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<p class="MsoNormal"><span style="font-size:12.0pt">Herbert<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal">In tests/monadfail/MonadFailErrors.stderr I see<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="Code">MonadFailErrors.hs:59:5: error:<o:p></o:p></p>
<p class="Code"> No instance for (MonadFail Identity)<o:p></o:p></p>
<p class="Code"> arising from a do statement<o:p></o:p></p>
<p class="Code"> with the failable pattern ‘Newtype x’<o:p></o:p></p>
<p class="Code"> In a stmt of a 'do' block: Newtype x <- undefined<o:p></o:p></p>
<p class="Code"> In the expression:<o:p></o:p></p>
<p class="Code"> do { Newtype x <- undefined;<o:p></o:p></p>
<p class="Code"> undefined }<o:p></o:p></p>
<p class="Code"> In an equation for ‘newtypeMatch’:<o:p></o:p></p>
<p class="Code"> newtypeMatch<o:p></o:p></p>
<p class="Code"> = do { Newtype x <- undefined;<o:p></o:p></p>
<p class="Code"> undefined }<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code">MonadFailErrors.hs:67:5: error:<o:p></o:p></p>
<p class="Code"> No instance for (MonadFail Identity)<o:p></o:p></p>
<p class="Code"> arising from a do statement<o:p></o:p></p>
<p class="Code"> with the failable pattern ‘Data x’<o:p></o:p></p>
<p class="Code"> In a stmt of a 'do' block: Data x <- undefined<o:p></o:p></p>
<p class="Code"> In the expression:<o:p></o:p></p>
<p class="Code"> do { Data x <- undefined;<o:p></o:p></p>
<p class="Code"> undefined }<o:p></o:p></p>
<p class="Code"> In an equation for ‘singleConMatch’:<o:p></o:p></p>
<p class="Code"> singleConMatch<o:p></o:p></p>
<p class="Code"> = do { Data x <- undefined;<o:p></o:p></p>
<p class="Code"> undefined }<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="MsoNormal">But ‘Newtype’ and ‘Data’ are single constructor types and hence cannot fail. So why are these errors happening?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’m tiding up your implementation (deleting code!) and accidentally fixed this. So I wanted to check that it is indeed wrong as-is.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
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