This replaces `prefix :: Absolute -> Path' -> Path` with a couple
alternatives:
- `prefixAbs :: Absolute -> Relative -> Absolute`,
- `maybePrefix :: Path' -> Path' -> Maybe Path'`, and
- `prefix :: Path' -> Relative -> Path'`.
The previous `prefix` could fail to prefix (covered by either the new
`prefix` or `maybePrefix`, depending on whether you want to guarantee
success or capture failure), always threw away the knowledge that the
result was necessarily `Absolute` (covered by `prefixAbs`), and then
always returned an ambiguous result type (covered by all three
replacements).
Then it also provides `prefixRel` as the complement of `prefixAbs` (both
of which are used in the implementation of `prefix`).
Similar changes are made in the replacements for `prefixName :: Absolute
-> Name -> Name`. First, we don’t currently have absolute/relative
variants of `Name`, so we can generalize the first argument to `Path'`.
Then `maybePrefixName :: Path' -> Name -> Maybe Name` exposes the case
where prefixing can’t succeed, and `prefixNameIfRel :: Path' -> Name ->
Name` handles the common case of using the original `Name` if it can’t
be prefixed. Both of these new functions also preserve the `Position` of
the new `Name`, whereas the old implementation always returned a
`Relative` `Name`, despite knowing when it was `Absolute`. And
`prefixName2 :: Path -> Name -> Name` has been removed as there is no
ambiguous variant of `Name` (as `Split` is to `Split'`), so prefixing
with a `Path` isn’t particularly meaningful.
Finally, `nameFromSplit'` is added as a dual to `splitFromName'` to make
it possible to operate on the `Path'` portion of a `Name` without introducing partiality.
These new operations are then propagated through the code, and enable a
couple other type changes: `StructuredArgument.ShallowListEntry` and
`StructuredArgument.SearchResult` now take a `Path'` prefix rather than
the `Path.Absolute` and `Path` prefixes they took previously. This fixes
the absolute `Name` issue in `ls` results.
In a separate part of the change this PR was extracted from,
`Path.prefixName` ensures the result is absolute, but that is more
involved than other changes here, so this uses the previous technique of
calling `Name.makeAbsolute` afterward.
I don’t think this changes anything important, but the `Name` _is_
always absolute after `Path.prefixName`, so this just ensures that it’s
tracked.
This function previously converted the `Name` to a `Path'`, so
recovering the `Name` became partial. It now goes through `Split'`
instead of `Path'`, preserving the guaranteed segment.
