> you can fuse any lawful composition of functions

each of those requires manually written rewrite rules or specific library support. It's not a universal property that falls out of laziness - it's careful engineering per data type. Transducers work over any reducing function by construction, not by optimization rules that may or may not fire.

> it's the same problem

It is not. Take a transducer like `(comp (filter odd?) (map inc) (take 5))`. You can apply this to a vector, a lazy seq, a core.async channel, or a custom step function you wrote five minutes ago. The transformation is defined once, independent of source and destination. In Haskell, fusing over a list is one thing. Applying that same composed transformation to a conduit, a streaming pipeline, an io-streams source, and a pure fold requires different code or different typeclass machinery for each. You can absolutely build this abstraction in Haskell (the foldl library gets close), but it's not free - it's a library with design choices, just like transducers are.

You're third claim is basically the "skill issue" defense. Two Haskell Simons - Marlow, and Jones, and also Edward Kmett have all written and spoken about the difficulty of reasoning about space behavior in lazy Haskell. If the people who build the compiler and its core libraries acknowledge it as a real trade-off, dismissing it as FUD from people who "don't know the basics" is not an argument. It's gatekeeping.

Come on, how can you fail to see the difference between: "Haskell can express similar things" with "Haskell gives you this for free"?

Why do you eliminate a library-based solution from the equation if it can actually prove the point that there's no difference in intent as long as my runtime is already lazy by default?

> It is not. Take a transducer like `(comp (filter odd?) (map inc) (take 5))`. You can apply this to a vector, a lazy seq, a core.async channel, or a custom step function you wrote five minutes ago. In Haskell, fusing over a list is one thing. Applying that same composed transformation to a conduit, a streaming pipeline, an io-streams source, and a pure fold requires different code or different typeclass machinery for each.

You can do that only because Clojure doesn't care whether the underlying iterable is to be processed by a side-effectful evaluation. That doesn't negate the fact that the underlying evaluation has a useless notion of "transducer". I said "fuse" in my previous comment to demonstrate that further comptime optimisations are possible that eliminate some transient steps altogether. If you don't need that you can just rely on generic lazy composition of functions that you define once over type classes' constraints.

`IsList` + `OverloadedLists` already exist. Had Haskell had a single type class for all iterable implicitly side-effectful data, you would have got the same singly-written algorithm without a single notion of a transducer. Let that sink in: it's not the transducer that's useful, it the differentiation between pure and side-effectful evaluations that allow your compiler to perform even better optimisations with out-of-order evaluations of pure stuff, as well as eliminating parts of inner steps within the composed step function, as opposed to focusing just on the reducing step-function during the composition. It's not a useful abstraction to have if you care about better precision and advanced optimisations coming from the ability to distinguish pure stuff from non-pure stuff.

Haskell aside, if your goal is to just compose reusable algorithms, a call-by-need runtime + currying + pointfree notation get you covered, you don't need a notion of transducers that exist on their own (outside of the notion of foldable interfaces) to be able to claim exactly the same benefits.

> Two Haskell Simons - Marlow, and Jones, and also Edward Kmett have all written and spoken about the difficulty of reasoning about space behavior in lazy Haskell.

There's a difference between what the people said in the past, and the things the crowd claims the people meant about laziness and space leaks. We can go over individual statements and see if they hold the same "negative" meaning that you say is there.