# 5. Building existing projects So far, we have been building simple projects. Let us now take an open source package and try to build it. We will be ambitious and use [yackage](https://hackage.haskell.org/package/yackage), a local package server using [Yesod](http://www.yesodweb.com/). To get the code, we will use the `stack unpack` command run from the root directory for all our Haskell projects: ~~~text stack unpack yackage Unpacked yackage-0.8.1 to /yackage-0.8.1/ ~~~ You can also unpack to a directory of your liking instead of the current one by commanding: ~~~text stack unpack yackage --to ~~~ This will create a `yackage-0.8.1` directory inside ``. We will change to that directory, with the command: ~~~text cd yackage-0.8.1 ~~~ ## The `stack init` command This new directory does not have a `stack.yaml` file, so we need to make one first. We could do it by hand, but let us be lazy instead with the `stack init` command: ~~~text stack init # init output ... ~~~ `stack init` does quite a few things for you behind the scenes: * Finds all of the Cabal files in your current directory and subdirectories (unless you use `--ignore-subdirs`) and determines the packages and versions they require * Finds the best combination of snapshot and package flags that allows everything to compile with minimum external dependencies * It tries to look for the best matching snapshot from latest LTS, latest nightly, other LTS versions in that order Assuming it finds a match, it will write your `stack.yaml` file, and everything will work. !!! note The `yackage` package does not currently support Hpack, but you can also use `hpack-convert` should you need to generate a `package.yaml` file. ### Excluded Packages Sometimes multiple packages in your project may have conflicting requirements. In that case `stack init` will fail, so what do you do? You could manually create `stack.yaml` by omitting some packages to resolve the conflict. Alternatively you can ask `stack init` to do that for you by specifying `--omit-packages` flag on the command line. Let us see how that works. To simulate a conflict we will use `acme-missiles-0.3` in `yackage` and we will also copy `yackage.cabal` to another directory and change the name of the file and package to `yackage-test`. In this new package we will use `acme-missiles-0.2` instead. Let us see what happens when we command `stack init` again: ~~~text stack init --force --omit-packages # init failure output ... ~~~ Looking at `stack.yaml`, you will see that the excluded packages have been commented out under the `packages` key. In case wrong packages are excluded you can uncomment the right one and comment the other one. Packages may get excluded due to conflicting requirements among user packages or due to conflicting requirements between a user package and the snapshot compiler. If all of the packages have a conflict with the compiler then all of them may get commented out. When packages are commented out you will see a warning every time you run a command which needs the configuration file. The warning can be disabled by editing the configuration file and removing it. ### Using a specific snapshot Sometimes you may want to use a specific snapshot for your project instead of `stack init` picking one for you. You can do that by using `stack init --snapshot `. You can also init with a compiler snapshot if you do not want to use a Stackage snapshot. That will result in all of your project's dependencies being put under the `extra-deps` section. ### Installing the compiler Stack will automatically install the compiler when you run `stack build` but you can manually specify the compiler by running `stack setup `. ### Miscellaneous and diagnostics _Add selected packages_: If you want to use only selected packages from your project directory you can do so by explicitly specifying the package directories on the command line. _Duplicate package names_: If multiple packages under the directory tree have same name, `stack init` will report those and automatically ignore one of them. _Ignore subdirectories_: By default `stack init` searches all the subdirectories for Cabal files. If you do not want that then you can use `--ignore-subdirs` command line switch. _Cabal warnings_: `stack init` will show warnings if there were issues in reading a Cabal file. You may want to pay attention to the warnings as sometimes they may result in incomprehensible errors later on during dependency solving. _Package naming_: If the `name` field defined in a Cabal file does not match with the Cabal file name then `stack init` will refuse to continue. _User warnings_: When packages are excluded or external dependencies added Stack will show warnings every time the configuration file is loaded. You can suppress the warnings by editing the configuration file and removing the warnings from it. If you command: ~~~text stack build ~~~ you may see something like this: ~~~text Warning: Warnings (added by new or init): Some packages were found to be incompatible with the snapshot and have been left commented out in the packages section. Warning (added by new or init): Specified snapshot could not satisfy all dependencies. Some external packages have been added as dependencies. You can omit this message by removing it from the project-level configuration file. ~~~