title: Stack - How do I (force?) install an old version of a package
body: <p>So I'm trying to install <code>ghc-mod-5.4.0.0</code>, which crashes with a bunch of other installation:</p>
  
  <pre><code>Error: While constructing the build plan, the following exceptions were encountered:
  
  In the dependencies for ghc-mod-5.4.0.0:
      async-2.1.1 must match &lt;2.1 &amp;&amp; &gt;=2.0.2 (latest applicable is 2.0.2)
      cabal-helper-0.7.3.0 must match &lt;0.7 &amp;&amp; &gt;=0.6.1.0 (latest applicable is 0.6.3.1)
      cereal-0.5.4.0 must match &lt;0.5 &amp;&amp; &gt;=0.4 (latest applicable is 0.4.1.1)
      directory-1.3.0.0 must match &lt;1.3 (latest applicable is 1.2.7.1)
      extra-1.5.1 must match ==1.4.* (latest applicable is 1.4.12)
      ghc-8.0.2 must match &gt;=7.4 &amp;&amp; &lt;7.11
      haskell-src-exts-1.18.2 must match &lt;1.18 &amp;&amp; &gt;=1.16.0.1 (latest applicable is 1.17.1)
      pipes-4.3.2 must match ==4.1.* (latest applicable is 4.1.9)
      process-1.4.3.0 must match &lt;1.3 (latest applicable is 1.2.3.0)
      time-1.6.0.1 must match &lt;1.6 (latest applicable is 1.5.0.1)
      transformers-0.5.2.0 must match &lt;0.5 (latest applicable is 0.4.3.0)
  
  Plan construction failed.
  </code></pre>
  
  <p>So how do I force it to install that version? Is there a simple way to purge the stack installation cache or something like that?</p>