Here is the identity transducer that does nothing but pretending to be a transducer:
// identity transducer (2x for illustrative purpose)constfoo=f=>(x,y)=>f(x,y);// ^^^^^^^^^^^^^^^^^ partially applied fooconstbar=f=>(x,y)=>f(x,y);// ^^^^^^^^^^^^^^^^^ partially applied bar// reducer (or rather monoid operation)constappend=(x,y)=>x.concat([y]);// function compositionconstcomp=(f,g)=>x=>f(g(x));[1,2,3,4,5].reduce(comp(foo,bar)(append),[]);// [1,2,3,4,5]
The trick is that when we apply comp(foo, bar) (append), foo's f becomes the partially applied bar and bar's f becomes append. reduce then applies the partially applied foo with (x, y), which in turn applies the partially applied bar, which in turn applies append. This is the reason why transducers compose from left-to-right.
I'm honestly still having a hard time wrapping my brain when it comes to this. I mean, it's easy to understand normal compose and pipe flow of data. But, it's challenging for me to 100% track the flow of reducers in the transducers.
Thanks for the example. Took me a lot of reread :)
Ya, it's hard stuff. If we dig even deeper the reason for all this confusing behavior is the composition itself. comp doesn't return a value, but another composed function. I think you already mentioned this in your post. This is sometimes called abstraction over arity, because this new composed function expects more arguments than comp = (f, g) => x => f(g(x)) has originally required. Whenever this happens, the order of evaluation switches to left-to-right, not only with transducers.
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Here is the identity transducer that does nothing but pretending to be a transducer:
The trick is that when we apply
comp(foo, bar) (append),foo'sfbecomes the partially appliedbarandbar'sfbecomesappend.reducethen applies the partially appliedfoowith(x, y), which in turn applies the partially appliedbar, which in turn appliesappend. This is the reason why transducers compose from left-to-right.I'm honestly still having a hard time wrapping my brain when it comes to this. I mean, it's easy to understand normal compose and pipe flow of data. But, it's challenging for me to 100% track the flow of reducers in the transducers.
Thanks for the example. Took me a lot of reread :)
Ya, it's hard stuff. If we dig even deeper the reason for all this confusing behavior is the composition itself.
compdoesn't return a value, but another composed function. I think you already mentioned this in your post. This is sometimes called abstraction over arity, because this new composed function expects more arguments thancomp = (f, g) => x => f(g(x))has originally required. Whenever this happens, the order of evaluation switches to left-to-right, not only with transducers.