Allow mixing ranges and values in collection expressions

docs/release-notes/.FSharp.Compiler.Service/11.0.200.md

@@ -1,5 +1,6 @@
 ### Added
 
+* Ranges can be mixed with values in list, array, sequence and computation expressions under `--langversion:preview`. ([RFC FS-1031](https://github.com/fsharp/fslang-design/blob/main/RFCs/FS-1031-mixing-ranges-and-values-in-sequences.md), [PR #20759](https://github.com/dotnet/fsharp/pull/20759))
 * F# Interactive gains a JSON-RPC server mode, `--fsi-server-jsonrpc:<pipe name>`, in which a host submits interactions over a named pipe and receives structured results — diagnostics with positions, escaping exceptions, the values each interaction bound, and the session's own process id — instead of recovering them by looking for a `SERVER-PROMPT>` marker in the output text. Program output continues to flow through the redirected console streams. The pipe admits only the user running the session; `--fsi-server-client-pid:<pid>` names the host process whose exit ends the session. `FsiEvaluationSession` exposes both options as `JsonRpcServerPipeName` and `JsonRpcClientProcessId`. The mode is part of the .NET fsi only. ([PR #20396](https://github.com/dotnet/fsharp/pull/20396))
 
 ### Fixed
@@ -11,6 +12,7 @@
 * Fix `NativePtr.stackalloc` used inside the `with` handler, filter or `finally` block of a `try` expression producing an assembly that throws `InvalidProgramException` at method load. Direct or mandatory-inline allocations are now reported at compile time as error FS3924; calls to non-inline allocation helpers, including handler-local helpers, remain legal and retain their method boundary during inlining and let-binding elimination. ([Issue #20295](https://github.com/dotnet/fsharp/issues/20295), [PR #20491](https://github.com/dotnet/fsharp/pull/20491))
 * Preserve SRTP code generation when extension constraint solutions are disabled. ([PR #20773](https://github.com/dotnet/fsharp/pull/20773))
 * `FsiEvaluationSession.EvalInteraction` and `EvalInteractionNonThrowing` evaluate every `;;`-separated interaction in the text they are given, as their documentation has always said, instead of silently discarding everything after the first. ([PR #20396](https://github.com/dotnet/fsharp/pull/20396))
+* Fix the counted-loop lowering of integral ranges: a range with a single `unativeint` element is no longer treated as empty, a mutable range bound is read before the later bounds are evaluated, and a `nativeint` range with the smallest step no longer yields extra elements. ([PR #20759](https://github.com/dotnet/fsharp/pull/20759))
 * Prevent `InvalidProgramException` when runtime async awaits a `Task<T>` upcast to `Task`, including through `ConfigureAwait`. ([Issue #20686](https://github.com/dotnet/fsharp/issues/20686), [PR #20726](https://github.com/dotnet/fsharp/pull/20726))
 * Struct/reference anonymous-record mismatches now name anonymous records rather than tuples. ([Issue #6110](https://github.com/dotnet/fsharp/issues/6110), [PR #20681](https://github.com/dotnet/fsharp/pull/20681))
 * Preserve constraint inference order when explicit generic arguments share a caller type parameter, avoiding spurious FS0001 errors with multiply implemented generic interfaces. ([PR #20491](https://github.com/dotnet/fsharp/pull/20491))

docs/release-notes/.Language/preview.md

@@ -4,6 +4,8 @@ Features assigned to F# 11.2 are listed in [11.2](https://fsharp.github.io/fshar
 
 ### Added
 
+* Ranges can be mixed with values in list, array, sequence and computation expressions, as in `[ -3; 1..10; 19 ]`; the range splices its elements like `yield!`. ([Language suggestion #1031](https://github.com/fsharp/fslang-suggestions/issues/1031), [RFC FS-1031](https://github.com/fsharp/fslang-design/blob/main/RFCs/FS-1031-mixing-ranges-and-values-in-sequences.md), [PR #20759](https://github.com/dotnet/fsharp/pull/20759))
+
 ### Fixed
 
 ### Changed

src/Compiler/Checking/Expressions/CheckArrayOrListComputedExpressions.fs

@@ -16,7 +16,6 @@ let TcArrayOrListComputedExpression (cenv: TcFileState) env (overallTy: OverallT
     let g = cenv.g
 
     // The syntax '[ n .. m ]' and '[ n .. step .. m ]' is not really part of array or list syntax.
-    // It could be in the future, e.g. '[ 1; 2..30; 400 ]'
     //
     // The elaborated form of '[ n .. m ]' is 'List.ofSeq (seq (op_Range n m))' and this shouldn't change
     match RewriteRangeExpr comp with
@@ -99,7 +98,8 @@ let TcArrayOrListComputedExpression (cenv: TcFileState) env (overallTy: OverallT
                 | _ -> SynExpr.ArrayOrList(isArray, elems, m)
 
             TcExprUndelayed cenv overallTy env tpenv replacementExpr
-        | _ ->
+        | SimpleSemicolonSequenceWithRanges cenv comp
+        | comp ->
 
             let genCollElemTy = NewInferenceType g
 

src/Compiler/Checking/Expressions/CheckComputationExpressions.fs

@@ -2391,6 +2391,8 @@ let rec TryTranslateComputationExpression
             Some(translatedCtxt callExpr)
 
         | SynExpr.YieldOrReturnFrom((true, _), synYieldExpr, mFull, { YieldOrReturnFromKeyword = m }) ->
+            let synYieldExpr = rewriteYieldFromRange cenv synYieldExpr
+
             let yieldFromExpr =
                 mkSourceExpr synYieldExpr ceenv.sourceMethInfo ceenv.builderValName
 
@@ -2451,6 +2453,7 @@ let rec TryTranslateComputationExpression
             if ceenv.isQuery && not isYield then
                 error (Error(FSComp.SR.tcReturnMayNotBeUsedInQueries (), m))
 
+            let synYieldOrReturnExpr = rejectRangeOperand cenv synYieldOrReturnExpr
             requireBuilderMethod methName ceenv m m
 
             let yieldOrReturnCall =
@@ -2464,6 +2467,8 @@ let rec TryTranslateComputationExpression
 
             Some(translatedCtxt yieldOrReturnCall)
 
+        | BareRangeExpr _ -> TryTranslateComputationExpression ceenv firstTry q varSpace (mkRangeSplice comp) translatedCtxt
+
         | _ -> None)
 
 and ConsumeCustomOpClauses
@@ -2746,6 +2751,8 @@ and TranslateComputationExpressionBind
 /// The inner option indicates if a custom operation is involved inside
 and convertSimpleReturnToExpr (ceenv: ComputationExpressionContext<'a>) comp varSpace innerComp =
     match innerComp with
+    | SynExpr.YieldOrReturn((false, _), RangeAsSingleValue ceenv.cenv, _, _) -> None
+
     | SynExpr.YieldOrReturn((false, _), returnExpr, m, _) ->
         let returnExpr =
             SynExpr.DebugPoint(DebugPointAtLeafExpr.Yes(false, m), false, returnExpr)
@@ -2868,6 +2875,7 @@ and isSimpleExpr ceenv comp =
     | SynExpr.YieldOrReturnFrom _ -> false
     | SynExpr.YieldOrReturn _ -> false
     | SynExpr.DoBang _ -> false
+    | BareRangeExpr _ -> false
     | _ -> true
 
 and TranslateComputationExpression (ceenv: ComputationExpressionContext<'a>) firstTry q varSpace comp translatedCtxt =

src/Compiler/Checking/Expressions/CheckExpressionsOps.fs

@@ -18,6 +18,7 @@ open FSharp.Compiler.TypedTree
 open FSharp.Compiler.TypedTreeBasics
 open FSharp.Compiler.TypedTreeOps
 open FSharp.Compiler.SyntaxTreeOps
+open FSharp.Compiler.SyntaxTrivia
 
 let TryAllowFlexibleNullnessInControlFlow isFirst (g: TcGlobals.TcGlobals) ty =
     match isFirst, g.checkNullness, GetTyparTyIfSupportsNull g ty with
@@ -181,6 +182,33 @@ let RewriteRangeExpr synExpr =
         Some otherExpr
     | _ -> None
 
+[<return: Struct>]
+let (|BareRangeExpr|_|) synExpr =
+    RewriteRangeExpr synExpr |> ValueOption.ofOption
+
+let (|RangeAsSingleValue|_|) (cenv: TcFileState) synExpr =
+    match synExpr with
+    | BareRangeExpr _ -> cenv.g.langVersion.SupportsFeature LanguageFeature.AllowMixedRangesAndValuesInSeqExpressions
+    | _ -> false
+
+let mkRangeSplice (synRange: SynExpr) =
+    let m = synRange.Range
+    SynExpr.YieldOrReturnFrom((true, false), synRange, m, { YieldOrReturnFromKeyword = m })
+
+let rewriteYieldFromRange (cenv: TcFileState) synExpr =
+    match synExpr with
+    | BareRangeExpr rangeCall ->
+        checkLanguageFeatureAndRecover cenv.g.langVersion LanguageFeature.AllowMixedRangesAndValuesInSeqExpressions synExpr.Range
+        rangeCall
+    | _ -> synExpr
+
+let rejectRangeOperand (cenv: TcFileState) operand =
+    match operand with
+    | RangeAsSingleValue cenv ->
+        errorR (Error(FSComp.SR.tcRangeIsNotASingleValue (), operand.Range))
+        arbExpr ("rangeAsSingleValue", operand.Range)
+    | _ -> operand
+
 /// Check if a computation or sequence expression is syntactically free of 'yield' (though not yield!)
 let YieldFree (_cenv: TcFileState) expr =
     let rec YieldFree expr =
@@ -223,7 +251,8 @@ let inline IsSimpleSemicolonSequenceElement expr cenv acceptDeprecated =
     | SynExpr.Do _
     | SynExpr.MatchBang _
     | SynExpr.While _
-    | SynExpr.WhileBang _ -> false
+    | SynExpr.WhileBang _
+    | BareRangeExpr _ -> false
     | _ -> true
 
 [<TailCall>]
@@ -247,6 +276,39 @@ let rec TryGetSimpleSemicolonSequenceOfComprehension expr acc cenv acceptDepreca
 let (|SimpleSemicolonSequence|_|) cenv acceptDeprecated cexpr =
     TryGetSimpleSemicolonSequenceOfComprehension cexpr [] cenv acceptDeprecated
 
+[<return: Struct>]
+let (|SimpleSemicolonSequenceWithRanges|_|) cenv cexpr =
+    let isRange = RewriteRangeExpr >> Option.isSome
+
+    let yieldValue expr =
+        SynExpr.YieldOrReturn((true, false), expr, expr.Range, SynExprYieldOrReturnTrivia.Zero)
+
+    let sequential head tail =
+        SynExpr.Sequential(
+            DebugPointAtSequential.SuppressNeither,
+            true,
+            head,
+            tail,
+            unionRanges head.Range tail.Range,
+            SynExprSequentialTrivia.Zero
+        )
+
+    let rec elements acc expr =
+        match expr with
+        | SynExpr.Sequential(isTrueSeq = true; expr1 = head; expr2 = tail) -> elements (head :: acc) tail
+        | _ -> List.rev (expr :: acc)
+
+    match elements [] cexpr with
+    | elems when
+        List.exists isRange elems
+        && List.forall (fun e -> isRange e || IsSimpleSemicolonSequenceElement e cenv false) elems
+        ->
+        elems
+        |> List.map (fun e -> if isRange e then e else yieldValue e)
+        |> List.reduceBack sequential
+        |> ValueSome
+    | _ -> ValueNone
+
 let elimFastIntegerForLoop (spFor, spTo, id, start: SynExpr, dir, finish: SynExpr, innerExpr, m: range) =
     let mOp = (unionRanges start.Range finish.Range).MakeSynthetic()
 

src/Compiler/Checking/Expressions/CheckSequenceExpressions.fs

@@ -379,6 +379,13 @@ let TcSequenceExpression (cenv: TcFileState) env tpenv comp (overallTy: OverallT
 
         | SynExpr.YieldOrReturnFrom(flags = (isYield, _); expr = synYieldExpr; trivia = { YieldOrReturnFromKeyword = m }) ->
             let env = { env with eIsControlFlow = false }
+
+            let synYieldExpr =
+                if isYield then
+                    rewriteYieldFromRange cenv synYieldExpr
+                else
+                    synYieldExpr
+
             let resultExpr, genExprTy, tpenv = TcExprOfUnknownType cenv env tpenv synYieldExpr
 
             if not isYield then
@@ -397,6 +404,14 @@ let TcSequenceExpression (cenv: TcFileState) env tpenv comp (overallTy: OverallT
 
             Some(resultExpr, tpenv)
 
+        | SynExpr.YieldOrReturn(flags = (isYield, _); expr = RangeAsSingleValue cenv as synRange; range = m) ->
+            if isYield then
+                rejectRangeOperand cenv synRange |> ignore
+            else
+                errorR (Error(FSComp.SR.tcSeqResultsUseYield (), m))
+
+            tryTcSequenceExprBody env genOuterTy tpenv (mkRangeSplice synRange)
+
         | SynExpr.YieldOrReturn(flags = (isYield, _); expr = synYieldExpr; range = m) ->
             let env = { env with eIsControlFlow = false }
             let genResultTy = NewInferenceType g
@@ -418,6 +433,8 @@ let TcSequenceExpression (cenv: TcFileState) env tpenv comp (overallTy: OverallT
 
             Some(resultExpr, tpenv)
 
+        | BareRangeExpr _ -> tryTcSequenceExprBody env genOuterTy tpenv (mkRangeSplice comp)
+
         | _ -> None
 
     and tcSequenceExprBody env (genOuterTy: TType) tpenv comp =

src/Compiler/FSComp.txt

@@ -1731,6 +1731,7 @@ featureUseBangBindingValueDiscard,"Allows use! _ = ... in computation expression
 featureBetterAnonymousRecordParsing,"Support for better anonymous record parsing"
 featureScopedNowarn,"Support for scoped enabling / disabling of warnings by #warn and #nowarn directives, also inside modules"
 featureErrorOnInvalidDeclsInTypeDefinitions,"Error when invalid declarations are used in type definitions."
+featureAllowMixedRangesAndValuesInSeqExpressions,"Allow mixed ranges and values in sequence expressions, e.g. seq {{ 1..10; 20 }}"
 featureAllowLetOrUseBangTypeAnnotationWithoutParens,"Allow let! and use! type annotations without requiring parentheses"
 3874,lexWarnDirectiveMustBeFirst,"#nowarn/#warnon directives must appear as the first non-whitespace characters on a line"
 3875,lexWarnDirectiveMustHaveArgs,"Warn directives must have warning number(s) as argument(s)"
@@ -1804,4 +1805,5 @@ featureTypeArgumentDependencyOrdering,"ordering explicit type arguments by their
 3922,ilRuntimeAsyncSequenceNotStaticallyKnown,"The runtime-async sequence recipe could not be statically lowered. Supply a statically known delayed sequence recipe through an inline builder."
 3923,tcMethodRequiresNamedArguments,"The method '%s' requires named arguments. Use named-argument syntax, e.g. 'MethodName(argumentName = value)'."
 3924,chkNativePtrStackallocInHandler,"'NativePtr.stackalloc' cannot be used inside the 'with' handler, filter or 'finally' block of a 'try' expression. The 'localloc' IL instruction this emits is rejected inside an exception-handling region, producing a 'System.InvalidProgramException' when the method is loaded."
+3925,tcRangeIsNotASingleValue,"A range expression cannot be used as a single value here. Use 'yield!' to splice the range, or wrap it in a list or sequence expression to use it as one value."
 featureReraiseInComputationExpressions,"calls to 'reraise' in a computation expression handler"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -45,6 +45,7 @@ type LanguageFeature =
     | BetterAnonymousRecordParsing
     | ScopedNowarn
     | ErrorOnInvalidDeclsInTypeDefinitions
+    | AllowMixedRangesAndValuesInSeqExpressions
     | AllowTypedLetUseAndBang
     | ReturnFromFinal
     | MoreConcreteTiebreaker
@@ -172,6 +173,7 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
                 // F# preview
                 // Unfinished features that still need work before they can be assigned a release language version.
                 LanguageFeature.FromEndSlicing, previewVersion // Unfinished features --- needs work
+                LanguageFeature.AllowMixedRangesAndValuesInSeqExpressions, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -295,6 +297,7 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
         | LanguageFeature.BetterAnonymousRecordParsing -> FSComp.SR.featureBetterAnonymousRecordParsing ()
         | LanguageFeature.ScopedNowarn -> FSComp.SR.featureScopedNowarn ()
         | LanguageFeature.ErrorOnInvalidDeclsInTypeDefinitions -> FSComp.SR.featureErrorOnInvalidDeclsInTypeDefinitions ()
+        | LanguageFeature.AllowMixedRangesAndValuesInSeqExpressions -> FSComp.SR.featureAllowMixedRangesAndValuesInSeqExpressions ()
         | LanguageFeature.AllowTypedLetUseAndBang -> FSComp.SR.featureAllowLetOrUseBangTypeAnnotationWithoutParens ()
         | LanguageFeature.ReturnFromFinal -> FSComp.SR.featureReturnFromFinal ()
         | LanguageFeature.MoreConcreteTiebreaker -> FSComp.SR.featureMoreConcreteTiebreaker ()

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -36,6 +36,7 @@ type LanguageFeature =
     | BetterAnonymousRecordParsing
     | ScopedNowarn
     | ErrorOnInvalidDeclsInTypeDefinitions
+    | AllowMixedRangesAndValuesInSeqExpressions
     | AllowTypedLetUseAndBang
     | ReturnFromFinal
     | MoreConcreteTiebreaker

src/Compiler/Optimize/LowerComputedCollections.fs

@@ -199,6 +199,16 @@ let LowerComputedListOrArraySeqExpr tcVal g amap m collectorTy overallSeqExpr =
                 Result.Ok (flag, exprR)
             | Result.Error msg -> Result.Error msg
 
+        | IntegralRange g (rangeTy, (start, step, finish)) when
+            not isUninteresting
+            && g.langVersion.SupportsFeature LanguageFeature.LowerIntegralRangesToFastLoops
+            && typeEquiv g (mkSeqTy g rangeTy) collectorSeqTy ->
+            let m = expr.Range
+            let exprR =
+                mkOptimizedRangeLoop g (m, m, m, DebugPointAtWhile.No) (rangeTy, expr) (start, step, finish) (fun _count mkLoop ->
+                    mkLoop (fun _idx elem -> mkCallCollectorAdd tcVal g infoReader m collExpr elem))
+            Result.Ok (false, exprR)
+
         // yield! e ---> (for x in e -> x)
 
         | arbitrarySeqExpr ->