Hi,<br><br><div class="gmail_quote">On 11 March 2012 15:57, Peter Hall <span dir="ltr"><<a href="mailto:peter.hall@memorphic.com">peter.hall@memorphic.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Can someone remind me why I can't do this:<br>
<br>
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data Digit = D0 | D1 | D2 | D3 | D4 | D5 | D6 | D7 | D8 | D9<br>
deriving (Eq, Ord, Show)<br>
<br>
instance Num [Digit] where ... -- This isn't allowed<br></blockquote><div><br></div><div>Why is it not allowed? Yes, it needs FlexibleInstances, but this is a fairly common language extension so shouldn't be a big problem for most use cases.</div>
<div><br></div><div>The error message generated by ghc should tell you about this. Following is what I get for example:</div><div><br></div><div><div><font face="'courier new', monospace">Illegal instance declaration for `Num [Digit]'</font></div>
<div><font face="'courier new', monospace"> (All instance types must be of the form (T a1 ... an)</font></div><div><font face="'courier new', monospace"> where a1 ... an are *distinct type variables*,</font></div>
<div><font face="'courier new', monospace"> and each type variable appears at most once in the instance head.</font></div><div><font face="'courier new', monospace"> Use -XFlexibleInstances if you want to disable this.)</font></div>
<div><font face="'courier new', monospace"> In the instance declaration for `Num [Digit]'</font></div></div><div><br></div><div>HTH,</div><div>Ozgur</div></div><br>