The purpose of this page is to collect information about issues that arise when users either have an existing cabal project or another nonstandard setup such as a private hackage database.
New users may be confused by the fact that you must add dependencies to the package's cabal file, even in the case when you have already listed the package in the stack.yaml. In most cases, dependencies for your package that are in the Stackage snapshot need only be added to the cabal file. stack makes heavy use of Cabal the library under the hood. In general, your stack packages should also end up being valid cabal-install packages.
Any build command, bench, install, haddock, test, etc. takes a --flag option which passes flags to cabal. Another way to do this is using the flags field in a stack.yaml, with the option to specify flags on a per package basis.
Currently one needs to list all of your modules that interpret flags in the other-modules section of a cabal file. cabal-install has a different behavior currently and doesn't require that the modules be listed. This may change in a future release.
stack init or stack new will try to default to the current Haskell LTS present on https://www.stackage.org/snapshots if no snapshot has been previously used locally, and to the latest LTS snapshot locally used for a build otherwise. Using an incorrect resolver can cause a build to fail if the version of GHC it requires is not present.
In order to override the resolver entry at project initialization one can pass --prefer-lts or --prefer-nightly. These options will choose the latest LTS or nightly versions locally used. Alternatively the --resolver option can be used with the name of any snapshots on Stackage, or with lts or nightly to select the latest versions, disregarding previously used ones. This is not the default so as to avoid unnecessary recompilation time.
Stack has support for packages that reside in remote git locations. Please see the YAML configuration documentation for more information.
Working with a private Hackage is currently supported in certain situations. There exist special entries in stack.yaml that may help you. In a stack.yaml file, it is possible to add lines for packages in your database referencing the sdist locations via an http entry, or to use a Hackage entry.
The recommended stack workflow is to use git submodules instead of a private Hackage. Either by using git submodules and listing the directories in the packages section of stack.yaml, or by adding the private dependencies as git URIs with a commit SHA to the stack.yaml. This has the large benefit of eliminating the need to manage a Hackage database and pointless version bumps.
stack supports intra-package targets, similar to cabal build COMPONENTS for situations when you don't want to build every target inside your package.
Note: this does require prefixing the component name with the package name.