Can't speak for anyone else, obviously, but part of the reason that I got into functional programming is specifically because I found the syntax very expressive. I felt like I was able to directly express my intent instead of describing a sequence of steps and hope that my intent comes to fruition.
Different strokes and whatnot, not everyone likes functional programming and of course there are valid enough criticisms against it, but I've always appreciated how terse and simple it feels compared to imperative stuff.
Even with regards to testability, if your function is pure and not mucking with IO or something, then even using a multi line lambda shouldn't affect that much. You would test function calling it.
Keep in mind, Haskell doesn't really have "loops" like you'd get in Python; in Python you might not necessarily need the lambda because you might do your one-off logic inside a for loop or something. In Haskell you have map and filter and reduce and recursion, that's basically it.
weeell, you can still do stuff like this in Haskell:
import Data.Vector.Mutable
...
let n = length vec
forM_ [0 .. n-1] $ \i -> do
next <- if i < n-1 then read vec (i+1) else pure 0
modify vec (+ next) i -- increase this value by next value
it's just in many cases the functional machinery is so accessible that you don't need to reach for for-loops.
Different strokes and whatnot, not everyone likes functional programming and of course there are valid enough criticisms against it, but I've always appreciated how terse and simple it feels compared to imperative stuff.
Even with regards to testability, if your function is pure and not mucking with IO or something, then even using a multi line lambda shouldn't affect that much. You would test function calling it.
Keep in mind, Haskell doesn't really have "loops" like you'd get in Python; in Python you might not necessarily need the lambda because you might do your one-off logic inside a for loop or something. In Haskell you have map and filter and reduce and recursion, that's basically it.