<div dir="ltr"><div class="markdown-here-wrapper" style=""><p style="margin:0px 0px 1.2em!important">Dear all,</p>
<p style="margin:0px 0px 1.2em!important">I would like to bring our own Richard Eisenberg’s proposal, #283: Local modules, to your attention.<br><a href="https://github.com/ghc-proposals/ghc-proposals/pull/283">https://github.com/ghc-proposals/ghc-proposals/pull/283</a></p>
<p style="margin:0px 0px 1.2em!important">The proposal is an attempt at solving the long debated issue that we can’t export qualified names out of a module (<em>e.g.</em> we may have to repeat <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">import qualified Data.Map as Map</code> and <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">import qualified Data.Set as Set</code> in every module in a project, rather than simply calling <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">import MyPrelude</code> which would take care of both and more), to improve scope management, and sharing names between types in the same files, while making namespaces more convenient to use in the process.</p>
<p style="margin:0px 0px 1.2em!important">It’s a pretty ambitious proposal, with quite a few interlocking pieces. Yet, in my opinion, they fit pretty well together. And I’m, as a potential consumer, very enthusiastic about it.</p>
<p style="margin:0px 0px 1.2em!important">Some highlight (though do read the whole thing):</p>
<ul style="margin:1.2em 0px;padding-left:2em">
<li style="margin:0.5em 0px">We can define modules inside of other modules, with the syntax <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">module Foo (some, optional, exports) where …</code></li>
<li style="margin:0.5em 0px">Like in the rest of Haskell, modules are no more than namespaces. Therefore you can open two <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">module Foo</code>, it will just merge the names in them</li>
<li style="margin:0.5em 0px">Values in local modules can be referred to, qualified, in the encompassing module</li>
<li style="margin:0.5em 0px">Local modules can be exported, importing them will import the name as qualified</li>
<li style="margin:0.5em 0px">A new <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">import module Foo</code> statement can be used in <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">let</code> and <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">where</code> clauses, it brings everything of the form <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Foo.x</code> into scope unqualified for the scope of the <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">let</code> or <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">where</code></li>
</ul>
<p style="margin:0px 0px 1.2em!important">Additionally, there is a small point that is not integral to the rest of the proposal, where defining, say, a type</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code class="hljs language-haskell" style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline;white-space:pre;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important;display:block;overflow-x:auto;padding:0.5em;color:rgb(51,51,51);background:rgb(248,248,248) none repeat scroll 0% 0%"><span class="hljs-typedef"><span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">data</span> <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">T</span></span>
  = <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">A</span>
  | <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">B</span>
</code></pre>
<p style="margin:0px 0px 1.2em!important">Would create a local module named <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T</code>, such that the constructors could be referred to as <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.A</code> and <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.B</code>. I mention it separately, because it causes the only (small) backward incompatibility (that is Haskell 2010 programs which stop compiling when you turn on <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">-XLocalModules</code>).</p>
<p style="margin:0px 0px 1.2em!important">I’m rather in favour of keeping it in, but it’s worth mentioning.</p>
<p style="margin:0px 0px 1.2em!important">I pretty much want all of these features in my daily programming, so, as I said above, I’m very enthusiastic for all of this. And I’m pretty happy with the realisation. Hence my recommending acceptance.</p>
<p style="margin:0px 0px 1.2em!important">/Arnaud</p>
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