OCamlで例えば次のようなCohttpの簡単なコードでhttpsのサイトをGETしようとしたらエラーが出た。

main.ml

let as_string uri =
  Client.get (Uri.of_string uri) >>= fun (_resp, body) ->
  Cohttp_lwt.Body.to_string body

エラー内容:

Fatal error: exception Failure("No SSL or TLS support compiled into Conduit")
Raised at Lwt.Miscellaneous.poll in file "src/core/lwt.ml", line 3068, characters 20-29
Called from Lwt_main.run.run_loop in file "src/unix/lwt_main.ml", line 31, characters 10-20
Called from Lwt_main.run in file "src/unix/lwt_main.ml", line 118, characters 8-13
Re-raised at Lwt_main.run in file "src/unix/lwt_main.ml", line 124, characters 4-13
Called from Dune__exe__Main in file "src/main.ml", line 18, characters 2-24

opam install lwt_ssl しようとしても次のようなエラーになるのは関係があるかもしれない。

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><>  🐫 
[ERROR] The compilation of conf-libssl failed at "/Users/mube/.opam/opam-init/hooks/sandbox.sh build sh -ex ./homebrew.sh check".

#=== ERROR while compiling conf-libssl.4 ======================================#
# context     2.0.7 | macos/x86_64 | ocaml-base-compiler.4.14.0 | https://opam.ocaml.org#13c823a5
# path        ~/.opam/default/.opam-switch/build/conf-libssl.4
# command     ~/.opam/opam-init/hooks/sandbox.sh build sh -ex ./homebrew.sh check
# exit-code   1
# env-file    ~/.opam/log/conf-libssl-94721-d6d332.env
# output-file ~/.opam/log/conf-libssl-94721-d6d332.out
### output ###
# ++ brew --prefix openssl
# + brew_pkg_config=/usr/local/opt/openssl@3/lib/pkgconfig
# + case "$1" in
# + test 1 '!=' 1
# + export PKG_CONFIG_PATH=/usr/local/opt/openssl@3/lib/pkgconfig:
# + PKG_CONFIG_PATH=/usr/local/opt/openssl@3/lib/pkgconfig:
# + pkg-config --print-errors --exists openssl
# Package openssl was not found in the pkg-config search path.
# Perhaps you should add the directory containing `openssl.pc'
# to the PKG_CONFIG_PATH environment variable
# No package 'openssl' found



<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><>  🐫 
┌─ The following actions failed
│ λ build conf-libssl 4
└─ 
╶─ No changes have been performed

The packages you requested declare the following system dependencies. Please make sure they are installed before retrying:
    openssl

どうやら、自分のMac環境のopensslが古いようだ。brewでupgradeすることで解決した。

brew install openssl