How to write complex / multi-line hlint rules?

Question

What's the way to match the following code-pattern...

do 
  x <- createModel a b
  case x of
    Left e -> throwM $ ValidationErrors e
    Right y -> ...

...and suggest the following replacement:

withThrow $ createModel a b

I tried the following, but it doesn't work:

hint: {lhs: "do {x <- createModel v w; case x of Left e -> throwM $ ValidationErrors e}", rhs: "withThrow $ createModel v w"}

Answer 1

The problem is that HLint matching is expression based, whereas the rule you're trying to define is really statement based - you want to match the two statements anywhere adjacently in a do. It's possible HLint could be modified to do that, and you think that would be useful, please raise an HLint issue.