Should a module importing itself be disallowed?

[paulyoung]

We use PureScript with webpack via purs-loader and circular-dependency-plugin, and started seeing this warning:

WARNING in Circular dependency detected:
bower_components/purescript-formatters/src/Data/Formatter/Parser/Number.purs -> bower_components/purescript-formatters/src/Data/Formatter/Parser/Number.purs

Sure enough, purescript-formatters has an import statement for itself with a function defined in the same module:

https://github.com/slamdata/purescript-formatters/blob/859e9f5c7d507441d495870acbb06380940eda97/src/Data/Formatter/Parser/Number.purs#L12

Should this be disallowed?

[paulyoung]

@natefaubion pointed out that this import doesn't appear in the generated JavaScript in Try PureScript, so maybe this is just an issue for purescript-formatters.

[natefaubion]

The compiler allows the import, but you can't reference anything in it (if qualified), and no code is generated for it. purs-loader does it's own import parsing to build dependencies for Webpack to follow, and it's adding the self reference as a dependency, which is how the plugin picks it up. I think the compiler should probably error on the import, though.

[damncabbage]

A possible alternative: could importing and referring to the current module just be syntax sugar?

module Abc where

import Abc as Abc
import Def as Def

foo :: String
foo = "hi"

bar :: String
bar = Def.qux <> Abc.foo -- `Abc.foo` just turns into `foo` on compilation.

It seems ridiculous at face value, but it's useful in some code-generation scenarios where it's a pain to keep track of the "current" module. If you always blindly import what you need in each place (and if the tree of imports doesn't contain any cycles outside of self-references), then you don't have to do the current-context accounting.

(I'm aware this use-case probably isn't enough to swing it. We're taking advantage of the above behaviour for some Haskell codegen, and switched to open-imports-with-compiler-warnings-disabled for the PureScript side of things until we can do a proper job of the accounting.)