<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; color: rgb(0, 0, 0); font-size: 14px; font-family: Calibri, sans-serif; "><div>Looks great; I care and have no improvements to offer; +1 from me.</div><div><br></div><div>Chris</div><div><br></div><span id="OLK_SRC_BODY_SECTION"><div style="font-family:Calibri; font-size:11pt; text-align:left; color:black; BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0in; PADDING-LEFT: 0in; PADDING-RIGHT: 0in; BORDER-TOP: #b5c4df 1pt solid; BORDER-RIGHT: medium none; PADDING-TOP: 3pt"><span style="font-weight:bold">From: </span> Simon Peyton-Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>><br><span style="font-weight:bold">Date: </span> Monday, 14 January 2013 18:09<br><span style="font-weight:bold">To: </span> glasgow-haskell-users <<a href="mailto:glasgow-haskell-users@haskell.org">glasgow-haskell-users@haskell.org</a>><br><span style="font-weight:bold">Subject: </span> Newtype wrappers<br></div><div><br></div><div xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"><meta name="Generator" content="Microsoft Word 14 (filtered medium)"><style><!--
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</o:shapelayout></xml><![endif]--><div lang="EN-GB" link="blue" vlink="purple"><div class="WordSection1"><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; ">Friends<o:p></o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; ">I’d like to propose a way to “promote” newtypes over their enclosing type. Here’s the writeup<o:p></o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "> <a href="http://hackage.haskell.org/trac/ghc/wiki/NewtypeWrappers">
http://hackage.haskell.org/trac/ghc/wiki/NewtypeWrappers</a><o:p></o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; ">Any comments? Below is the problem statement, taken from the above page.<o:p></o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; ">I’d appreciate<o:p></o:p></span></p><p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l2 level1 lfo4"><!--[if !supportLists]--><span style=""><span style="mso-list:Ignore">·<span style="font-style: normal; font-variant: normal; font-weight: normal; font-size: 7pt; line-height: normal; font-family: 'Times New Roman'; ">
</span></span></span><!--[endif]--><span style="font-family: Verdana, sans-serif; ">A sense of whether you care. Does this matter?<o:p></o:p></span></p><p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l2 level1 lfo4"><!--[if !supportLists]--><span style=""><span style="mso-list:Ignore">·<span style="font-style: normal; font-variant: normal; font-weight: normal; font-size: 7pt; line-height: normal; font-family: 'Times New Roman'; ">
</span></span></span><!--[endif]--><span style="font-family: Verdana, sans-serif; ">Improvements to the design I propose<o:p></o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; ">Simon<o:p></o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "><o:p> </o:p></span></p><p class="MsoNormal"><b><span style="font-family: Verdana, sans-serif; ">The problem<o:p></o:p></span></b></p><p>Suppose we have <o:p></o:p></p><pre>newtype Age = MkAge Int<o:p></o:p></pre><p>Then if <tt><span style="font-size:10.0pt">n :: Int</span></tt>, we can convert
<tt><span style="font-size:10.0pt">n</span></tt> to an <tt><span style="font-size:10.0pt">Age</span></tt> thus:
<tt><span style="font-size:10.0pt">MkAge n :: Age</span></tt>. Moreover, this conversion is a type conversion only, and involves no runtime instructions whatsoever. This cost model -- that newtypes are free -- is important to Haskell programmers, and encourages
them to use newtypes freely to express type distinctions without introducing runtime overhead.
<o:p></o:p></p><p>Alas, the newtype cost model breaks down when we involve other data structures. Suppose we have these declarations
<o:p></o:p></p><pre>data T a = TLeaf a | TNode (Tree a) (Tree a)<o:p></o:p></pre><pre>data S m a = SLeaf (m a) | SNode (S m a) (S m a)<o:p></o:p></pre><p>and we have these variables in scope <o:p></o:p></p><pre>x1 :: [Int]<o:p></o:p></pre><pre>x2 :: Char -> Int<o:p></o:p></pre><pre>x3 :: T Int<o:p></o:p></pre><pre>x4 :: S IO Int<o:p></o:p></pre><p>Can we convert these into the corresponding forms where the <tt><span style="font-size:10.0pt">Int</span></tt> is replaced by
<tt><span style="font-size:10.0pt">Age</span></tt>? Alas, not easily, and certainly not without overhead.
<o:p></o:p></p><ul type="disc"><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
For <tt><span style="font-size:10.0pt">x1</span></tt> we can write <tt><span style="font-size:10.0pt">map MkAge x1 :: [Age]</span></tt>. But this does not follow the newtype cost model: there will be runtime overhead from executing the
<tt><span style="font-size:10.0pt">map</span></tt> at runtime, and sharing will be lost too. Could GHC optimise the
<tt><span style="font-size:10.0pt">map</span></tt> somehow? This is hard; apart from anything else, how would GHC know that
<tt><span style="font-size:10.0pt">map</span></tt> was special? And it it gets worse.
<o:p></o:p></li></ul><ul type="disc"><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
For <tt><span style="font-size:10.0pt">x2</span></tt> we'd have to eta-expand: <tt><span style="font-size:10.0pt">(\y -> MkAge (x2 y)) :: Char -> Age</span></tt>. But this isn't good either, because eta exapansion isn't semantically valid (if
<tt><span style="font-size:10.0pt">x2</span></tt> was bottom, <tt><span style="font-size:10.0pt">seq</span></tt> could distinguish the two). See
<a href="http://hackage.haskell.org/trac/ghc/ticket/7542" title="bug: GHC doesn't optimize (strict) composition with id (new)">
#7542</a> for a real life example. <o:p></o:p></li></ul><ul type="disc"><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l3 level1 lfo3">
For <tt><span style="font-size:10.0pt">x3</span></tt>, we'd have to map over <tt><span style="font-size:10.0pt">T</span></tt>, thus <tt><span style="font-size:10.0pt">mapT MkAge x3</span></tt>. But what if
<tt><span style="font-size:10.0pt">mapT</span></tt> didn't exist? We'd have to make it. And not all data types have maps.
<tt><span style="font-size:10.0pt">S</span></tt> is a harder one: you could only map over S-values if
<tt><span style="font-size:10.0pt">m</span></tt> was a functor. There's a lot of discussion abou this on
<a href="http://hackage.haskell.org/trac/ghc/ticket/2110" title="feature request: Rules to eliminate casted id's (new)">
#2110</a>. <o:p></o:p></li></ul><p class="MsoNormal"><span style="font-family: Verdana, sans-serif; "><o:p> </o:p></span></p></div></div></div>
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