combinatorial explosion of recursive unions definitions.

[sledorze]

TypeScript Version: 2.5.2

Code

type A = { tag: 'a', pa: ALL}
type B = { tag: 'b', pb: ALL}
type C = { tag: 'c', pc: ALL}
type D = { tag: 'd', pd: ALL}
type E = { tag: 'e', pe: ALL}
type ALL = A | B | C | D | E

Expected behavior:

When hovering on pa ... pe members, we see a few types, ideally just 'ALL'.

Actual behavior:

When hovering on pa ... pe members, we see an explosion of types.

[DanielRosenwasser]

I'm not seeing this in the playground or the nightlies.

[sledorze]

Daniel Rosenwasser (@DanielRosenwasser) I've seen that in VSCode, can you try in it?
( in the playground that looks like what I would expect )

I have two concerns:

• It lags terribly when editing.
• I'm afraid this pattern could lead to some OOM when compiling.
  for information, in VSCode, the class version does not exhibit the issue.

class CA { tag: 'a'; pa: CALL }
class CB { tag: 'b'; pb: CALL }
class CC { tag: 'c'; pc: CALL }
class CD { tag: 'd'; pc: CALL }
class CE { tag: 'e'; pc: CALL }
type CALL = CA | CB | CC | CD | CE

[mhegazy]

Can you share some more context. like a bigger code sample?

[sledorze]

Mohamed Hegazy (@mhegazy) Daniel Rosenwasser (@DanielRosenwasser) oh that's strange!

I found out it happens when I do an import like this at the top of the file:

import * as t from 'io-ts' 

type A = { tag: 'a', pa: ALL}
type B = { tag: 'b', pb: ALL}
type C = { tag: 'c', pc: ALL}
type D = { tag: 'd', pd: ALL}
type E = { tag: 'e', pe: ALL}
type ALL = A | B | C | D | E

Here's a repo exhibiting the issue, this one with latest TS 2.6.0-dev.20170926 ( happens on both that version and 2.5.2, I've not checked others ) :
https://github.com/sledorze/tsInferenceIssue

[sledorze]

Actually, I'm even not requiring io-ts, I've updated the repo with a version that just tries to import an empty module.

emptymodule.ts

export { }

test.ts

import * as t from './emptymodule'

type ZA = { tag: 'a', pa: ALL }
type ZB = { tag: 'b', pb: ALL }
type ZC = { tag: 'c', pc: ALL }
type ZD = { tag: 'd', pc: ALL }
type ZE = { tag: 'e', pc: ALL }
type ALL = ZA | ZB | ZC | ZD | ZE

[sledorze]

Another variations happens with just

export { }

type ZA = { tag: 'a', pa: ALL }
type ZB = { tag: 'b', pb: ALL }
type ZC = { tag: 'c', pc: ALL }
type ZD = { tag: 'd', pc: ALL }
type ZE = { tag: 'e', pc: ALL }
type ALL = ZA | ZB | ZC | ZD | ZE

Here's a playground link to it:
https://www.typescriptlang.org/play/#src=export%20%7B%7D%0D%0A%0D%0Atype%20A%20%3D%20%7B%20tag%3A%20'a'%2C%20pa%3A%20ALL%20%7D%0D%0Atype%20B%20%3D%20%7B%20tag%3A%20'b'%2C%20pb%3A%20ALL%7D%0D%0Atype%20C%20%3D%20%7B%20tag%3A%20'c'%2C%20pc%3A%20ALL%7D%0D%0Atype%20D%20%3D%20%7B%20tag%3A%20'd'%2C%20pd%3A%20ALL%7D%0D%0Atype%20E%20%3D%20%7B%20tag%3A%20'e'%2C%20pe%3A%20ALL%7D%0D%0Atype%20ALL%20%3D%20A%20%7C%20B%20%7C%20C%20%7C%20D%20%7C%20E%0D%0A

[sledorze]

Also this one exhibit the error

type ZA = { tag: 'a', pa: ALL }
type ZB = { tag: 'b', pb: ALL }
type ZC = { tag: 'c', pc: ALL }
type ZD = { tag: 'd', pc: ALL }
type ZE = { tag: 'e', pc: ALL }
type ALL = ZA | ZB | ZC | ZD | ZE
export { ALL }

whereas this one not

type ZA = { tag: 'a', pa: ALL }
type ZB = { tag: 'b', pb: ALL }
type ZC = { tag: 'c', pc: ALL }
type ZD = { tag: 'd', pc: ALL }
type ZE = { tag: 'e', pc: ALL }
export type ALL = ZA | ZB | ZC | ZD | ZE

So there's a kind of 'workaround' so far..

[sledorze]

Mohamed Hegazy (@mhegazy) Daniel Rosenwasser (@DanielRosenwasser) enough information to emit a judgment if either this is a bug or a by design limitation?

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[DanielRosenwasser]

This is definitely undesirable behavior, but can you give us more context on how you're running into this in the first place? Specifically, what is the motivating use-case?