Incorrect foldr hint

[hw202207]

I meant this suggestion

<td>Use foldr</td>
<td>
LHS:
<code>
foldr f z (concat x)
</code>
<br>
RHS:
<code>
foldr (foldr f) z x
</code>

Here is my reasoning

• Assume
  • foldr has type (a -> b -> b) -> b -> [a] -> b for the sake of simpler reasoning.
  • concat :: [[a]] -> [a]
• Given this code foldr f z (concat x), if it compile, we got
  • x :: [[a]]
  • z :: b
  • f :: a -> b -> b
• Assume the RHS foldr (foldr f) z x is correct,
  • foldr f has to have type [a] -> b -> b given x :: [[a]] from previous step.
  • but actually foldr f :: b -> [a] -> b given f :: a -> b -> b from previous step.
• So the RHS could compile if write in this
  • foldr (flip $ foldr f) z x
  • However I dont see this is simpler than the LHS.

Which version report this hint?

• the latest master build but not v3.5

[gruhn]

You're right and I think the same applies to:

Use foldr

LHS: foldr f z (concatMap g x) RHS:foldr (foldr f . g) z x

Suggestion

I removed both rules: #1520