Add -prelude-is flag

I would like GHC to have a -prelude-is flag, which would specify an alternate Prelude to use instead of the default Haskell prelude.

The benefit this would have is that this would give alternate preludes a more first-class status. Instead of having to import an alternative prelude everywhere, you could just have a ghc-options: -prelude-is=... flag in your *.cabal file, and have a different prelude be used. This is important for my own work, as I highly prefer other preludes for my non-library development; I think this is a feature which will be very useful as Haskell develops and we try to figure out how to get rid of the warts in the current Prelude.

Additional discussion here: https://groups.google.com/forum/#!topic/haskell-cafe/NCVzXZcxNmQ

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Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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I'm not against this.

Simon

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I'm sorry I'm late to the conversation, but I wanted to remark that with GHC HEAD, there is an alternative implementation path to this feature, which is reusing the new module renaming support. Essentially, instead of saying -prelude-is, you can say -package "base hiding (Prelude)" -package "my-prelude (MyPrelude as Prelude)". (Well, actually, the hiding modifier is not supported yet, but it wouldn't be hard to add.) Obviously this set of flags is longer, so -prelude-is might still be a handy shortcut, but an FYI!

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ezyang, if I implement this using the module renaming support, will that work if the new prelude MyPrelude isn't in a package? For example, if I just have two files named A.hs and B.hs, the current implementation allows you to write ghc -prelude-is A B.hs to use the module A as a prelude, even though you have no package structure at all.

Not out of the box, but there is a related feature we're planning on adding which is -alias MyPrelude Prelude, which gives a module from any package another name. Of course, you could also just name your local file Prelude...

mentioned in issue #9764 (closed)

Tbh, I wouldn't like seeing the ghc-options: -is-prelude foobar approach spread, as it would be a rather adhoc GHC-specific hack IMO, and also require if impl(ghc >= ...) guards if older GHCs are to be supported

Moreover, I don't understand why the current implicit Prelude approach requires "having to import an alternative prelude everywhere", and how -is-prelude would help here. (I'd also like to point to comment:#9590 (closed):20)

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Related	→ #9590 (closed)

Fwiw, here's a different way how to replace the default Prelude with the current tooling that would IMO reduce the need for an -is-prelude option:

https://github.com/hvr/base-noprelude

The basic idea is to have base sans the Prelude module, and then you just need to provide the Prelude module either locally in the Package, or by importing some other package that provides the Prelude module.

For instance, there's the extended-prelude package example:

https://github.com/hvr/base-noprelude/tree/master/examples/extended-prelude

which can be then used simply by setting build-depends: base-noprelude, extended-prelude

PS: This is basically a variant of ticket:9499#comment:87379 that works already now

Agree with hvr's comments.

An quick-and-dirty hack that also works (I don't entirely understand why it works, but it does) is just to add a file Prelude.hs to your project consisting of

module Prelude (module My.Fancy.Prelude) where
import My.Fancy.Prelude

Apparently this overrides the Prelude module from base for the scope of your project, even for the implicit Prelude import in modules that don't have an explicit one.

closed

Closing, since the Diff is abandoned, and an alternative prelude can already be specified without compiler modifications (see ticket:9499#comment:99924 and ticket:9499#comment:100243).

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