Structuring a cabal file so as to provide a unified `dev` component.

fragment.cabal

-- A demonstration, using a fragment of an unpublished library as-is.

-- Split out fields shared by all components into a common stanza:
common shared
  build-depends:      base
  default-language:   Haskell2010
  default-extensions: ImportQualifiedPost
                      GeneralisedNewtypeDeriving
                      DerivingVia
                      DeriveTraversable
                      LexicalNegation
                      NamedFieldPuns
                      BangPatterns
                      LambdaCase
                      BlockArguments
                      FlexibleInstances
                      FlexibleContexts
                      TupleSections
  ghc-options:        -Wall

-- Split `hs-source-dirs`, `other-modules` and /external/ `build-depends` from each component into common stanzas:

common main
  hs-source-dirs:     src
  build-depends:      cairo
library
  import:             shared, main
  exposed-modules:    Graphics.Rendering.Cairo.Declarative
                      Graphics.Rendering.Cairo.Declarative.Internal

common     sanity
  hs-source-dirs:     test
  other-modules:      Sanity
test-suite sanity
  import:             shared, sanity
  type:               exitcode-stdio-1.0
  main-is:            Main.hs
  build-depends:      cairo-declarative

common     example
  hs-source-dirs:     example
  other-modules:      Example
executable example
  import:             shared, example
  main-is:            Main.hs
  build-depends:      cairo-declarative

-- Finally, introduce a new `dev` component importing all the common stanzas:
library dev
  import:             shared, main, sanity, example
  -- When using `dev`, this warning may be issued, but should be ignored.
  ghc-options:        -Wno-missing-home-modules

-- You can now use `cabal repl dev` to effectively obtain multi-repl, whether
-- directly or in `ghciwatch`. Note that only the listed `other-modules` and
-- their transitive dependencies will be loaded on start, but the others can
-- be loaded on need, as if using `--repl-no-load`.