do
-        _ <- try (symbol "[")
-        t <- appTerm sp
-        _ <- char ']'
-        return t
+    tryAppTerm =
+      withSpan $ \sp ->
+        between
+          (symbol "[" <?> "opening bracket '['")
+          (char ']' <?> "closing bracket ']'")
+          (appTerm sp)
 
     tryTermInParens :: Parser PTerm
     tryTermInParens =
-      withSpan $ \sp -> do
-        _ <- try (symbol "(")
-        t <-
-          choice
-            [ try (symbol "builtin") *> builtinTerm sp
-            , try (symbol "lam") *> lamTerm sp
-            , try (symbol "constr") *> constrTerm sp -- "constr" must come before "con"
-            , try (symbol "con") *> conTerm sp
-            , try (symbol "delay") *> delayTerm sp
-            , try (symbol "force") *> forceTerm sp
-            , try (symbol "error") *> errorTerm sp
-            , try (symbol "constr") *> constrTerm sp
-            , try (symbol "case") *> caseTerm sp
-            ]
-        _ <- char ')'
-        return t
+      withSpan $ \sp ->
+        between
+          (symbol "(" <?> "opening parenthesis '('")
+          (char ')' <?> "closing parenthesis ')'")
+          ( choice
+              [ symbol "builtin" *> builtinTerm sp
+              , symbol "lam" *> lamTerm sp
+              , symbol "constr" *> constrTerm sp -- "constr" must come before "con"
+              , symbol "con" *> conTerm sp
+              , symbol "delay" *> delayTerm sp
+              , symbol "force" *> forceTerm sp
+              , symbol "error" *> errorTerm sp
+              , symbol "case" *> caseTerm sp
+              ]
+              <?> "term keyword (builtin, lam, constr, con, delay, force, error, case)"
+          )
 
 -- | Parser for UPLC programs.
 program :: Parser (UPLC.Program PLC.Name PLC.DefaultUni PLC.DefaultFun SrcSpan)

plutus-core/untyped-plutus-core/test/Parser/Golden/bracket-mismatch.golden

@@ -0,0 +1,6 @@
+test:1:18:
+  |
+1 | (program 1.1.0 [(var x))
+  |                  ^^^^^^^
+unexpected "var x))"
+expecting term keyword (builtin, lam, constr, con, delay, force, error, case)

plutus-core/untyped-plutus-core/test/Parser/Golden/invalid-keyword.golden

@@ -0,0 +1,6 @@
+test:1:17:
+  |
+1 | (program 1.1.0 (foo x))
+  |                 ^^^^^^^
+unexpected "foo x))"
+expecting term keyword (builtin, lam, constr, con, delay, force, error, case)

plutus-core/untyped-plutus-core/test/Parser/Golden/list-element-type-mismatch.golden

@@ -0,0 +1,6 @@
+test:3:64:
+  |
+3 |  (force (builtin mkCons)) (con integer 4) (con (list integer) [true]) ]
+  |                                                                ^
+unexpected 't'
+expecting '+', '-', ']', or integer

plutus-core/untyped-plutus-core/test/Parser/Golden/missing-builtin-operand.golden

@@ -0,0 +1,106 @@
+test:1:24:
+  |
+1 | (program 1.1.0 (builtin))
+  |                        ^
+Unknown built-in function '' at test:1:24.
+Parsable functions are  [ addInteger
+                        , andByteString
+                        , appendByteString
+                        , appendString
+                        , bData
+                        , blake2b_224
+                        , blake2b_256
+                        , bls12_381_G1_add
+                        , bls12_381_G1_compress
+                        , bls12_381_G1_equal
+                        , bls12_381_G1_hashToGroup
+                        , bls12_381_G1_multiScalarMul
+                        , bls12_381_G1_neg
+                        , bls12_381_G1_scalarMul
+                        , bls12_381_G1_uncompress
+                        , bls12_381_G2_add
+                        , bls12_381_G2_compress
+                        , bls12_381_G2_equal
+                        , bls12_381_G2_hashToGroup
+                        , bls12_381_G2_multiScalarMul
+                        , bls12_381_G2_neg
+                        , bls12_381_G2_scalarMul
+                        , bls12_381_G2_uncompress
+                        , bls12_381_finalVerify
+                        , bls12_381_millerLoop
+                        , bls12_381_mulMlResult
+                        , byteStringToInteger
+                        , chooseData
+                        , chooseList
+                        , chooseUnit
+                        , complementByteString
+                        , consByteString
+                        , constrData
+                        , countSetBits
+                        , decodeUtf8
+                        , divideInteger
+                        , dropList
+                        , encodeUtf8
+                        , equalsByteString
+                        , equalsData
+                        , equalsInteger
+                        , equalsString
+                        , expModInteger
+                        , findFirstSetBit
+                        , fstPair
+                        , headList
+                        , iData
+                        , ifThenElse
+                        , indexArray
+                        , indexByteString
+                        , insertCoin
+                        , integerToByteString
+                        , keccak_256
+                        , lengthOfArray
+                        , lengthOfByteString
+                        , lessThanByteString
+                        , lessThanEqualsByteString
+                        , lessThanEqualsInteger
+                        , lessThanInteger
+                        , listData
+                        , listToArray
+                        , lookupCoin
+                        , mapData
+                        , mkCons
+                        , mkNilData
+                        , mkNilPairData
+                        , mkPairData
+                        , modInteger
+                        , multiplyInteger
+                        , nullList
+                        , orByteString
+                        , quotientInteger
+                        , readBit
+                        , remainderInteger
+                        , replicateByte
+                        , ripemd_160
+                        , rotateByteString
+                        , scaleValue
+                        , serialiseData
+                        , sha2_256
+                        , sha3_256
+                        , shiftByteString
+                        , sliceByteString
+                        , sndPair
+                        , subtractInteger
+                        , tailList
+                        , trace
+                        , unBData
+                        , unConstrData
+                        , unIData
+                        , unListData
+                        , unMapData
+                        , unValueData
+                        , unionValue
+                        , valueContains
+                        , valueData
+                        , verifyEcdsaSecp256k1Signature
+                        , verifyEd25519Signature
+                        , verifySchnorrSecp256k1Signature
+                        , writeBits
+                        , xorByteString ]

plutus-core/untyped-plutus-core/test/Parser/Golden/missing-closing-bracket.golden

@@ -0,0 +1,6 @@
+test:1:69:
+  |
+1 | (program 1.1.0 [(builtin addInteger) (con integer 1) (con integer 2))
+  |                                                                     ^
+unexpected ')'
+expecting '`', closing bracket ']', opening bracket '[', or opening parenthesis '('

plutus-core/untyped-plutus-core/test/Parser/Golden/missing-closing-paren.golden

@@ -0,0 +1,6 @@
+test:1:24:
+  |
+1 | (program 1.1.0 (lam x (var x))
+  |                        ^^^^^^^
+unexpected "var x))"
+expecting term keyword (builtin, lam, constr, con, delay, force, error, case)

plutus-core/untyped-plutus-core/test/Parser/Golden/missing-con-operands.golden

@@ -0,0 +1,6 @@
+test:1:20:
+  |
+1 | (program 1.1.0 (con))
+  |                    ^^
+unexpected "))"
+expecting type name (integer, bytestring, string, unit, bool, list, array, pair, data, value, bls12_381_G1_element, bls12_381_G2_element, bls12_381_mlresult, or type application)

plutus-core/untyped-plutus-core/test/Parser/Golden/type-name-typo.golden

@@ -0,0 +1,6 @@
+test:2:38:
+  |
+2 | [ (builtin integerToByteString) (con boot True) (con integer 0) (con integer 712372356934756347862573452345342345) ]
+  |                                      ^^^^^^^^^^^^^^^^^^^^
+unexpected "boot True) (con inte"
+expecting type name (integer, bytestring, string, unit, bool, list, array, pair, data, value, bls12_381_G1_element, bls12_381_G2_element, bls12_381_mlresult, or type application)