<div dir="ltr"><div style><span style="font-family:arial,sans-serif;font-size:13px">Ah, so is the idea, then, to use <i>op()</i> when `n` wasn't actually constructed formally, but rather "assembled" by the user, so as to match the type of the accessor function normally supplied as the argument to the constructor?</span></div>
<div style><span style="font-family:arial,sans-serif;font-size:13px"><br></span></div><div><span style="font-family:arial,sans-serif;font-size:13px"><br></span></div><span style="font-family:arial,sans-serif;font-size:13px">On 6/7/2013 4:51 PM, Tom Ellis wrote:</span><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">> On Fri, Jun 07, 2013 at 04:05:09PM -0400, Joe Q wrote:</span><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">>> The phantom parameter solves the same problem as scoped type variables.</span><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">>> Granted, if you find yourself in that kind of polymorphic soup you have</span><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">>> deeper problems...</span><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">> I don't understand this. Scoped type variables are used when you want to</span><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">> use a type variable from the top level within the body of a function. If</span><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">> you use "op" and specify a particular constructor then you don't have a</span><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">> variable but a concrete instance of a type. But maybe I'm missing some more</span><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">> powerful way this can be used ...</span><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">></span><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">> Tom</span><br style="font-family:arial,sans-serif;font-size:13px"><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">You can use scoped type variables to correct an ambiguous type error.</span><br style="font-family:arial,sans-serif;font-size:13px">
<br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">You can think of op as a variation on asTypeOf, as documented here on</span><br style="font-family:arial,sans-serif;font-size:13px">
<a href="http://www.haskell.org/haskellwiki/Scoped_type_variables#Avoiding_Scoped_Type_Variables" target="_blank" style="font-family:arial,sans-serif;font-size:13px">http://www.haskell.org/haskellwiki/Scoped_type_variables#Avoiding_Scoped_Type_Variables</a><span style="font-family:arial,sans-serif;font-size:13px">.</span><br style="font-family:arial,sans-serif;font-size:13px">
<br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">If I tried to come up with an example that's specific to op, it would</span><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">only be horribly contrived.</span><br style="font-family:arial,sans-serif;font-size:13px"><div><span style="font-family:arial,sans-serif;font-size:13px"><br></span></div>
<div><span style="font-family:arial,sans-serif;font-size:13px"><br></span></div></div>