# Paket 'why' command

In this entry I introduce a brand new command called "why" in Paket dependency manager. It was inspired by a recent project of Facebook, Yarn and aims to display user-friendly reason for a specific package to be under dependency management. The command is available in Paket 3.26.

## Inspiration

Lately Facebook has released a new JavaScript dependency manager on top of NPM. It's called Yarn and as per the docs, it is supposed to provide

"Fast, reliable, and secure dependency management."

The product comes with a plenty of commands, one of which is called "why".

Steffen's tweet was a motiviation for me to contribute once again to Paket, also cause I would find interest in such feature myself.

## Implementation

If a specified package is listed in paket.lock file (which basically means it's under Paket management control), then reason for its presence is one of the following:

  1: 2: 3: 4: 5: 6: 7: 8: 9: 10: 11:  // In context of FAKE project dependencies type Reason = // e.g. Argu - specified in paket.dependencies // is not a dependency of any other package | TopLevel // e.g. Microsoft.AspNet.Razor - specified in paket.dependencies // but also a dependency of other package(s) | Direct of DependencyChain list // e.g. Microsoft.AspNet.Mvc - not specified in paket.dependencies // a dependency of other package(s) | Transitive of DependencyChain list 
1. TopLevel stands for a package which is not a dependency of any other packages controlled by Paket (hence "top-level"). It must be listed in paket.dependencies - we call these "direct" dependencies.
2. Direct is also a direct dependency however contrary to TopLevel, there's at least one other package managed by Paket that depends on Direct. Note that TopLevel will always be a direct package, but Direct won't always be "top-level".
3. Transitive means an "indirect" dependency. The sole reason it's kept track of by Paket is because it's a dependency of some other package.

## Example

As example, let's have a look at dependencies of FAKE project:

 1: 2: 3: 4: 5: 6: 7: 8: 9:  source http://nuget.org/api/v2 nuget Knockout nuget Argu nuget Nancy.Viewengines.Razor nuget Microsoft.AspNet.Razor 2.0.30506 nuget Microsoft.AspNet.WebPages 2.0.30506 // more dependencies ...

The paket.dependencies file for FAKE enlists some dependencies - we say that these are "direct" dependencies.

Dependency resolution for FAKE project can be found in paket.lock:

  1: 2: 3: 4: 5: 6: 7: 8: 9: 10: 11: 12: 13: 14: 15: 16: 17: 18: 19: 20:  NUGET remote: http://www.nuget.org/api/v2 Argu (2.1) AspNetMvc (4.0.20710) Microsoft.AspNet.Mvc (>= 4.0.20710 < 4.1) Knockout (0.0.1) AspNetMvc (>= 4.0) Microsoft.AspNet.Mvc (4.0.40804) Microsoft.AspNet.Razor (>= 2.0.20710) Microsoft.AspNet.WebPages (>= 2.0.20710) Microsoft.AspNet.Razor (2.0.30506) Microsoft.AspNet.WebPages (2.0.30506) Microsoft.AspNet.Razor (>= 2.0.20710) Microsoft.Web.Infrastructure (>= 1.0) Nancy.Viewengines.Razor (1.4.3) Microsoft.AspNet.Razor Microsoft.AspNet.Razor (>= 2.0.30506) - framework: >= net40 Nancy (>= 1.4.3) // more resolution entries ...

Note that above is just an excerpt from the whole file. While paket.lock is very precise on the dependencies resolution, it might be hard for a human to comprehend what is the dependency chain for a given package.

Thanks to the "why" command, we can get a nice overview of dependencies resolved by Paket:

In case of Argu, it turns out to be a top-level (hence also direct) dependency in FAKE.

Microsoft.AspNet.Razor on the other hand is not a top-level dependency. For example there's Microsoft.AspNet.Mvc which depends on Microsoft.AspNet.Razor package. Even though it's not top-level, it is still a direct dependency, because as we saw before it's included into paket.dependencies manifest.

By default "why" command will display only shortest dependency chain from any top-level package down to the specified one. If however we're interested in more details, we can print more details by adding --details argument to the command:

The --details flag displays also information about version requirements as well as framework constraints if any.

What about transitive dependencies? In FAKE project, Microsoft.AspNet.Mvc is an example of transitive dependency:

## Summary

The new "why" command allows to easily determine why a given package is under Paket control. This in turn can help us better understand our dependencies within a project and enable smoother management.

The command is now available in Paket 3.26.

Till next time!

type Reason =
| TopLevel
| Direct of obj
| Transitive of obj

Full name: paketwhycommand.Reason
union case Reason.TopLevel: Reason
union case Reason.Direct: obj -> Reason
type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>
union case Reason.Transitive: obj -> Reason