<div dir="ltr">Are you speculating that THIH probably intentionally leaves how to resolve the particular instance for e.g. `Show Bool` as an exercise for the reader, instead only implementing that Bool satisfies the shape expected for the class head?</div><div class="gmail_extra"><br><div class="gmail_quote">On 3 May 2017 at 19:59, Brandon Allbery <span dir="ltr"><<a href="mailto:allbery.b@gmail.com" target="_blank">allbery.b@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Doesn't this more or less mirror the problem with instance visibility that leads to the orphan instance problem?</div><div class="gmail_extra"><br><div class="gmail_quote"><div><div class="h5">On Wed, May 3, 2017 at 2:46 PM, Christopher Done <span dir="ltr"><<a href="mailto:chrisdone@gmail.com" target="_blank">chrisdone@gmail.com</a>></span> wrote:<br></div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="h5"><div dir="ltr"><div class="m_-5913802395378429451m_-6643315683928402302markdown-here-wrapper"><p style="margin:0px 0px 1.2em!important">I’ve been working through Typing Haskell in Haskell in detail (<a href="https://gist.github.com/chrisdone/0075a16b32bfd4f62b7b" target="_blank">https://gist.github.com/chris<wbr>done/0075a16b32bfd4f62b7b</a>), and I noticed that if I change e.g. SourcePrelude.hs from the Hackage package (<a href="http://hackage.haskell.org/package/thih" target="_blank">http://hackage.haskell.org/pa<wbr>ckage/thih</a>) to have a declaration 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="m_-5913802395378429451m_-6643315683928402302hljs m_-5913802395378429451m_-6643315683928402302language-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-wrap;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="m_-5913802395378429451m_-6643315683928402302hljs-string" style="color:rgb(221,17,68)">"printZ"</span>,
<span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Just</span> (<span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Forall</span> []
([] :=>
(tString))),
[([],
ap [econst showMfun, econst ((<span class="m_-5913802395378429451m_-6643315683928402302hljs-string" style="color:rgb(221,17,68)">"Z"</span> :>: toScheme (<span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">TCon</span> (<span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Tycon</span> <span class="m_-5913802395378429451m_-6643315683928402302hljs-string" style="color:rgb(221,17,68)">"Nat"</span> <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Star</span>))))])])],
</code></pre>
<p style="margin:0px 0px 1.2em!important">In other words</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="m_-5913802395378429451m_-6643315683928402302hljs m_-5913802395378429451m_-6643315683928402302language-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-wrap;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="m_-5913802395378429451m_-6643315683928402302hljs-title" style="color:rgb(153,0,0);font-weight:bold">printChar</span> = show (<span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Z</span> :: <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Nat</span>)
</code></pre>
<p style="margin:0px 0px 1.2em!important">Where Z is neither defined nor an instance of <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">Show</code>, it type checks just fine. If I replace it with e.g. <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">toScheme (TVar(Tyvar "k" Star))</code> i.e. “some thing of type <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">k</code>“, then it complains that the instance is ambiguous, which is true.</p>
<p style="margin:0px 0px 1.2em!important">So it seems that THIH will accept any type as an instance of any class without complaint as long as all the type variables have been filled in with the correct kind. This came as a surprise to me, as I’m using it in a project. There is apparently code in the type checker that pays attention to the class environment, and you provide instances to it, but it doesn’t seem to care what instances are provided or not.</p>
<p style="margin:0px 0px 1.2em!important">I believe I’ll have to do some extra work to check that the instances actually exist and are full resolved. I can’t tell whether this is a bug or intended behavior. I tried emailing the author of THIH but his email bounced back. Frankly the actual interesting bit of the type checker — the bit where we resolve class instances — is actually the hardest bit for me to read and the bit I’d like more exposition on.</p>
<p style="margin:0px 0px 1.2em!important">I’ve written a tokenizer, parser, renamer and thanks to THIH an inferer, and a stepper in a small dialect of Haskell I’m calling Duet. There’s an example here: <a href="http://chrisdone.com/toys/duet-gamma/" target="_blank">http://chrisdone.com/toys/duet<wbr>-gamma/</a></p>
<p style="margin:0px 0px 1.2em!important">You can see the problem illustrated if you type e.g. the following in the top-left:</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="m_-5913802395378429451m_-6643315683928402302hljs m_-5913802395378429451m_-6643315683928402302language-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-wrap;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="m_-5913802395378429451m_-6643315683928402302hljs-typedef"><span class="m_-5913802395378429451m_-6643315683928402302hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">data</span> <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Nat</span> = <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Z</span> | <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">S</span> <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Nat</span></span>
<span class="m_-5913802395378429451m_-6643315683928402302hljs-title" style="color:rgb(153,0,0);font-weight:bold">main</span> = show <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Z</span>
</code></pre>
<p style="margin:0px 0px 1.2em!important">The type inferred is:</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="m_-5913802395378429451m_-6643315683928402302hljs m_-5913802395378429451m_-6643315683928402302language-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-wrap;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="m_-5913802395378429451m_-6643315683928402302hljs-title" style="color:rgb(153,0,0);font-weight:bold">main</span> = ((show :: (<span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Show</span> <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Nat</span>) => <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Nat</span> -> <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">String</span>) (<span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Z</span> :: <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Nat</span>) :: (<span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Show</span> <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">Nat</span>) => <span class="m_-5913802395378429451m_-6643315683928402302hljs-type" style="color:rgb(68,85,136);font-weight:bold">String</span>)
</code></pre>
<p style="margin:0px 0px 1.2em!important">Which is at least enough information for me to do a lookup and insert an instance dictionary. But I sort of would have expected the type checker to say “no instance”. It does explicitly say that you have to do your own dependency analysis and renamer, but the instance part isn’t clear to me. Oh, and you also have to write your own kind inference for data type declarations. Who knew implementing a Haskell was so involved?</p>
<p style="margin:0px 0px 1.2em!important">So beware future travelers… THIH is awesome, but these are the things I’ve found missing.</p>
<p style="margin:0px 0px 1.2em!important">Ciao!</p>
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