@nwolverson -- I'm curious if you have any thoughts on this from the language server side of things; I'm not really sure what is and isn't possible with psc-ide to make this work.

As per discussion on discord I'd suggest avoiding proxying the purs ide requests directly simply for the reason that it likely allows read/write access to the entire filesystem as the current user, and probably allows more significant DoS potential than a more directed endpoint. However returning an unparsed blob of json to be parsed on the client side seems like a legitimate option if it avoids re-encoding results & can reuse code.

It is surprising that the request would take 2s, it would seem the only extra overhead is an HTTP request, you are spawning the purs binary from the shell as well as the Haskell code. Are you actually looking at the same request in the same workspace? I see groupReexports and maxResults on the left, I'd also prefer to see a better timing (eg just directly using time).
Certainly making the socket request that purs ide client makes directly could be faster but that's something to shave off milliseconds I'd assume.

Happy to discuss the details of the completion results themselves later - but that request looks sane at least

This is surprising to me—these are the results from this commit 0bdd1fd (which is back to being a GET request, encoding the JSON on the server)

❯ hyperfine --warmup 1 "curl -Gs --data-urlencode 'q=app' localhost:8081/complete"
Benchmark #1: curl -Gs --data-urlencode 'q=app' localhost:8081/complete
  Time (mean ± σ):     173.3 ms ±  10.2 ms    [User: 7.3 ms, System: 11.3 ms]
  Range (min … max):   155.2 ms … 188.8 ms    16 runs

so, pretty snappy, but removing this line

❯ hyperfine --warmup 1 "curl -Gs --data-urlencode 'q=app' localhost:8081/complete"
Benchmark #1: curl -Gs --data-urlencode 'q=app' localhost:8081/complete
  Time (mean ± σ):      2.339 s ±  0.118 s    [User: 5.0 ms, System: 8.3 ms]
  Range (min … max):    2.209 s …  2.590 s    10 runs

I added that line because the previous iteration with the POST request was hanging and that seemed to fix it, but I don't fully understand the behavior there.

Benchmark #1: curl -s 'http://localhost:8081/complete' -X POST --data-binary '{ "command": "complete", "params": { "filters": [{ "filter": "prefix", "params": { "search": "app" } }] } }' -H 'content-type: application/json'
  Time (mean ± σ):     117.7 ms ±  10.5 ms    [User: 4.7 ms, System: 7.0 ms]
  Range (min … max):   100.6 ms … 137.3 ms    21 runs

Benchmark #1: echo '{ "command": "complete", "params": { "filters": [{ "filter": "prefix", "params": { "search": "app" } }] } }' | purs ide client -p 8082
  Time (mean ± σ):      69.8 ms ±   3.2 ms    [User: 7.3 ms, System: 16.2 ms]
  Range (min … max):    65.1 ms …  81.3 ms    33 runs