How are "pnpMode" and "pnpFallbackMode" related ?

[Robin-Hoodie]

The following is my understanding from the documentation on the yarnrc file on the pnpMode and pnpFallbackMode config params:

The config param pnpMode will determine if packages are ever allowed to require dependencies that they don't list as a dependency themselves.

• In "strict" mode (the default), no packages will be allowed to require dependencies they don't list as a dependency themselves.
• In "loose" mode, they will be allowed to require packages under some circumstances.

For "loose" mode is where the config param pnpFallbackMode comes into play, which will determine which kind of packages are allowed to require dependencies they don't list as a dependency themselves:

• "none": this will not allow packages to require a dependency they don't list as a dependency
• "dependencies-only": This will allow your workspace's dependencies to require a dependency that would have been present in a traditional "node_modules"-install
• "all": the same as "dependencies-only", but also applies to your workspace's code itself

I'm assuming I'm wrong in my understanding here, as a few things don't make sense:

• If pnpFallbackMode only applies when pnpMode is set to loose, I would find it quite strange that this does not seem to be documented explicitly anywhere

• If pnpFallbackMode set to none doesn't allow packages to require a dependency they don't list as a dependency themselves, that's basically the same thing as pnpMode set to strict, making this value redundant

• Why does pnpFallbackMode even have a default value, if a value of strict for pnpMode would not even make it relevant ? (I guess this one could make sense if it was documented that pnpFallbackMode is only relevant for pnpMode of loose)

--

Just to clarify, I am aware of the extra documentation on PnP loose mode and PnPFallbackMode, though that doesn't explain my points above

[arcanis]

pnpFallbackMode was a thing long before the loose mode, and has an effect even in strict mode (although I want to deprecate it someday). Check the last paragraph from this page for details: https://yarnpkg.com/features/pnp/#different-behaviours-based-on-workspace--not-a-workspace

[arcanis]

It's a slight deviation. Yes, this fallback exists even in strict mode for now, but it'll eventually disappear. In the meantime, its effect is relatively small and gave us some breathing room in terms of compatibility back when we needed it (for instance, I initially implemented it because Babel wasn't able to load its plugins).

[Robin-Hoodie]

Which deviation? I'm still not really sure how pnpFallbackMode affects pnpMode to be honest

[Robin-Hoodie]

Perhaps a few specific questions that would help me understand things better:

• Does using pnpMode=loose and pnpFallbackMode=none make any sense ?
• Does using pnpMode=strict and pnpFallbackMode=all make any sense?
• For pnpFallbackMode=dependencies-only: what is the difference between the behaviour of pnpMode=strict and pnpMode=loose?

[arcanis]

Does using pnpMode=loose and pnpFallbackMode=none make any sense ?

Nope

Does using pnpMode=strict and pnpFallbackMode=all make any sense?

Yeah but ... don't. Basically you would only do this if you want to break the encapsulation a little, but not entirely. Weird.

For pnpFallbackMode=dependencies-only: what is the difference between the behaviour of pnpMode=strict and pnpMode=loose

The set of packages that your dependencies can read by bypassing the hoisting. Even in loose mode your application (ie your own code, that you wrote yourself) isn't allowed to access unlisted dependencies. Only with pnpFallbackMode=all will your application be allowed to access dependencies through hoisting.

[Robin-Hoodie]

Right, thanks for clarifying the first 2 examples, though I'm honestly still fuzzy on some details

Let's say I have a dependency on package "A" and package "B".
Package "A" has a dependency on package "C"

The following piece of code is part of my application code/workspace:

require('C")

Yarn will refuse to require "C" for the following settings

• pnpMode=strict, pnpFallbackMode will not make this work, even if it's set to all
• pnpMode=loose, pnpFallbackMode=none|dependencies-only

This will only work by setting pnpMode=loose and pnpFallbackMode=all

The following piece of code is part of package "A":

require("B")

Yarn will refuse to require "B" if pnpFallbackMode is set to none

It will work if pnpFallbackMode is set to "dependencies-only" or "all", regardless of what pnpMode is set to

Is that correct?

[merceyz]

To put it differently pnpMode controls what is in the fallback pool, pnpFallbackMode controls what is allowed to access the fallback pool.