project (and potentially also inject, we haven’t verified`, will cause undefined behavior, which we noticed & verified in a production system.
The bug only appears with optimizations enabled (only tested -O2), not in ghci
I don’t have a minimal repro (yet), but I can describe what we did:
There was one function which loaded some data in IO, constructed a ist of rec, then before returning used project to reduce unnecessary fields in every list element:
loadData :: IO (Record '[ "foo" := Text ])
loadData = do
listOfRes <- <fetch data>
pure
$ map
(project @'["foo" := Text, "bar" := Int] @'["foo" := Text])
listOfResAnd another function that pulled a subset of fields from the record, like this:
requestBody = do
dat <- loadData
let mapDat d = (Json.object [
("foo", get #foo d)
])
pure $ Json.Array (map mapDat dat)Let’s say the final data was
[
{ foo = "xx" },
{ foo = "yy" },
{ foo = "zz" },
]Then what would be actually returned was (!!):
[
{ foo = "xx" },
{ foo = "xx" },
{ foo = "xx" },
]now, if you rewrote the function to deeply evaluate the projected record:
loadData :: IO (Record '[ "foo" := Text ])
loadData = do
listOfRes <- <fetch data>
print listOfRes -- <- deeply evaluate the record by printing it
pure
$ map
(project @'["foo" := Text, "bar" := Int] @'["foo" := Text])
listOfResIt would again return
[
{ foo = "xx" },
{ foo = "yy" },
{ foo = "zz" },
]When we rewrote the original function to construct a new record instead of projecting:
loadData :: IO (Record '[ "foo" := Text ])
loadData = do
listOfRes <- <fetch data>
pure
$ map
(\res -> rcons #foo (get #foo res) rnil)
listOfResThe problem went away.
project(and potentially alsoinject, we haven’t verified`, will cause undefined behavior, which we noticed & verified in a production system.The bug only appears with optimizations enabled (only tested
-O2), not in ghciI don’t have a minimal repro (yet), but I can describe what we did:
There was one function which loaded some data in
IO, constructed a ist ofrec, then before returning usedprojectto reduce unnecessary fields in every list element:And another function that pulled a subset of fields from the record, like this:
Let’s say the final data was
[ { foo = "xx" }, { foo = "yy" }, { foo = "zz" }, ]Then what would be actually returned was (!!):
[ { foo = "xx" }, { foo = "xx" }, { foo = "xx" }, ]now, if you rewrote the function to deeply evaluate the
projected record:It would again return
[ { foo = "xx" }, { foo = "yy" }, { foo = "zz" }, ]When we rewrote the original function to construct a new record instead of
projecting:The problem went away.