`if` inside `case`

[miniBill]

What the rule should do: Infer the value from a simple case statement to use it to simplify if expressions

What problems does it solve: It helps during refactorings, especially when switching from if to case, and copying the old content inside branches

Example of things the rule would report:

case category of
    "Rock" ->
        if category == "Rock" then a else b
-->
case category of
    "Rock" ->
        a

Should this be part of the Simplify rule or should it be a new rule? Part of simplify. It ties into the inference code.

I am looking for:

• Feedback
• Someone to implement it with/for me

[jfmengels]

We should also handle the opposite, when the case expression is inside the if:

if category == "Rock" then
  case category of
    "Rock" ->
        1
    _ ->
        2
else
   3
-->
if category == "Rock" then
  1
else
  3