reexport IsList class from a trustworthy module

Motivation

GHC's Safe Haskell extension relies to a large extent on inferring safety of modules automatically. As a concrete example, data-default used to be inferred safe.

This is no longer the case, because dlist-0.7.1 added an IsList instance for difference lists. The only way to do that currently is to import the relevant parts of GHC.Exts, and that module is not safe.

Of course the resulting non-safety of the Data.DList module can be fixed by adding a TrustWorthy pragma to the module and trusting the dlist package. But that is unsatisfactory, because there is nothing unsafe about providing an instance for the IsList class. This issue is bound to pop up in other packages as well.

Proposed Solution

(this is one of many possibilities)

Add a GHC.Exts.Safe module that is marked TrustWorthy and reexports the IsList class, and possibly other things from GHC.Exts that are safe to use.

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	core-libraries-committee@haskell.org
Operating system
Architecture

changed weight to 5

added Tfeature request Trac import core libraries labels

I agree that this deserves some attention. I always feel dirty importing GHC.Exts, and the non-dirty things from there should have other places to be pulled from. I'm specifically thinking of Constraint, but I'm sure there are things other than just IsList and Constraint that should find new homes.

Trac metadata

Trac field	Value
CC	core-libraries-committee@haskell.org, eir@cis.upenn.edu, oerjan → core-libraries-committee@haskell.org, dterei, eir@cis.upenn.edu, ekmett, oerjan

added 1 deleted label

This issue also seems to block marking the `sorted-list` package as safe. Does the IsList class (and its instances) depend on any unsafe functionality? If not, could a new marked-Safe module named GHC.Exts.List be created so that people can safely import this subset? GHC.Exts.List could then be re-exported from GHC.Exts for backwards-compatibility.

Trac metadata

Trac field	Value
CC	core-libraries-committee@haskell.org, dterei, eir@cis.upenn.edu, ekmett, oerjan → core-libraries-committee@haskell.org, dterei, eir@cis.upenn.edu, ekmett, kostmo, oerjan

added Safe Haskell label

added Pnormal label

mentioned in issue #19127 (closed)

This was done in !8124 (merged).

closed