Possible to compile less of large dependencies?

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This is bad practice, please don’t do that.

library
  hs-source-dirs:
      src
  exposed-modules:
      Data.Password.Validate
  if flag(argon2)
    exposed-modules:
        Data.Password.Argon2
  if flag(bcrypt)
    exposed-modules:
        Data.Password.Bcrypt
  if flag(pbkdf2)
    exposed-modules:
        Data.Password.PBKDF2
  if flag(scrypt)
    exposed-modules:
        Data.Password.Scrypt

What people forget here is that you can’t enable flags of dependencies through the cabal file.

If someone wrote a library depending on Data.Password.Scrypt, which happens to be in your depgraph, and the user disabled -scrypt, you’ll get an obscure compilation failure. If your feature flag is off by default, then that actually makes your package broken for hackage.

Exposed API should not depend on flags: Educate (or prevent) users from making flag-dependent API · Issue #8128 · haskell/cabal · GitHub

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