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<p>Constraints aren't considered when instance selection is
performed. I.e. you can't have both:<br>
instance Foo a where ...<br>
instance Show a => Foo a where ....<br>
</p>
<p>But you can use default method signatures:
<a class="moz-txt-link-freetext" href="https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#default-method-signatures">https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#default-method-signatures</a><br>
</p>
<br>
<div class="moz-cite-prefix">On 29/06/2017 19:27, Jean Lopes wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAKeoKsh2RogDHp7h50hf6Lf11fn_iDnNT+1yyGOqfJb=K1naFg@mail.gmail.com">
<div dir="auto">It appear more like Show, you can add a
constraint to the generic instance
<div dir="auto"><br>
</div>
<div dir="auto">instance Show a => Foo a where</div>
<div dir="auto"> bar x = show x<br>
<div dir="auto"><br>
</div>
<div dir="auto"><br>
</div>
<div dir="auto"><br>
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</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">Em 29 de jun de 2017 2:16 PM, "Silent
Leaf" <<a href="mailto:silent.leaf0@gmail.com"
moz-do-not-send="true">silent.leaf0@gmail.com</a>>
escreveu:<br type="attribution">
<blockquote class="quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">hi,
<div><br>
</div>
<div>say i have the following typeclass:</div>
<div><br>
</div>
<div>class Foo a where</div>
<div> bar :: a -> String</div>
<div><br>
</div>
<div>looks a lot like the Read typeclass, right? (at least
i think it should?)</div>
<div>well say it's a different meaning (in other terms i
can't or do not want to use Read, but i'd like to
implement a default version of bar for those instances
that also implement Read. is there a way to do so?</div>
</div>
<br>
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