Proc desugaring PSA

[cfhammill]

The proc notation introduced by the Arrows language extension desugars in a very inefficient way, adding many irrelevant steps to the computation graph. A simple example, here a is the proc version and b is the point-free variant

{-# LANGUAGE Arrows, OverloadedStrings #-}
import Control.Funflow
import Control.Funflow.Pretty
import Path
import Data.Default

a :: SimpleFlow (Path a Dir) (String)
a = proc d -> do
  stepIO' (def {name = Just "reading file"}) (readFile . toFilePath) -< d

b :: SimpleFlow (Path a Dir) (String)
b = stepIO' (def {name = Just "reading file"}) (readFile . toFilePath)

print $ ppFlow a
print $ ppFlow b

Which outputs

-- a
(unlabeled step >>> (unlabeled step >>> (unlabeled step >>> (unlabeled step >>> reading file))))
-- b
reading file

There is a library arrowp-qq that provides a quasiquoter version of the proc notation that does a better job of desugaring

{-# LANGUAGE TemplateHaskell, QuasiQuotes #-}
import Control.Arrow.QuasiQuoter

c :: SimpleFlow (Path a Dir) (String)
c =  [proc| d -> do
              stepIO' (def {name = Just "reading file"}) (readFile . toFilePath) -< d
              |]

print $ ppFlow c

(unlabeled step >>> reading file)

unfortunately the package hasn't been actively developed recently, so I had to modify it slightly to get it work.

For rendering workflows as graphs, using something like the quasiquoter proc notation may be useful for generating human readable graphs.

[dorranh]

I'm actually not sure if this issue is addressed now that we are using kernmantle's Rope to represent flows in funflow2, and it is worth investigating.