<div dir="ltr"><div class="markdown-here-wrapper" style=""><p style="margin:0px 0px 1.2em!important">Hi all,</p>
<p style="margin:0px 0px 1.2em!important">Do we have already a syntax for ‘foo that also contains the typed value like TExp?</p>
<p style="margin:0px 0px 1.2em!important">I have e.g. an AST that I want to do more static checks on it that aren’t as convenient to do in the type system. Here’s an example:</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)"><span class="hljs-comment" style="color:rgb(153,153,136);font-style:italic">-- | Check the grammar spec to produce a grammar.</span>
<span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">checkGrammar</span> :: (<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">SchemaName</span>, [(<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">SchemaName</span>, <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Schema</span>)]) -> <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Q</span> <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Exp</span>
<span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">checkGrammar</span> (toplevel, rules) =
<span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">if</span> <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">M</span>.size rulesMap /= length rules
<span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">then</span> error <span class="hljs-string" style="color:rgb(221,17,68)">"Duplicate rule names in grammar."</span>
<span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">else</span> lift (<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Grammar</span> {grammarToplevel = toplevel, grammarRules = rulesMap})
<span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">where</span>
rulesMap = <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">M</span>.fromList rules
<span class="hljs-comment" style="color:rgb(153,153,136);font-style:italic">-- | Grammar for Haskell.</span>
<span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">grammar</span> :: <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Grammar</span>
<span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">grammar</span> = $(checkGrammar $ runDefine $ <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">mdo</span>
<span class="hljs-comment" style="color:rgb(153,153,136);font-style:italic">-- General expression</span>
expression <- rule <span class="hljs-string" style="color:rgb(221,17,68)">"Expression"</span> (<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">ChoiceSchema</span> [variable, constructor, parentheses
,tuple, <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">let</span>', application, string])
application <- rule <span class="hljs-string" style="color:rgb(221,17,68)">"Application"</span> (<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">CompositeSchema</span> [expression, expression])
parentheses <- rule <span class="hljs-string" style="color:rgb(221,17,68)">"Parentheses"</span> (<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">CompositeSchema</span> [openParenSchema, expression, closeParenSchema])
...
pure expression)
</code></pre>
<p style="margin:0px 0px 1.2em!important">Here I do a trivial check for duplicates. After I’ve checked the<br>expression at compile-time, I Lift it so that it can be used at<br>runtime. That’s pretty good. But some types like (a -> b) don’t Lift. So<br>an alternative would be:</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)"><span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">grammar</span> = $(checkGrammar_take2 thename 'thename)
</code></pre>
<p style="margin:0px 0px 1.2em!important">In which checkGrammar_take2 would:</p>
<ol style="margin:1.2em 0px;padding-left:2em">
<li style="margin:0.5em 0px">Use thename at compile-time for a check.</li>
<li style="margin:0.5em 0px">If the check passes, then return (VarE thename)</li>
</ol>
<p style="margin:0px 0px 1.2em!important">E.g.</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)"><span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">checkGrammar_take2</span> value name = <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">if</span> valueFine value <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">then</span> varE name <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">else</span>
<span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">error</span> <span class="hljs-string" style="color:rgb(221,17,68)">"value isn't fine"</span>
</code></pre>
<p style="margin:0px 0px 1.2em!important">That’s actually quite a good solution because it avoids a lift, and I<br>didn’t transform the AST. It’s also more efficient than lifting.</p>
<p style="margin:0px 0px 1.2em!important">But there’s no checked relationship between thename and ‘thename.<br>checkGrammar_take2 has no way of knowing that they refer to the same<br>thing. See?</p>
<p style="margin:0px 0px 1.2em!important">Hence, if I could get e.g. `thename to produce both the value and a<br>name for the value, that would be cool. My gist doesn’t go this<br>far. It might look like this:</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)"><span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">checkGrammar_take2</span> namedValue = <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">if</span> valueFine (getValue namedValue) <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">then</span> getExp namedValue <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">else</span> error <span class="hljs-string" style="color:rgb(221,17,68)">"value isn't fine"</span>
</code></pre>
<p style="margin:0px 0px 1.2em!important">and call it like:</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)"><span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">mygrammar</span> = checkGrammar_take2 `thename
</code></pre>
<p style="margin:0px 0px 1.2em!important">So the semantics would be roughly similar to</p>
<p style="margin:0px 0px 1.2em!important">[|| thename ||] :: TExp a</p>
<p style="margin:0px 0px 1.2em!important">but you’d get</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)">`thename :: <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Named</span> a
</code></pre>
<p style="margin:0px 0px 1.2em!important">where</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)"><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">Named</span> a = <span class="hljs-container">{ <span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">namedThing</span> :: <span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">a</span>, <span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">nameOfThing</span> :: <span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Name</span> }</span></span>
</code></pre>
<p style="margin:0px 0px 1.2em!important">I feel like the more DSLs I design, the more I’d like something like this to perform my static checks.</p>
<p style="margin:0px 0px 1.2em!important">Cheers,</p>
<p style="margin:0px 0px 1.2em!important">Chris</p>
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