Not sure if this helps, but when you have <br><br>move :: Elements a => a<br><br>It doesn't mean move will return some unknown Elements type, it means you can choose any type which is a member of elements and move will return that type.<br>
<br>This means that<br><code style="background-color: rgb(255, 255, 187);"></code><br>Burn :: Elements a => a<br><br>implies <br><br>Burn :: Water<br><br>Which is wrong.<br><br>3 :: Double<br>3 :: Integer<br><br>are both correct on the other hand.<br>
<br>/J<br><br><br><div class="gmail_quote">On 20 August 2010 10:58, Michael Bradley <span dir="ltr"><<a href="mailto:michael.bradley@hotmail.co.uk">michael.bradley@hotmail.co.uk</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div>
Hello all!<br><br>I have been experimenting with data and class declarations and came across a problem. I have been trying to do this:<br>
<br>
<div style="margin: 5px 20px 20px;">
<div style="margin-bottom: 2px;">Code:</div>
<pre dir="ltr" style="margin: 0px; padding: 6px; border: 1px inset; width: 750px; min-height: 194px; text-align: left; overflow: auto;">data Fire = Burn | Ember<br>data Water = Bubble | WaterGun<br><br>class Elements a<br>
<br>instance Elements Fire<br>instance Elements Water<br><br>data Elemental = Elemental { name :: String,<br> move :: (Elements a) => a<br> }</pre>
</div>So, the idea was that "move" in the data constructor "Elemental"
would be able to take any value from either the type Fire or Water.
However, the error message tells me this is an illegal polymorphic type.<br>
<br>
Therefore, I tried creating a function that could read my value for me
after "show" was applied to the move. Hence, the data declaration for
Elemental could now assign "move" to a String. The function looked like
this:<br>
<br>
<div style="margin: 5px 20px 20px;">
<div style="margin-bottom: 2px;">Code:</div>
<pre dir="ltr" style="margin: 0px; padding: 6px; border: 1px inset; width: 750px; min-height: 50px; text-align: left; overflow: auto;">getMove :: (Elements b) => String -> b<br>getMove x = read x :: (Elements a) => a</pre>
</div>This will not work either, as the function "read" complains of
ambiguity in the letter a. I also tried this (amongst other attempts)<br>
<br>
<div style="margin: 5px 20px 20px;">
<div style="margin-bottom: 2px;">Code:</div>
<pre dir="ltr" style="margin: 0px; padding: 6px; border: 1px inset; width: 750px; min-height: 114px; text-align: left; overflow: auto;">getMove :: (Elements b) => String -> b<br>getMove "Burn" = Burn<br>
getMove "Ember" = Ember<br>getMove "Bubble" = Bubble<br>getMove "WaterGun" = WaterGun<br>getMove _ = error "Unknown move!"</pre>
</div>The above caused the function to infer the type Fire, and then
complain about the type Water. So, how can I either create a function
that can return multiple types like I am trying to above, or is there a
way to adjust the data declaration for Elemental?<br>
<br>
Also, I have noticed that <code style="background-color: rgb(255, 255, 187);">3 :: (Num a) => a</code> will work but <code style="background-color: rgb(255, 255, 187);">Burn :: (Elements a) => a</code> causes an ambiguity error. Why is this the case? <img src="" alt="" title="Confused" border="0"><br>
<br>
Please help!<br><br>Mike<br>
<br> </div>
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<br></blockquote></div><br>