Control.Lens.Internal.Doctest

This module exists for the sole purpose of redefining the head and tail functions (which are normally provided by the Prelude) so that they can be used in the doctests of Data.Data.Lens.

The head and tail functions are partial, and as of GHC 9.8, there is a -Wx-partial warning (implied by -Wall) that triggers any time you use either of these functions. This is a fairly reasonable default in most settings, but there are a handful of doctests in Data.Data.Lens that do in fact rely on head and tail being partial functions. These doctests demonstrate that various functions in Data.Data.Lens can recover from exceptions that are thrown due to partiality (see, for instance, the upon function).

One possible workaround would be to disable -Wx-partial. We don't want to disable the warning for all code in lens, however—we only want to disable it for a particular group of doctests. It is rather tricky to achieve this level of granularity, unfortunately. This is because tools like cabal-docspec rely on GHCi to work, and the statefulness of GHCi's :set command means that disabling -Wx-partial might leak into other modules' doctests, which we don't want.

Instead, we opt to redefine our own versions of head and tail here, which do not trigger any -Wx-partial warnings, and use them in the Data.Data.Lens doctests. This has no impact on anyone reading the doctests, as these functions will look indistinguishable from the head and tail functions in the Prelude. One consequence of this design is that we must export the Control.Lens.Internal.Doctest module, as GHCi (and therefore cabal-docspec) won't be able to import it otherwise. Despite this technical oddity, this module should be thought of as internal to lens.