UCS/UPS tracking issue

[LPTK]

Issue to track the parts of the UCS implementation that are still incomplete or wrong – please update with more cases as you find them.

Translation

• Support virtual constructors for pattern matching against tuples. Note as of Jan 2024: We implemented simple tuple pattern in New UCS desugarer and rudimentary PreTyper #194 and we will support advanced tuple patterns, for example, splice tuple patterns, in the near future.
• Do not duplicate default cases and insert local function definitions instead.
  • Note as of Jan 2024: We paved the road for this in New UCS desugarer and rudimentary PreTyper #194 and we will implement this feature in the near future.
  • Note as of Nov 2025: This has been solved in Unify patterns in UCS and UPS #336. Instead of using local functions, we use Block and Break.

Locations

• Track scrutinee locations with Scrutinee.Source. (Note: it was removed in new desugarer.)

• No location reported in:

  fun testF(x) = if x is
    Foo(a) then a
    Foo(a) then a
  //│ ╔══[WARNING] duplicated branch
  //│ ╙──

Reporting

• Warn when an is test has a single pattern
  Though maybe we shouldn't warn when it's conjuncted with other things, as then the is may be used to bind expressions, as in if xs is x :: xs and mapPartition(f, xs) is (as, bs) and ...

[LPTK]

• hygiene

  :NewParser
  :NewDefs
  :NoJS
  
  class Union<Region>(a: Region, b: Region)
  //│ class Union[Region](a: Region, b: Region)
  
  // * [FIXME:UCS] unhygienically desugars to:
  // | | | | Desugared term: case x of { Union => let x = (x).a in let y = (x).b in x }
  fun hmm(x) =
    if x is Union(x, y) then x
  //│ fun hmm: Union[{b: anything} & 'a] -> 'a
  
  fun hmm(x) =
    if x is Union(z, y) then x
  //│ fun hmm: Union['Region] -> Union['Region]

[LPTK]

• There's a problem witth he builtin true/false in the current UCS impl

:NewParser
:NewDefs

module Nil
class Cons[A](head: A, tail: Cons[A] | Nil)
//│ module Nil()
//│ class Cons[A](head: A, tail: Cons[A] | Nil)

// FIXME
fun filtermap(f, xs) = if xs is
  Nil then Nil
  Cons(y, ys) and f(ys) is
    false then filtermap(f, ys)
    true then Cons(y, filtermap(f, ys))
    [true, z] then Cons(y, filtermap(f, ys))
//│ ╔══[ERROR] identifier not found: ys
//│ ║  l.26: 	  Cons(y, ys) and f(ys) is
//│ ╙──      	                    ^^
//│ ╔══[ERROR] Type mismatch in application:
//│ ║  l.26: 	  Cons(y, ys) and f(ys) is
//│ ║        	                  ^^^^^^^^
//│ ║  l.27: 	    false then filtermap(f, ys)
//│ ║        	^^^^^^^^^
//│ ╟── reference of type `false` is not an instance of type `number`
//│ ║  l.27: 	    false then filtermap(f, ys)
//│ ╙──      	    ^^^^^
//│ ╔══[ERROR] Type mismatch in application:
//│ ║  l.26: 	  Cons(y, ys) and f(ys) is
//│ ║        	                  ^^^^^^^^
//│ ║  l.27: 	    false then filtermap(f, ys)
//│ ║        	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
//│ ║  l.28: 	    true then Cons(y, filtermap(f, ys))
//│ ║        	^^^^^^^^
//│ ╟── reference of type `true` is not an instance of type `number`
//│ ║  l.28: 	    true then Cons(y, filtermap(f, ys))
//│ ╙──      	    ^^^^
//│ ╔══[ERROR] type identifier not found: Tuple#2
//│ ╙──
//│ fun filtermap: ((error | Cons['A] | Nil) -> number & (Cons['A] | Nil) -> error, Cons['A0] | Nil,) -> (Cons['A1] | Nil | error)
//│   where
//│     'A := 'A0
//│     'A0 <: nothing
//│ Code generation encountered an error:
//│   unknown match case: Tuple#2

module True
module False
//│ module True()
//│ module False()

class Pair[A, B](lhs: A, rhs: B)
//│ class Pair[A, B](lhs: A, rhs: B)

fun filtermap(f, xs) = if xs is
  Nil then Nil
  Cons(y, ys) then if f(y) is
    True then filtermap(f, ys)
    False then Cons(y, filtermap(f, ys))
    Pair(True, z) then Cons(z, filtermap(f, ys))
//│ fun filtermap: ('head -> (False | Pair[anything, 'A] | True), Cons['head] | Nil,) -> (Cons['A] | Nil)
//│   where
//│     'head <: 'A

[chengluyu]

• Fix unhygienic bindings in HygienicBindings.mls and shared/src/test/diff/ucs/Hygiene.mls
• Should report warnings in shared/src/test/diff/ucs/Wildcard.mls
• Proper errors in shared/src/test/diff/ucs/PlainConditionals.mls
• Some parser failures in shared/src/test/diff/ucs/ParseFailures.mls
• Support or in shared/src/test/diff/ucs/Or.mls
• Leading and in shared/src/test/diff/ucs/LeadingAnd.mls
• Incomplete example in shared/src/test/diff/ucs/JSON.mls

[chengluyu]

• Warning message regression. It should report the first duplicated branch.
  https://github.com/chengluyu/mlscript/blob/ee1ab193234e9d59ad0ce3fa82e2c922efd8d56e/shared/src/test/diff/ucs/DirectLines.mls#L50-L63
• Show terms properly, not using the internal debug string. https://github.com/chengluyu/mlscript/blob/ee1ab193234e9d59ad0ce3fa82e2c922efd8d56e/shared/src/test/diff/ucs/Exhaustiveness.mls#L51

[chengluyu]

• Give warnings for non-conditional UCS. See

  mlscript/shared/src/test/diff/nu/BadUCS.mls

  Line 75 in ee1ab19

  fun foo0(x, y) = if x is y then 0

  Update as of the introduction of new desugarer in Jan 2024: This case is interesting, it means that we bind x to y which shadows parameter y, but someone may think it means if x == y then 0.

• Precise scrutinee tracking and merge explicit let bindings:

  mlscript/shared/src/test/diff/ucs/HygienicBindings.mls

  Line 31 in 66d41f2

  // FIXME: Precise scrutinee identification (easy)

[LPTK]

• One more hygienic bindings thing to make sure and address

  mlscript/shared/src/test/diff/ucs/CrossBranchCapture.mls

  Lines 8 to 12 in 687f39d

  	// * FIXME should be rejected
  	fun process(e) =
  	if e is
  	Numb(n) and n > 0 then n
  	Numb(m) then n

• Warn against shadowing pattern variables

  mlscript/shared/src/test/diff/ucs/CrossBranchCapture.mls

  Lines 49 to 52 in 687f39d

  	// * FIXME should warn, be rejected, or compare both values for equality
  	fun process(e) =
  	if e is
  	Pair(Numb(n), Numb(n)) then n

[LPTK]

• It seems that this should work, but is currently warned about due to an overly restrictive implementation:

  fun fold(opt, k, d) = if opt is
    None then d
    refined(Some) then k(opt)
    else k(opt)
  //│ ╔══[WARNING] inconsistent refined pattern
  //│ ║  l.67:     None then d
  //│ ║            ^^^^
  //│ ╟── pattern `Some` is refined
  //│ ║  l.68:     refined(Some) then k(opt)
  //│ ║                    ^^^^
  //│ ╟── but pattern `None` is not refined
  //│ ║  l.67:     None then d
  //│ ╙──          ^^^^

[LPTK]

• Currently, or is not supported – see:

  mlscript/hkmc2/shared/src/test/mlscript/ucs/general/LogicalConnectives.mls

  Lines 39 to 43 in ac5ce9c

  	:fixme
  	true or test(42)
  	//│ ╔══[ERROR] Logical `or` is not yet supported.
  	//│ ║ l.40: true or test(42)
  	//│ ╙── ^^^^^^^^