mlscript UCS/UPS tracking issue

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

[x] Support virtual constructors for pattern matching against tuples. Note as of Jan 2024: We implemented simple tuple pattern in #194 and we will support advanced tuple patterns, for example, splice tuple patterns, in the near future.
[x] Do not duplicate default cases and insert local function definitions instead.
- Note as of Jan 2024: We paved the road for this in #194 and we will implement this feature in the near future.
- Note as of Nov 2025: This has been solved in #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.)

[x] 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 ...

Nov 22 '22 15:11 LPTK

[x] 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]

Feb 25 '23 16:02 LPTK

[x] 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

Mar 03 '23 06:03 LPTK

[ ] 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
[x] Some parser failures in shared/src/test/diff/ucs/ParseFailures.mls
[ ] Support or in shared/src/test/diff/ucs/Or.mls
[x] Leading and in shared/src/test/diff/ucs/LeadingAnd.mls
[x] Incomplete example in shared/src/test/diff/ucs/JSON.mls

May 05 '23 06:05 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

May 12 '23 18:05 chengluyu

[x] Give warnings for non-conditional UCS. See https://github.com/hkust-taco/mlscript/blob/ee1ab193234e9d59ad0ce3fa82e2c922efd8d56e/shared/src/test/diff/nu/BadUCS.mls#L75

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: https://github.com/hkust-taco/mlscript/blob/66d41f279648dd67507d442fa96330bec17075e7/shared/src/test/diff/ucs/HygienicBindings.mls#L31

May 17 '23 09:05 chengluyu

[ ] One more hygienic bindings thing to make sure and address

https://github.com/hkust-taco/mlscript/blob/687f39d8ebf4d649622e40ecba2af6ac09c3400f/shared/src/test/diff/ucs/CrossBranchCapture.mls#L8-L12
[ ] Warn against shadowing pattern variables

https://github.com/hkust-taco/mlscript/blob/687f39d8ebf4d649622e40ecba2af6ac09c3400f/shared/src/test/diff/ucs/CrossBranchCapture.mls#L49-L52

Jul 18 '23 03:07 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
//│ ╙──          ^^^^

Feb 19 '24 14:02 LPTK