70e8dba1

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Check a repository against one rule

hopinion reads every module of every cabal component and reports what the rules
have to say about it. One rule ships, HsNoCustomShowRead from
haskell-style.md, so that the machinery is exercised by something real rather
than by a demonstration.

The shape it is built on:

A rule says there is something to fix or there is not. There is no third
answer, because a checker whose silence can mean "I could not tell" is a
checker whose silence means nothing. A module that did not parse is a failure
of the run rather than a module with nothing wrong in it, and a module whose
source is a preprocessor's input is counted as unread.

A rule is a value in a registry rather than an instance of a class: no generic
code is polymorphic in a check type, so a record of functions is the right
encoding. Adding a rule is a module and a line in a list, plus a directory of
resources that becomes a golden test per file with no test code to write.

Comments, spans and declarations are read from the token stream and the parse
tree rather than from exact-print annotations, because that placement answers
"where must this be reprinted" and not "what is this about".

Nothing here is built for a rule that does not exist yet. One level, no fact
store, no cross-process split, and nothing reads what the compiler wrote down,
because one module-level rule needs none of them. The build is what holds that
line: -Wunused-packages, -Wunused-top-binds and weeder run as errors, and they
are what removed the last of the machinery this rule turned out not to need.

It cannot be adopted yet, and the rule it ships is the reason: haskell-style.md
allows a written Show for data that is secret, and there is no way to say so.
Suppressions come next, with the rule that makes them unavoidable.

Timeline