WIP RFC FS-1092: Anonymous Type-tagged Union types

[dsyme]

Replaces #10566.

Implements RFC https://github.com/fsharp/fslang-design/blob/master/RFCs/FS-1092-anonymous-type-tagged-unions.md

Builds on #10884

Wet paint

[dsyme]

I set up auto-flow from feature/auto-widen to this branch here: https://github.com/dotnet/roslyn-tools/blob/master/src/GitHubCreateMergePRs/config.xml

[Happypig375]

Can we get the "Candidate for F# vNext" here?

[cartermp]

The base of this is still an experiment (for example: it's a draft) and nowhere near the quality of other vNext-style PRs. Once design quirks are worked out I think we can proceed with labeling it as such. I expect quite a bit of churn in both places until we get at something that's considered reviewable.

[dsyme]

@Swoorup I updated this by merging in the base branch

[chkn]

Looks like there is an issue when a function type is used as one of the union types:

> type IntOrFn = (int | unit -> int);;
type IntOrFn = (int | (unit -> int))

> let foo :IntOrFn = 5;;
val foo : IntOrFn = 5

> let foo :IntOrFn = fun () -> 5;;

  let foo :IntOrFn = fun () -> 5;;
  -------------------^^^^^^^^^^^

stdin(6,20): error FS0002: This function takes too many arguments, or is used in a context where a function is not expected

[chkn]

Also, flexible types seem to work for let bound functions (which is great), but not for static methods:

> type IntOrStr = (int | string);;
type IntOrStr = (int | string)

> let mer (fn : unit -> #IntOrStr) = fn();;
val mer : fn:(unit -> 'a) -> 'a when 'a :> IntOrStr

> type Foof = static member Boom(fn : unit -> #IntOrStr) = fn();;
Process terminated. Assertion failed.
Unexpected exception raised in compiler: buildGenParam: multiple base types
System.Exception: buildGenParam: multiple base types
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.buildGenParamsPass1b@1405.Invoke(Int32 i, ILGenericParameterDef gp) in /fsharp/src/fsharp/absil/ilreflect.fs:line 1414
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.buildGenParamsPass1b(cenv cenv, emEnv emEnv, Type[] genArgs, FSharpList`1 gps) in /fsharp/src/fsharp/absil/ilreflect.fs:line 1405
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.buildMethodPass2(cenv cenv, ILTypeRef tref, TypeBuilder typB, emEnv emEnv, ILMethodDef mdef) in /fsharp/src/fsharp/absil/ilreflect.fs:line 1544
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.emEnv@1819-2.Invoke(emEnv emEnv, ILMethodDef mdef) in /fsharp/src/fsharp/absil/ilreflect.fs:line 1819
   at Microsoft.FSharp.Collections.ArrayModule.Fold[T,TState](FSharpFunc`2 folder, TState state, T[] array) in /fsharp/src/fsharp/FSharp.Core/array.fs:line 901
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.buildTypeDefPass2(cenv cenv, FSharpList`1 nesting, emEnv emEnv, ILTypeDef tdef) in /fsharp/src/fsharp/absil/ilreflect.fs:line 1819
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.emEnv@1825-5.Invoke(emEnv emEnv, ILTypeDef tdef) in /fsharp/src/fsharp/absil/ilreflect.fs:line 1825
   at Microsoft.FSharp.Collections.ListModule.Fold[T,TState](FSharpFunc`2 folder, TState state, FSharpList`1 list) in /fsharp/src/fsharp/FSharp.Core/list.fs:line 221
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.buildTypeDefPass2(cenv cenv, FSharpList`1 nesting, emEnv emEnv, ILTypeDef tdef) in /fsharp/src/fsharp/absil/ilreflect.fs:line 1825
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.buildModuleTypePass2(cenv cenv, emEnv emEnv, ILTypeDef tdef) in /fsharp/src/fsharp/absil/ilreflect.fs:line 2019
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.emEnv@2032-9.Invoke(emEnv emEnv, ILTypeDef tdef) in /fsharp/src/fsharp/absil/ilreflect.fs:line 2032
   at Microsoft.FSharp.Collections.ListModule.Fold[T,TState](FSharpFunc`2 folder, TState state, FSharpList`1 list) in /fsharp/src/fsharp/FSharp.Core/list.fs:line 221
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.buildModuleFragment(cenv cenv, emEnv emEnv, AssemblyBuilder asmB, ModuleBuilder modB, ILModuleDef m) in /fsharp/src/fsharp/absil/ilreflect.fs:line 2032
   at FSharp.Compiler.AbstractIL.ILRuntimeWriter.emitModuleFragment(ILGlobals ilg, Boolean emitTailcalls, emEnv emEnv, AssemblyBuilder asmB, ModuleBuilder modB, ILModuleDef modul, Boolean debugInfo, FSharpFunc`2 resolveAssemblyRef, FSharpFunc`2 tryFindSysILTypeRef) in /fsharp/src/fsharp/absil/ilreflect.fs:line 2103
   at FSharp.Compiler.Interactive.Shell.FsiDynamicCompiler.ProcessCodegenResults(CompilationThreadToken ctok, ErrorLogger errorLogger, FsiDynamicCompilerState istate, IncrementalOptimizationEnv optEnv, TcState tcState, TcConfig tcConfig, FSharpList`1 prefixPath, Boolean showTypes, Boolean isIncrementalFragment, String fragName, FSharpList`1 declaredImpls, IlxAssemblyGenerator ilxGenerator, IlxGenResults codegenResults) in /fsharp/src/fsharp/fsi/fsi.fs:line 1159
   at FSharp.Compiler.Interactive.Shell.FsiDynamicCompiler.ProcessInputs(CompilationThreadToken ctok, ErrorLogger errorLogger, FsiDynamicCompilerState istate, FSharpList`1 inputs, Boolean showTypes, Boolean isIncrementalFragment, Boolean isInteractiveItExpr, FSharpList`1 prefixPath) in /fsharp/src/fsharp/fsi/fsi.fs:line 1257
   at FSharp.Compiler.Interactive.Shell.FsiDynamicCompiler.EvalParsedDefinitions(CompilationThreadToken ctok, ErrorLogger errorLogger, FsiDynamicCompilerState istate, Boolean showTypes, Boolean isInteractiveItExpr, FSharpList`1 defs) in /fsharp/src/fsharp/fsi/fsi.fs:line 1390
   at FSharp.Compiler.Interactive.Shell.clo@2212-597.Invoke(FsiDynamicCompilerState istate) in /fsharp/src/fsharp/fsi/fsi.fs:line 2224
   at FSharp.Compiler.Interactive.Shell.FsiInteractionProcessor.InteractiveCatch[a](ErrorLogger errorLogger, FSharpFunc`2 f, a istate) in /fsharp/src/fsharp/fsi/fsi.fs:line 2122
   at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ErrorLogger.EmitDiagnostic(ErrorLogger x, Exception exn, FSharpDiagnosticSeverity severity) in /fsharp/src/fsharp/ErrorLogger.fs:line 372
   at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ErrorLogger.ErrorRecovery(ErrorLogger x, Exception exn, Range m) in /fsharp/src/fsharp/ErrorLogger.fs:line 408
   at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ErrorLogger.StopProcessingRecovery(ErrorLogger x, Exception exn, Range m) in /fsharp/src/fsharp/ErrorLogger.fs:line 422
   at FSharp.Compiler.ErrorLogger.stopProcessingRecovery(Exception exn, Range m) in /fsharp/src/fsharp/ErrorLogger.fs:line 482
   at FSharp.Compiler.Interactive.Shell.FsiInteractionProcessor.InteractiveCatch[a](ErrorLogger errorLogger, FSharpFunc`2 f, a istate) in /fsharp/src/fsharp/fsi/fsi.fs:line 2122
   at FSharp.Compiler.Interactive.Shell.FsiInteractionProcessor.ExecInteraction(CompilationThreadToken ctok, TcConfig tcConfig, FsiDynamicCompilerState istate, ParsedScriptInteraction action, ErrorLogger errorLogger) in /fsharp/src/fsharp/fsi/fsi.fs:line 2212
   at FSharp.Compiler.Interactive.Shell.FsiInteractionProcessor.execParsedInteractions(CompilationThreadToken ctok, TcConfig tcConfig, FsiDynamicCompilerState istate, FSharpOption`1 action, ErrorLogger errorLogger, FSharpOption`1 lastResult, CancellationToken cancellationToken) in /fsharp/src/fsharp/fsi/fsi.fs:line 2357
   at FSharp.Compiler.Interactive.Shell.FsiInteractionProcessor.executeParsedInteractions(CompilationThreadToken ctok, TcConfig tcConfig, FsiDynamicCompilerState istate, FSharpOption`1 action, ErrorLogger errorLogger, FSharpOption`1 lastResult, CancellationToken cancellationToken) in /fsharp/src/fsharp/fsi/fsi.fs:line 2368
   at FSharp.Compiler.Interactive.Shell.clo@2398-599.Invoke(CompilationThreadToken ctok, TcConfig tcConfig, FsiDynamicCompilerState istate) in /fsharp/src/fsharp/fsi/fsi.fs:line 2399
   at FSharp.Compiler.Interactive.Shell.FsiInteractionProcessor.mainThreadProcessAction[a,b](a ctok, FSharpFunc`2 action, b istate) in /fsharp/src/fsharp/fsi/fsi.fs:line 2381
   at FSharp.Compiler.Interactive.Shell.FsiInteractionProcessor.mainThreadProcessParsedInteractions(CompilationThreadToken ctok, ErrorLogger errorLogger, FSharpOption`1 action, FsiDynamicCompilerState istate, CancellationToken cancellationToken) in /fsharp/src/fsharp/fsi/fsi.fs:line 2398
   at FSharp.Compiler.Interactive.Shell.res@2458-60.Invoke(CompilationThreadToken ctok, FsiDynamicCompilerState istate) in /fsharp/src/fsharp/fsi/fsi.fs:line 2458
   at FSharp.Compiler.Interactive.Shell.clo@2102-596.Invoke(Unit unitVar0) in /fsharp/src/fsharp/fsi/fsi.fs:line 2111
   at <StartupCode$FSharp-Compiler-Interactive-Settings>.$Fsiaux.FSharp-Compiler-Interactive-IEventLoop-Invoke@50-1.Invoke(Unit x) in /fsharp/src/fsharp/fsiaux.fs:line 50
   at <StartupCode$FSharp-Compiler-Interactive-Settings>.$Fsiaux.run@41-4.Invoke(FSharpFunc`2 f) in /fsharp/src/fsharp/fsiaux.fs:line 41
   at <StartupCode$FSharp-Compiler-Interactive-Settings>.$Fsiaux.run@39.Invoke(Unit unitVar0) in /fsharp/src/fsharp/fsiaux.fs:line 41
   at FSharp.Compiler.Interactive.SimpleEventLoop.FSharp.Compiler.Interactive.IEventLoop.Run() in /fsharp/src/fsharp/fsiaux.fs:line 48
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
   at FSharp.Compiler.Interactive.Shell.Utilities.callInstanceMethod0[a](Object obj, Type[] typeArgs, String nm) in /fsharp/src/fsharp/fsi/fsi.fs:line 131
   at Sample.FSharp.Compiler.Interactive.Main.evaluateSession@228-4.EventLoopRun() in /fsharp/src/fsharp/fsi/fsimain.fs:line 246
   at FSharp.Compiler.Interactive.Shell.runLoop@2707.Invoke(Unit unitVar0) in /fsharp/src/fsharp/fsi/fsi.fs:line 2712
   at FSharp.Compiler.Interactive.Shell.DriveFsiEventLoop(FsiEvaluationSessionHostConfig fsi, FsiConsoleOutput fsiConsoleOutput) in /fsharp/src/fsharp/fsi/fsi.fs:line 2728
   at FSharp.Compiler.Interactive.Shell.FsiEvaluationSession.Run() in /fsharp/src/fsharp/fsi/fsi.fs:line 3182
   at Sample.FSharp.Compiler.Interactive.Main.evaluateSession(String[] argv) in /fsharp/src/fsharp/fsi/fsimain.fs:line 298
   at Sample.FSharp.Compiler.Interactive.Main.MainMain(String[] argv) in /fsharp/src/fsharp/fsi/fsimain.fs:line 342
Abort trap: 6

[Swoorup]

I doubt flexible type should be used that way. I can't see it making sense, at least not right now, as generics wouldn't be supported in this feature.

[chkn]

@Swoorup this is the problem I'm trying to solve:

> type IntOrStr = (int | string);;
type IntOrStr = (int | string)

> let fn = fun () -> 5;;
val fn : unit -> int

> // with flexible types  
- let foo (fn : unit -> #IntOrStr) = fn();;
val foo : fn:(unit -> 'a) -> 'a when 'a :> IntOrStr

> foo fn;;
val it : int = 5

> // without flexible types  
- let bar (fn : unit -> IntOrStr) = fn();;
val bar : fn:(unit -> IntOrStr) -> IntOrStr

> bar fn;;

  bar fn;;
  ----^^

stdin(6,5): error FS0001: Type mismatch. Expecting a
    'unit -> IntOrStr'    
but given a
    'unit -> int'    
The type 'IntOrStr' does not match the type 'int'

[Swoorup]

@chkn It looks more like a problem for auto-widening (as future improvements). Ideally you shouldn't need flexible type for this case.

[chkn]

This creates a compatibility consideration for libraries that publish erased union types. It might not be a concern, but I think it's important to be aware of:

Library1

type Foo = obj

Library2 (references Library1)

type Bar = (* ... *)
type Problematic = (Foo | Bar) // where `Foo` is from Library1

Due to this conditional, the Bar part of Problematic will be discarded, keeping only obj aliased through Foo. But if Library1 is later updated to change the type of Foo, any code compiled against the updated Library1 and old Library2 will find that the Problematic type no longer accepts Bars.

[Swoorup]

Wouldn't that be same issue as the following:

Library1

type Foo = int

Library2 (references Library1)

let handle (foo: Foo) = 
   foo + 1

Later Library1 is updated to so that Foo is aliased to String? handle doesn't handle string. I don't know enough about how .net handles transitive dependencies in depth.

[chkn]

Yeah you'd definitely expect that sort of breakage with the Foo type. But I wouldn't expect that using Problematic with Bar would also be broken in that case.

[Swoorup]

But you would just recompile the code no in that case?

[chkn]

Sure, if you could. But it might be a dependency you don't have the source for

[Swoorup]

I don't see, how this is different to other issues that arise out of dependency version mismatch. You'd only have the issue if you publish a package using a version and then republish it using the same version, causing the breakage.

[En3Tho]

@Swoorup @chkn Do you guys need help with this?

[Swoorup]

@Swoorup @chkn Do you guys need help with this?

Pattern matching, (giving out warnings when matches are incomplete or overmatching) is an area that needs to be looked after. At the moment, it is just based on :? pattern match which doesn't do much. There are other tickets which are addressing areas of pattern matching, so probably best holding for now.

[Swoorup]

Any reasons for this being closed? @dsyme

[dsyme]

Hmm looks like it got auto-closed when the branch it was targeting got deleted. I'll retarget to main

[dsyme]

Actually looks like feature/erased-unions got deleted by mistake