The central part of transducers are 3-arity reducing procedures.
result-so-far either with or
without transforming it first.
result-so-far and input to produce
a new result-so-far.
In the case of a summing + reducer, the reducer would produce, in
arity order: 0, result-so-far, (+ result-so-far
input). This happens to be exactly what the regular + does.
A transducer is a one-arity procedure that takes a reducer and produces a reducing function that behaves as follows:
A simple example is as following:
(list-transduce (tfilter odd?) + '(1 2 3 4 5))
This first returns a transducer filtering all odd
elements, then it runs + without arguments to retrieve its
identity. It then starts the transduction by passing + to the
transducer returned by (tfilter odd?) which returns a reducing
function. It works not unlike reduce from SRFI 1, but also checks
whether one of the intermediate transducers returns a "reduced" value
(implemented as a SRFI 9 record), which means the reduction finished
early.
Because transducers compose and the final reduction is only executed in
the last step, composed transducers will not build any intermediate
result or collections. Although the normal way of thinking about
application of composed functions is right to left, due to how the
transduction is built it is applied left to right. (compose
(tfilter odd?) (tmap sqrt)) will create a transducer that first filters
out any odd values and then computes the square root of the rest.
Even though transducers appear to be somewhat of a generalisation of
map and friends, this is not really true. Since transducers don’t
know in which context they are being used, some transducers must keep
state where their collection-specific counterparts do not. The
transducers that keep state do so using hidden mutable state, and as
such all the caveats of mutation, parallelism, and multi-shot
continuations apply. Each transducer keeping state is clearly described
as doing so in the documentation.
Reducers exported from the transducers module are named as in their SRFI-1 counterpart, but prepended with an r. Transducers also follow that naming, but are prepended with a t.