njlr (Jan 12 2024 at 20:38): njlr (Jan 12 2024 at 20:38):Here is my program:
app "automatons"
packages { pf: "https://github.com/roc-lang/basic-cli/releases/download/0.7.1/Icc3xJoIixF3hCcfXrDwLCu4wQHtNdPyoJkEbkgIElA.tar.br" }
imports [pf.Stdout]
provides [main] to pf
Automaton i o : [ Arrow (i -> o), Step (i -> (Automaton i o, o)) ]
# Get the output and next Automaton for a given input
step : i, Automaton i o -> (Automaton i o, o)
step = \input, automaton ->
when automaton is
Arrow f -> (automaton, f input)
Step f -> f input
# Run an Automaton on a list of inputs till completion
run : List i, Automaton i o -> List o
run = \inputs, automaton ->
loop = \remaining, current ->
when remaining is
[] -> []
[x, .. as xs] ->
(next, output) = step x current
List.append (run xs next) output
# loop inputs automaton
List.reverse (loop inputs automaton)
# Automaton that doubles the input
doubler = Arrow (\x -> x * 2)
main =
inputs = [ 1, 2, 3, 4, 5 ]
output = run inputs doubler
outputStr = output |> List.map Num.toStr |> Str.joinWith ", "
Stdout.line outputStr
# Without List.reverse
# 10, 8, 6, 4, 2
# With List.reverse
# 2, 6, 10, 8, 4
# I would expect
# 2, 4, 6, 8, 10
Expected and actual in the comments at the end.
Removing List.reverse gets rid of the weirdness, although is not correct implementation-wise.
njlr (Jan 12 2024 at 20:40):Unless I'm missing something stupid I can file on GitHub
Brendan Hansknecht (Jan 13 2024 at 00:23):I think it is working correctly:
[/tmp/test.roc:50] list = []
[/tmp/test.roc:52] out = []
[/tmp/test.roc:50] list = [10]
[/tmp/test.roc:52] out = [10]
[/tmp/test.roc:50] list = [10, 8]
[/tmp/test.roc:52] out = [8, 10]
[/tmp/test.roc:50] list = [8, 10, 6]
[/tmp/test.roc:52] out = [6, 10, 8]
[/tmp/test.roc:50] list = [6, 10, 8, 4]
[/tmp/test.roc:52] out = [4, 8, 10, 6]
[/tmp/test.roc:50] list = [4, 8, 10, 6, 2]
[/tmp/test.roc:52] out = [2, 6, 10, 8, 4]
looks like an impl bug for your code
Brendan Hansknecht (Jan 13 2024 at 00:23):probably reversing more than you want
njlr (Jan 13 2024 at 08:49):In my code above, if I swap these two lines (in the run function):
List.reverse (loop inputs automaton)
# 2, 6, 10, 8, 4
loop inputs automaton
# 10, 8, 6, 4, 2
Then the final output is not the reverse of the other, which I think is odd.
Maybe I'm missing something?
Hannes Nevalainen (Jan 13 2024 at 10:08):You are recursively calling your “run” function, so you are doing a lot of reversing.
Not just on the final list as I think you intended.
Try putting the reverse inside
main instead.
njlr (Jan 13 2024 at 10:16):Ah yes! Thank you
I was recursively calling run where I should have been calling loop
This works:
run : List i, Automaton i o -> List o
run = \inputs, automaton ->
loop = \remaining, current ->
when remaining is
[] -> []
[x, .. as xs] ->
(next, output) = step x current
List.append (loop xs next) output
loop inputs automaton
|> List.reverse
njlr has marked this topic as resolved.
Last updated: Jul 26 2025 at 12:14 UTC