Safe Haskell | None |
---|---|

Language | Haskell2010 |

## Synopsis

- class Commute p => CommuteNoConflicts p where
- commuteNoConflicts :: (p :> p) wX wY -> Maybe ((p :> p) wX wY)

- mergeNoConflicts :: (Invert p, CommuteNoConflicts p) => (p :\/: p) wX wY -> Maybe ((p :/\: p) wX wY)

# Documentation

class Commute p => CommuteNoConflicts p where Source #

It is natural to think of conflicting patches `p`

and `q`

as
a parallel pair `p:/:q`

because this is how conflicting patches arise.
But then Darcs comes along and merges them anyway by converting one of
them to a conflictor. Thus, inside a sequence of patches we may see
them as a sequential pair `(p:>q')`

. In that case, `commute`

will always
succeed, as expressed by the `prop_mergeCommute`

law. `commuteNoConflicts`

is a restricted version of `commute`

that should fail in this case but
otherwise give the same result as `commute`

.

Primitive patch types have no conflictors, so for them we have
`commute == commuteNoConflicts`

.

Instances should obey the following laws:

- Symmetry

commuteNoConflicts (p:>q) == Just (q':>p') <=> commuteNoConflicts (q':>p') == Just (p':>q)

- Square-Commute (if an instance

exists)`Invert`

p

commuteNoConflicts (p:>q) == Just (q':>p') => commuteNoConflicts (invert p:>q') == Just (q:>invert p')

`commuteNoConflicts`

is a restriction of`commute`

commuteNoConflicts (p:>q) == Just r => commute (p:>q) == Just r

commuteNoConflicts :: (p :> p) wX wY -> Maybe ((p :> p) wX wY) Source #

An alternative to `commute`

to be used if correctness of your code
depends on the validity of the square-commute law, or to determine
whether patches are in conflict. A parallel pair of patches `p:/:q`

is conflicting if and only if `commuteNoConflicts(p^:>q)`

fails. Its
main use is so that we can define `mergeNoConflicts`

cleanly.

## Instances

PrimPatch prim => CommuteNoConflicts (RepoPatchV1 prim) Source # | |

Defined in Darcs.Patch.V1.Commute commuteNoConflicts :: (RepoPatchV1 prim :> RepoPatchV1 prim) wX wY -> Maybe ((RepoPatchV1 prim :> RepoPatchV1 prim) wX wY) Source # | |

PrimPatch prim => CommuteNoConflicts (RepoPatchV2 prim) Source # | |

Defined in Darcs.Patch.V2.RepoPatch commuteNoConflicts :: (RepoPatchV2 prim :> RepoPatchV2 prim) wX wY -> Maybe ((RepoPatchV2 prim :> RepoPatchV2 prim) wX wY) Source # | |

(SignedId name, StorableId name, PrimPatch prim) => CommuteNoConflicts (RepoPatchV3 name prim) Source # | |

Defined in Darcs.Patch.V3.Core commuteNoConflicts :: (RepoPatchV3 name prim :> RepoPatchV3 name prim) wX wY -> Maybe ((RepoPatchV3 name prim :> RepoPatchV3 name prim) wX wY) Source # |

mergeNoConflicts :: (Invert p, CommuteNoConflicts p) => (p :\/: p) wX wY -> Maybe ((p :/\: p) wX wY) Source #

The non-conflicting merge of `p:/:q`

tries to commute the inverse `p^`

of `p`

with `q`

. If it succeeds then the part of the result that corresponds
to `p^`

is re-inverted. This is also known as a "clean merge".

Note that to maintain consistency in the presence of conflictors we must use
use `commuteNoConflicts`

here and not `commute`

. Otherwise we run into
contradictions as explained below.

Concretely, suppose we use `commute`

here and that `q`

is a conflictor that
represents the primitive patch `r`

and conflicts (only) with (primitive
patch) `p^`

. That is, `q`

results from the conflicted
`merge(r:/:p^)=(s:/:q)`

, where `s`

is another conflictor. Now, according
to `prop_mergeCommute`

we get `commute(p^:>q)=Just(r:>s)`

, and thus
`mergeNoConflict(p:/:q)=Just(s^:/:r)`

in contradiction to our assumption
that `p^:/:q`

are in conflict i.e. `mergeNoConflict(p^:/:q)`

fails. (This
argument takes for granted that the addition of conflictors to prim patches
preserves their commute behavior. This is not yet stated as a law but all
implementations obviously adhere to it.)

As a side note, the fact that we now get an inverse conflictor `s^`

as part
of the result leads to further problems. For instance, whether our repo is
conflicted now depends on the order of patches: `(p:>r)`

is not conflicted,
but its commute `(q:>s^)`

obviously is. In fact, `(q:>s^)`

is nothing else
but the (identity-preserving) "force-commute" of `(p:>r)`

, see the thread at
https:/*lists.osuosl.org*pipermail*darcs-devel*2017-November/018403.html