Pattern synonyms rules

[xsebek]

I would like to use HLint to suggest using custom pattern synonyms.

My use case is that swarm defines several patterns for matching types in its language and I would like HLint to remind me about them when pattern matching.

Another less important example is the pattern for pressed Key:

pattern Key c = VtyEvent (EvKey (KChar c) [])

I would like to write a HLint rule like this:

- warning: {lhs: "VtyEvent (EvKey (KChar c) [])", rhs: "Key c"}

But it only works on the right-hand side and not on the left:

f (VtyEvent (EvKey (KChar c) [])) = c
-- No warning.

g c = VtyEvent (EvKey (KChar c) [])
-- Warning: Use Key
-- Found:
--   VtyEvent (EvKey (KChar c) [])
-- Perhaps:
--   Key c

I tried including the function and right-hand side in the rule:

- warning: {lhs: 'f (VtyEvent (EvKey (KChar c) [])) = x', rhs: 'f (Key c) = x'}  # does not parse

But HLint said that Aeson would not parse = (or ->).

Is there a way to write this rule now or would that be a new feature?

[ndmitchell]

Currently the LHS/RHS matches are restricted to expressions. People have asked for this to be generalised to types, #449, and doing so for patterns too would be a reasonable extension.