Tags: documentation

Haddock: disambiguating types and values

Haskell has separate namespaces for types and values. When types and data constructors share a name, Haddock, Haskell’s documentation generator, can get confused. In this post I show how to disambiguate types and values in Haddock documentation.

Demonstrating the problem §

For demonstration purposes I created a simple module, ACME.Disamb:

module ACME.Disamb (Foo(..), Bar(..), Quux, Xyxxy) where

data Foo = Foo

-- | A bar contains a 'Foo'.  Example:
--
-- @let bar = 'Bar' 'Foo'@
--
data Bar = Bar Foo

data Quux

class Xyxxy a

Note that Foo is the name of both a type and a data constructor. Same for Bar. Quux is a type with no constructor and Xyxxy is a class. The Haddock for type Bar contains ambiguous references to both Bar and Foo.

Let’s look at the HTML Haddock generated for each top-level declaration:

data Foo §

<div class="top">
  <p class="src">
    <span class="keyword">data</span> <a id="t:Foo" class="def">Foo</a>
    <a href="#t:Foo" class="selflink">#</a>
  </p>
  <div class="subs constructors">
    <p class="caption">Constructors</p>
    <table>
      <tbody>
        <tr>
          <td class="src"><a id="v:Foo" class="def">Foo</a></td>
          <td class="doc empty">&nbsp;</td>
        </tr>
      </tbody>
    </table>
  </div>
</div>

The element representing type Foo has id="t:Foo", whereas the constructor has id="v:Foo". These identifiers can be used as fragment identifiers in hyperlinks. Types and values are disambiguated through the t:… and v:… identifier prefixes.

data Bar §

<div class="top">
  <p class="src">
    <span class="keyword">data</span> <a id="t:Bar" class="def">Bar</a>
    <a href="#t:Bar" class="selflink">#</a>
  </p>
  <div class="doc">
    <p>
      A bar contains a
      <code><a href="ACME-Disamb.html#t:Foo" title="ACME.Disamb">Foo</a></code>. Example:
    </p>
    <pre>let bar = <code><a href="ACME-Disamb.html#t:Bar" title="ACME.Disamb">Bar</a></code> <code><a href="ACME-Disamb.html#t:Foo" title="ACME.Disamb">Foo</a></code></pre>
  </div>
  <!-- constructors elided -->
</div>

Here we can see that all references to Foo and Bar in the documentation I wrote all link to t:Foo or t:Bar. This is not what I intended. The usage example should refer to the data constructors.

In my example this is a minor nuisance, but recall that Foo could be the constructor of some other type. The type Foo could be unrelated!

data Quux §

<div class="top">
  <p class="src">
    <span class="keyword">data</span> <a id="t:Quux" class="def">Quux</a>
    <a href="#t:Quux" class="selflink">#</a>
  </p>
</div>

Quux has no constructor. As a result, there is no element with id="v:…".

class Xyxxy a §

<div class="top">
  <p class="src">
    <span class="keyword">class</span> <a id="t:Xyzzy" class="def">Xyzzy</a> a
    <a href="#t:Xyzzy" class="selflink">#</a>
  </p>
</div>

Type class names inhabit the type namespace. Therefore the corresponding element identifiers also use the t:… prefix.

The solution §

To refer explicitly to a type or value, prefix the reference with t or v. For example:

-- | A bar contains a 'Foo'.  Example:
--
-- @let bar = v'Bar' v'Foo'@
--
data Bar = Bar Foo

The resulting HTML


  <div class="doc">
    <p>
      A bar contains a
      <code><a href="ACME-Disamb.html#t:Foo" title="ACME.Disamb">Foo</a></code>. Example:
    </p>
    <pre>let bar = <code><a href="ACME-Disamb.html#v:Bar" title="ACME.Disamb">Bar</a></code> <code><a href="ACME-Disamb.html#v:Foo" title="ACME.Disamb">Foo</a></code></pre>
  </div>

This feature is available since haddock-2.23.0 (commit). The published user guide is out of date but you can read up-to-date documentation on GitHub.

Inter-module references §

Consider the following module:

-- | See also 'ACME.Disamb.Quux'.
module ACME.Disamb2 where

Unlike references within a module, inter-module references default to the value namespace:

<p class="caption">Description</p>
<div class="doc">
  <p>
    See also
    <code><a href="ACME-Disamb.html#v:Quux" title="ACME.Disamb">Quux</a></code>.
  </p>
</div>

Recall that Quux has no constructor. So the link doesn’t even target the wrong identifier; it targets a non-existent identifier.

The solution is the same: prefix the whole reference with t:

-- | See also t'ACME.Disamb.Quux'.
module ACME.Disamb2 where
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