<html><head></head><body><div class="yahoo-style-wrap" style="font-family:courier new, courier, monaco, monospace, sans-serif;font-size:13px;"><div dir="ltr" data-setdir="false"><font face="times new roman, new york, times, serif" size="3">Here is a simplified, self-contained version of some code I'm working on. I have a </font><font size="2">Ord</font><font face="times new roman, new york, times, serif" size="3"> type</font> Foo <font size="3" face="times new roman, new york, times, serif">that I want to be ordered primarily by</font> comp1<font face="times new roman, new york, times, serif" size="3">, and, in case of ties, secondarily by</font> comp2<font face="times new roman, new york, times, serif" size="3">. It is important to my users to know that the primary sorting will always be by </font><font size="2">comp1</font><font face="times new roman, new york, times, serif" size="3">. </font><font face="times new roman, new york, times, serif" size="3">Therefore, I am trying to use to the type system to articulate that</font> Foo<font face="times new roman, new york, times, serif" size="3"> <b>must</b> be ordered in a two-fold lexicographic way. My code is</font></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><div><div>module Lex2Test where</div><div><br></div><div>class Ord a => Primary a where</div><div> comp1 :: a -> a -> Ordering</div><div> comp1 = compare</div><div><br></div><div>class Ord a => Secondary a where</div><div> comp2 :: a -> a -> Ordering</div><div> comp2 = compare</div><div><br></div><div>class (Primary a, Secondary a) => Lex2 a</div><div><br></div><div>instance Lex2 a => Ord a where</div><div> compare x y = comp1 x y <> comp2 x y</div><div><font face="times new roman, new york, times, serif" size="3" style="color: var(--text-primary); background-color: var(--background-main);"><br></font></div><div dir="ltr" data-setdir="false"><font face="times new roman, new york, times, serif" size="3">However, when I load the code into ghci, I get</font></div><div dir="ltr" data-setdir="false"><font size="2"><br></font></div><div dir="ltr" data-setdir="false"><div><div><font size="2">Lex2Test.hs:13:10: error:</font></div><div><font size="2"> • The constraint ‘Lex2 a’</font></div><div><font size="2"> is no smaller than the instance head ‘Ord a’</font></div><div><font size="2"> (Use UndecidableInstances to permit this)</font></div><div><font size="2"> • In the instance declaration for ‘Ord a’</font></div><div><font size="2"> |</font></div><div><font size="2">13 | instance Lex2 a => Ord a where</font></div><div><font size="2"> | ^^^^^^^^^^^^^^^</font></div><div style="font-size: medium; font-family: "times new roman", "new york", times, serif;"><br></div></div><font face="times new roman, new york, times, serif" size="3">I feel I must be missing something. </font><font size="2">UndecidableInstances </font><font face="times new roman, new york, times, serif" size="3">seems too extreme for what I am trying to do. (I have never said that I want to go backwards in class inference from </font><font size="2">Ord</font><font face="times new roman, new york, times, serif" size="3"> to </font><font size="2">Lex2</font><font face="times new roman, new york, times, serif" size="3">.)</font></div></div><div dir="ltr" data-setdir="false"><font face="times new roman, new york, times, serif" size="3"><br></font></div><div dir="ltr" data-setdir="false"><font face="times new roman, new york, times, serif" size="3">If this were C++, I would be just trying to implement some virtual functions in terms of other virtual functions.</font></div><div dir="ltr" data-setdir="false"><font face="times new roman, new york, times, serif" size="3"><br></font></div><div dir="ltr" data-setdir="false"><font face="times new roman, new york, times, serif" size="3">Any comments would be appreciated.</font></div><br></div></div></body></html>