Cabal's HTTP Transport Choice Changes Where Proxy Settings Belong
cabal-install can use an external downloader. Its supported proxy behavior then depends on that selected transport and process environment.
A proxy problem in cabal-install can be a question about its selected HTTP transport. Cabal documents a transport choice, and an external curl or Wget downloader brings that tool's proxy interface with it.
Inspect the installed Cabal version and active transport before looking for a universal --no-proxy switch. Then configure the request through the selected downloader's documented environment or options. For public package sources reached through a permitted public HTTP intermediary, the HTTP proxy list offers endpoint candidates.
Do not place an export http_proxy=... shell statement into Cabal's configuration file. That file has its own syntax; it is not executed as your terminal shell.
Also keep repository and proxy addresses distinct. Changing the package repository selects another source of packages. Selecting a forwarding proxy changes the network path used to reach a source.
If a wrapper launches the downloader, inspect that process's actual environment. A variable shown in an unrelated terminal is weak evidence about a build service. Preserve the transport choice in a failure report so the next person knows which client's proxy behavior needs investigation.