Allow _.Property / _.MethodCall() / _.IndexerAccess[idx] shorthand for accessor functions

src/Compiler/Checking/CheckExpressions.fs

@@ -5524,7 +5524,15 @@ and TcExprUndelayed (cenv: cenv) (overallTy: OverallTy) env tpenv (synExpr: SynE
         TcNonControlFlowExpr env <| fun env ->
         CallExprHasTypeSink cenv.tcSink (m, env.NameEnv, overallTy.Commit, env.AccessRights)
         TcConstExpr cenv overallTy env m tpenv synConst
-
+    | SynExpr.DotLambda (synExpr, m, trivia) ->
+        if env.NameEnv.eUnqualifiedItems |> Map.containsKey "_arg1"
+        then
+            warning(Error(FSComp.SR.tcAmbiguousDiscardDotLambda(), trivia.UnderscoreRange))
+        let unaryArg = mkSynId trivia.UnderscoreRange (cenv.synArgNameGenerator.New())
+        let svar = mkSynCompGenSimplePatVar unaryArg
+        let pushedExpr = pushUnaryArg synExpr unaryArg
+        let lambda = SynExpr.Lambda(false, false, SynSimplePats.SimplePats([ svar ],[], svar.Range), pushedExpr, None, m, SynExprLambdaTrivia.Zero)
+        TcIteratedLambdas cenv true env overallTy Set.empty tpenv lambda
     | SynExpr.Lambda _ ->
         TcIteratedLambdas cenv true env overallTy Set.empty tpenv synExpr
 
@@ -8721,6 +8729,7 @@ and TcImplicitOpItemThen (cenv: cenv) overallTy env id sln tpenv mItem delayed =
         | SynExpr.Const _
         | SynExpr.Typar _
         | SynExpr.LongIdent _
+        | SynExpr.DotLambda _
         | SynExpr.Dynamic _ -> true
 
         | SynExpr.Tuple (_, synExprs, _, _)

src/Compiler/Checking/MethodCalls.fs

@@ -841,6 +841,7 @@ let InferLambdaArgsForLambdaPropagation origRhsExpr =
         match e with 
         | SynExpr.Lambda (body = rest) -> 1 + loop rest
         | SynExpr.MatchLambda _ -> 1
+        | SynExpr.DotLambda _ -> 1
         | _ -> 0
     loop origRhsExpr
 

src/Compiler/Driver/GraphChecking/FileContentMapping.fs

@@ -362,6 +362,7 @@ let visitSynExpr (e: SynExpr) : FileContentEntry list =
         | SynExpr.IndexFromEnd (expr, _) -> visit expr continuation
         | SynExpr.ComputationExpr (expr = expr) -> visit expr continuation
         | SynExpr.Lambda (args = args; body = body) -> visit body (fun bodyNodes -> visitSynSimplePats args @ bodyNodes |> continuation)
+        | SynExpr.DotLambda (expr = expr) -> visit expr continuation
         | SynExpr.MatchLambda (matchClauses = clauses) -> List.collect visitSynMatchClause clauses |> continuation
         | SynExpr.Match (expr = expr; clauses = clauses) ->
             visit expr (fun exprNodes ->

src/Compiler/FSComp.txt

@@ -1701,6 +1701,9 @@ featureStaticLetInRecordsDusEmptyTypes,"Allow static let bindings in union, reco
 3566,tcMultipleRecdTypeChoice,"Multiple type matches were found:\n%s\nThe type '%s' was used. Due to the overlapping field names\n%s\nconsider using type annotations or change the order of open statements."
 3567,parsMissingMemberBody,"Expecting member body"
 3568,parsMissingKeyword,"Missing keyword '%s'"
+3570,tcAmbiguousDiscardDotLambda,"The meaning of _ is ambiguous here. It cannot be used for a discarded variable and a function shorthand in the same scope."
+3571,parsUnderScoreDotLambdaNonAtomic," _. shorthand syntax for lambda functions can only be used with atomic expressions. That means expressions with no whitespace unless enclosed in parentheses."
+featureAccessorFunctionShorthand,"underscore dot shorthand for accessor only function"
 3569,chkNotTailRecursive,"The member or function '%s' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way."
 3570,tcStaticBindingInExtrinsicAugmentation,"Static bindings cannot be added to extrinsic augmentations. Consider using a 'static member' instead."
 3571,pickleFsharpCoreBackwardsCompatible,"Newly added pickle state cannot be used in FSharp.Core, since it must be working in older compilers+tooling as well. The time window is at least 3 years after feature introduction. Violation: %s . Context: \n %s "

src/Compiler/Facilities/LanguageFeatures.fs

@@ -55,6 +55,7 @@ type LanguageFeature =
     | LowercaseDUWhenRequireQualifiedAccess
     | InterfacesWithAbstractStaticMembers
     | SelfTypeConstraints
+    | AccessorFunctionShorthand
     | MatchNotAllowedForUnionCaseWithNoData
     | CSharpExtensionAttributeNotRequired
     | ErrorForNonVirtualMembersOverrides
@@ -150,6 +151,7 @@ type LanguageVersion(versionText) =
 
                 // F# preview
                 LanguageFeature.FromEndSlicing, previewVersion
+                LanguageFeature.AccessorFunctionShorthand, previewVersion
                 LanguageFeature.MatchNotAllowedForUnionCaseWithNoData, previewVersion
                 LanguageFeature.CSharpExtensionAttributeNotRequired, previewVersion
                 LanguageFeature.ErrorForNonVirtualMembersOverrides, previewVersion
@@ -275,6 +277,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.LowercaseDUWhenRequireQualifiedAccess -> FSComp.SR.featureLowercaseDUWhenRequireQualifiedAccess ()
         | LanguageFeature.InterfacesWithAbstractStaticMembers -> FSComp.SR.featureInterfacesWithAbstractStaticMembers ()
         | LanguageFeature.SelfTypeConstraints -> FSComp.SR.featureSelfTypeConstraints ()
+        | LanguageFeature.AccessorFunctionShorthand -> FSComp.SR.featureAccessorFunctionShorthand ()
         | LanguageFeature.MatchNotAllowedForUnionCaseWithNoData -> FSComp.SR.featureMatchNotAllowedForUnionCaseWithNoData ()
         | LanguageFeature.CSharpExtensionAttributeNotRequired -> FSComp.SR.featureCSharpExtensionAttributeNotRequired ()
         | LanguageFeature.ErrorForNonVirtualMembersOverrides -> FSComp.SR.featureErrorForNonVirtualMembersOverrides ()

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -45,6 +45,7 @@ type LanguageFeature =
     | LowercaseDUWhenRequireQualifiedAccess
     | InterfacesWithAbstractStaticMembers
     | SelfTypeConstraints
+    | AccessorFunctionShorthand
     | MatchNotAllowedForUnionCaseWithNoData
     | CSharpExtensionAttributeNotRequired
     | ErrorForNonVirtualMembersOverrides

src/Compiler/Service/FSharpParseFileResults.fs

@@ -292,6 +292,8 @@ type FSharpParseFileResults(diagnostics: FSharpDiagnostic[], input: ParsedInput,
             // Capture the body of a lambda, often nested in a call to a collection function
             | SynExpr.Lambda (body = body) when rangeContainsPos body.Range pos -> getIdentRangeForFuncExprInApp traverseSynExpr body pos
 
+            | SynExpr.DotLambda (expr = body) when rangeContainsPos body.Range pos -> getIdentRangeForFuncExprInApp traverseSynExpr body pos
+
             | SynExpr.Do (expr, range) when rangeContainsPos range pos -> getIdentRangeForFuncExprInApp traverseSynExpr expr pos
 
             | SynExpr.Assert (expr, range) when rangeContainsPos range pos -> getIdentRangeForFuncExprInApp traverseSynExpr expr pos
@@ -425,6 +427,7 @@ type FSharpParseFileResults(diagnostics: FSharpDiagnostic[], input: ParsedInput,
                 override _.VisitBinding(_path, defaultTraverse, binding) =
                     match binding with
                     | SynBinding(expr = SynExpr.Lambda _) when skipLambdas -> defaultTraverse binding
+                    | SynBinding(expr = SynExpr.DotLambda _) when skipLambdas -> defaultTraverse binding
 
                     // Skip manually type-annotated bindings
                     | SynBinding(returnInfo = Some (SynBindingReturnInfo _)) -> defaultTraverse binding
@@ -533,6 +536,7 @@ type FSharpParseFileResults(diagnostics: FSharpDiagnostic[], input: ParsedInput,
                     | SynBinding.SynBinding (expr = expr; range = range) when Position.posEq range.Start pos ->
                         match expr with
                         | SynExpr.Lambda _ -> Some range
+                        | SynExpr.DotLambda _ -> Some range
                         | _ -> None
                     | _ -> defaultTraverse binding
             }
@@ -667,6 +671,7 @@ type FSharpParseFileResults(diagnostics: FSharpDiagnostic[], input: ParsedInput,
                         match expr with
                         | SynExpr.ArbitraryAfterError _
                         | SynExpr.LongIdent _
+                        | SynExpr.DotLambda _
                         | SynExpr.LibraryOnlyILAssembly _
                         | SynExpr.LibraryOnlyStaticOptimization _
                         | SynExpr.Null _
@@ -828,6 +833,8 @@ type FSharpParseFileResults(diagnostics: FSharpDiagnostic[], input: ParsedInput,
 
                         | SynExpr.Lambda (body = bodyExpr) -> yield! walkExpr true bodyExpr
 
+                        | SynExpr.DotLambda (expr = bodyExpr) -> yield! walkExpr true bodyExpr
+
                         | SynExpr.Match (matchDebugPoint = spBind; expr = inpExpr; clauses = cl) ->
                             yield! walkBindSeqPt spBind
                             yield! walkExpr false inpExpr

src/Compiler/Service/ServiceInterfaceStubGenerator.fs

@@ -880,6 +880,8 @@ module InterfaceStubGenerator =
 
                 | SynExpr.Lambda (body = synExpr) -> walkExpr synExpr
 
+                | SynExpr.DotLambda (expr = synExpr) -> walkExpr synExpr
+
                 | SynExpr.MatchLambda (_isExnMatch, _argm, synMatchClauseList, _spBind, _wholem) ->
                     synMatchClauseList
                     |> List.tryPick (fun (SynMatchClause (resultExpr = e)) -> walkExpr e)

src/Compiler/Service/ServiceParseTreeWalk.fs

@@ -561,6 +561,8 @@ module SyntaxTraversal =
                         | None -> traverseSynExpr synExpr
                         | x -> x
 
+                | SynExpr.DotLambda (expr = e) -> traverseSynExpr e
+
                 | SynExpr.MatchLambda (_isExnMatch, _argm, synMatchClauseList, _spBind, _wholem) ->
                     synMatchClauseList
                     |> List.map (fun x -> dive x x.Range (traverseSynMatchClause path))

src/Compiler/Service/ServiceParsedInputOps.fs

@@ -752,6 +752,8 @@ module ParsedInput =
 
             | SynExpr.Lambda (body = e) -> walkExprWithKind parentKind e
 
+            | SynExpr.DotLambda (expr = e) -> walkExprWithKind parentKind e
+
             | SynExpr.MatchLambda (_, _, synMatchClauseList, _, _) -> List.tryPick walkClause synMatchClauseList
 
             | SynExpr.Match (expr = e; clauses = synMatchClauseList) ->
@@ -1739,6 +1741,7 @@ module ParsedInput =
             | SynExpr.Lambda (args = pats; body = e) ->
                 walkSimplePats pats
                 walkExpr e
+            | SynExpr.DotLambda (expr = e) -> walkExpr e
             | SynExpr.New (_, t, e, _)
             | SynExpr.TypeTest (e, t, _)
             | SynExpr.Upcast (e, t, _)

src/Compiler/SyntaxTree/SyntaxTree.fs

@@ -618,6 +618,8 @@ type SynExpr =
 
     | DotGet of expr: SynExpr * rangeOfDot: range * longDotId: SynLongIdent * range: range
 
+    | DotLambda of expr: SynExpr * range: range * trivia: SynExprDotLambdaTrivia
+
     | DotSet of targetExpr: SynExpr * longDotId: SynLongIdent * rhsExpr: SynExpr * range: range
 
     | Set of targetExpr: SynExpr * rhsExpr: SynExpr * range: range
@@ -755,6 +757,7 @@ type SynExpr =
         | SynExpr.DotIndexedGet (range = m)
         | SynExpr.DotIndexedSet (range = m)
         | SynExpr.DotGet (range = m)
+        | SynExpr.DotLambda (range = m)
         | SynExpr.DotSet (range = m)
         | SynExpr.Set (range = m)
         | SynExpr.DotNamedIndexedPropertySet (range = m)

src/Compiler/SyntaxTree/SyntaxTree.fsi

@@ -770,6 +770,9 @@ type SynExpr =
     /// F# syntax: expr.ident.ident
     | DotGet of expr: SynExpr * rangeOfDot: range * longDotId: SynLongIdent * range: range
 
+    /// F# syntax: _.ident.ident
+    | DotLambda of expr: SynExpr * range: range * trivia: SynExprDotLambdaTrivia
+
     /// F# syntax: expr.ident...ident <- expr
     | DotSet of targetExpr: SynExpr * longDotId: SynLongIdent * rhsExpr: SynExpr * range: range
 

src/Compiler/SyntaxTree/SyntaxTreeOps.fs

@@ -63,6 +63,42 @@ let mkSynSimplePatVar isOpt id =
 let mkSynCompGenSimplePatVar id =
     SynSimplePat.Id(id, None, true, false, false, id.idRange)
 
+let rec pushUnaryArg expr arg =
+    match expr with
+    | SynExpr.App (ExprAtomicFlag.Atomic, infix, SynExpr.Ident ident, x1, m1) ->
+        SynExpr.App(
+            ExprAtomicFlag.Atomic,
+            infix,
+            SynExpr.LongIdent(false, SynLongIdent(arg :: ident :: [], [ ident.idRange ], [ None ]), None, ident.idRange),
+            x1,
+            m1
+        )
+    | SynExpr.App (ExprAtomicFlag.Atomic,
+                   infix,
+                   SynExpr.LongIdent (isOptional, SynLongIdent (id, dotRanges, trivia), altNameRefCell, range),
+                   x1,
+                   m1) ->
+        SynExpr.App(
+            ExprAtomicFlag.Atomic,
+            infix,
+            SynExpr.LongIdent(isOptional, SynLongIdent(arg :: id, dotRanges, trivia), altNameRefCell, range),
+            x1,
+            m1
+        )
+    | SynExpr.App (ExprAtomicFlag.Atomic, infix, (SynExpr.App (_) as innerApp), x1, m1) ->
+        SynExpr.App(ExprAtomicFlag.Atomic, infix, (pushUnaryArg innerApp arg), x1, m1)
+    | SynExpr.App (ExprAtomicFlag.Atomic, infix, SynExpr.DotGet (synExpr, rangeOfDot, synLongIdent, range), x1, m1) ->
+        SynExpr.App(ExprAtomicFlag.Atomic, infix, SynExpr.DotGet((pushUnaryArg synExpr arg), rangeOfDot, synLongIdent, range), x1, m1)
+    | SynExpr.App (ExprAtomicFlag.Atomic, infix, innerExpr, x1, m1) ->
+        SynExpr.App(ExprAtomicFlag.Atomic, infix, pushUnaryArg innerExpr arg, x1, m1)
+    | SynExpr.Ident ident -> SynExpr.LongIdent(false, SynLongIdent(arg :: ident :: [], [ ident.idRange ], [ None ]), None, ident.idRange)
+    | SynExpr.LongIdent (isOptional, SynLongIdent (id, dotRanges, trivia), altNameRefCell, range) ->
+        SynExpr.LongIdent(isOptional, SynLongIdent(arg :: id, dotRanges, trivia), altNameRefCell, range)
+    | SynExpr.DotGet (synExpr, rangeOfDot, synLongIdent, range) -> SynExpr.DotGet(pushUnaryArg synExpr arg, rangeOfDot, synLongIdent, range)
+    | SynExpr.DotIndexedGet (objectExpr, indexArgs, dotRange, range) ->
+        SynExpr.DotIndexedGet(pushUnaryArg objectExpr arg, indexArgs, dotRange, range)
+    | _ -> expr
+
 let (|SynSingleIdent|_|) x =
     match x with
     | SynLongIdent ([ id ], _, _) -> Some id
@@ -792,6 +828,7 @@ let rec synExprContainsError inpExpr =
         | SynExpr.ArbitraryAfterError _ -> true
 
         | SynExpr.LongIdent _
+        | SynExpr.DotLambda _
         | SynExpr.Quote _
         | SynExpr.LibraryOnlyILAssembly _
         | SynExpr.LibraryOnlyStaticOptimization _

src/Compiler/SyntaxTree/SyntaxTreeOps.fsi

@@ -41,6 +41,8 @@ val mkSynSimplePatVar: isOpt: bool -> id: Ident -> SynSimplePat
 
 val mkSynCompGenSimplePatVar: id: Ident -> SynSimplePat
 
+val pushUnaryArg: expr: SynExpr -> arg: Ident -> SynExpr
+
 /// Match a long identifier, including the case for single identifiers which gets a more optimized node in the syntax tree.
 val (|LongOrSingleIdent|_|):
     inp: SynExpr -> (bool * SynLongIdent * SynSimplePatAlternativeIdInfo ref option * range) option

src/Compiler/SyntaxTree/SyntaxTrivia.fs

@@ -75,6 +75,13 @@ type SynExprLambdaTrivia =
 
     static member Zero: SynExprLambdaTrivia = { ArrowRange = None }
 
+[<NoEquality; NoComparison>]
+type SynExprDotLambdaTrivia =
+    {
+        UnderscoreRange: range
+        DotRange: range
+    }
+
 [<NoEquality; NoComparison>]
 type SynExprLetOrUseTrivia = { InKeyword: range option }
 

src/Compiler/SyntaxTree/SyntaxTrivia.fsi

@@ -119,6 +119,12 @@ type SynExprLambdaTrivia =
 
     static member Zero: SynExprLambdaTrivia
 
+/// Represents additional information for SynExpr.DotLambda
+[<NoEquality; NoComparison>]
+type SynExprDotLambdaTrivia =
+    { UnderscoreRange: range
+      DotRange: range }
+
 /// Represents additional information for SynExpr.LetOrUse
 [<NoEquality; NoComparison>]
 type SynExprLetOrUseTrivia =

src/Compiler/pars.fsy

@@ -365,6 +365,7 @@ let parse_error_rich = Some(fun (ctxt: ParseErrorContext<_>) ->
 %left pat_app
 %left pat_args
 %left PREFIX_OP
+%right dot_lambda
 %left DOT QMARK
 %left HIGH_PRECEDENCE_BRACK_APP
 %left HIGH_PRECEDENCE_PAREN_APP
@@ -4791,6 +4792,23 @@ argExpr:
         arg }
 
 atomicExpr:
+  | UNDERSCORE DOT atomicExpr %prec dot_lambda
+      { let mUnderscore  = rhs parseState 1
+        let mDot  = rhs parseState 2
+        parseState.LexBuffer.CheckLanguageFeatureAndRecover LanguageFeature.AccessorFunctionShorthand (unionRanges mUnderscore  mDot )
+        let expr, hpa = $3
+        let trivia: SynExprDotLambdaTrivia = { UnderscoreRange = mUnderscore ; DotRange = mDot  }
+        SynExpr.DotLambda(expr, unionRanges mUnderscore  expr.Range, trivia), false }
+
+  | UNDERSCORE DOT appExpr recover %prec dot_lambda
+      { let mUnderscore  = rhs parseState 1
+        let mDot  = rhs parseState 2
+        parseState.LexBuffer.CheckLanguageFeatureAndRecover LanguageFeature.AccessorFunctionShorthand (unionRanges mUnderscore  mDot )
+        reportParseErrorAt (rhs parseState 1) (FSComp.SR.parsUnderScoreDotLambdaNonAtomic())
+        let expr = $3
+        let trivia: SynExprDotLambdaTrivia = { UnderscoreRange = mUnderscore ; DotRange = mDot  }
+        SynExpr.DotLambda(expr, unionRanges mUnderscore  expr.Range, trivia), false }    
+
   | atomicExpr HIGH_PRECEDENCE_BRACK_APP atomicExpr
       { let arg1, _ = $1
         let arg2, hpa = $3