weeder/README.md

# Weeder

Weeder is an application to perform whole-program dead-code analysis. Dead code
is code that is written, but never reachable from any other code. Over the
lifetime of a project, this happens as code is added and removed, and leftover
code is never cleaned up. While GHC has warnings to detect dead code is a single
module, these warnings don't extend across module boundaries - this is where
Weeder comes in.

Weeder uses HIE files produced by GHC - these files can be thought of as source
code that has been enhanced by GHC, adding full symbol resolution and type
information. Weeder builds a dependency graph from these files to understand how
code interacts. Once all analysis is done, Weeder performs a traversal of this
graph from a set of roots (e.g., your `main` function), and determines which
code is reachable and which code is dead.

# Using Weeder

## Preparing Your Code for Weeder

To use Weeder, you will need to generate `.hie` files from your source code.

### Cabal

If you use Cabal, this is easily done by adding one line to your
`cabal.project.local` file:

``` cabal
program-options
  ghc-options: -fwrite-ide-info
```

Once this has been added, perform a full rebuild of your project:

``` shell
cabal clean
cabal build all
```

### Stack

If you use `stack`, add the following to your `stack.yaml`:

``` yaml
ghc-options:
  "$locals": -fwrite-ide-info
```

and rebuild:

``` shell
stack clean
stack build
```

### Nix

See [`weeder-nix`](https://github.com/NorfairKing/weeder-nix) for `weeder <-> nixpkgs` integration.

## Calling Weeder

To call Weeder, you first need to provide a configuration file, `weeder.toml`. Weeder uses
[TOML](https://toml.io/en/) as its configuration format.

`roots` is a list of regular expressions of symbols that are considered as
alive. If you're building an executable, the pattern `^Main.main$` is a
good starting point - specifying that `main` is a root. Weeder currently doesn't
add all exported functions as roots automatically but in many cases `main` from a
test suite could be a good workaround for that

`type-class-roots` configures whether or not Weeder should consider all instances
of type classes as roots. Defaults to `false`.

``` toml
roots = [ "^Main.main$" ]
type-class-roots = true
```

Now invoke the `weeder` executable, and - if your project has weeds - you will
see something like the following:

``` shell
$ weeder
src/Dhall/TH.hs:187: toDeclaration
src/Dhall/TH.hs:196: toNestedHaskellType
```

… which indicates the location of two unused symbols.
(Please note these warnings are just for demonstration and not necessarily weeds
in the Dhall project).

## Configuration options

| Name             | Default value                        | Description |
| ---------------- | ------------------------------------ | --- |
| roots            | `[ "Main.main", "^Paths_weeder.*" ]` | Any declarations matching these regular expressions will be considered as alive. |
| type-class-roots | `false`                              | Consider all instances of type classes as roots. Overrides `root-instances`. |
| root-instances   | `[ {class = '\.IsString$'}, {class = '\.IsList$'} ]` | Type class instances that match on all specified fields will be considered as roots. Accepts the fields `instance` matching on the pretty-printed type of the instance (visible in the output), `class` matching on its parent class declaration, and `module` matching on the module the instance is in. |
| unused-types     | `false`                              | Enable analysis of unused types. |

`root-instances` can also accept string literals as a shorthand for writing a table
containing only the `instance` field. See the following example from the test suite:

``` toml
root-instances = [ { module = "Spec.ConfigInstanceModules.Module1", instance = "Bounded T" }
                 , "Read T" 
                 , { module = "Spec.ConfigInstanceModules.Module3" }
                 , { class = '\.Enum$' }
                 , { module = "Spec.ConfigInstanceModules.Module2", class = '\.Show$' }
                 ]
```

## Exit codes

Weeder emits the following exit codes:

| Exit code | Cause |
| --- | --- |
|  0  | No weeds were found |
| 228 | One or more weeds found |
|  1  | Generic failing exit code |
|  2  | Failure to read HIE file due to GHC version mismatch |
|  3  | Failure to parse config file |
|  4  | No HIE files found |

# Tips

- You may want to add `^Paths_.*` to the roots in `weeder.toml` to ignore the
  `Paths_packageName` module automatically generated by Cabal.

- You can automatically write and use a default configuration file by calling 
  Weeder with the `--write-default-config` flag, if no configuration file is
  found.

- You can mandate explicitly specifying every option in the configuration by 
  calling Weeder with the `--no-default-fields` flag. This can prevent being
  caught off guard by new configuration options or changes to default values.

- To mark all instances in a module `M` as roots, add `{ module = "^M$" }`
  to `root-instances`.

# Limitations

Weeder currently has a few limitations:

## Overloaded syntax

On some versions of GHC, Weeder might report various type classes that are used
for syntax extensions as weeds. For example, `Num` and `IsString` classes might be
flagged as weeds if they are only used for overloaded literal syntax (that is,
the `fromInteger` and `fromString` methods).

You can add instances of specific type classes as roots with the `root-instances` 
field, or toggle whether Weeder considers all type class instances as roots with 
the `type-class-roots` configuration option.

## Type families

Weeder cannot yet analyse uses of type family instances. For this reason type
family instances will be marked as implicit roots if analysis of types is
enabled via `unused-types`.

## Template Haskell

Weeder is currently unable to parse the result of a Template Haskell splice. If
some Template Haskell code refers to other source code, this dependency won't be
tracked by Weeder, and thus Weeder might end up with false positives.