Options | Runtime | Use Show instance for results -- turned off is surprisingly unreliable

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I usually have 'Use Show instance' switched off (aka use 'built-in
printer'), so I can see (at least in crude form) imported or 'system'
datatypes that don't have show instances/also it's a bit more revealing if
I get exceptions.

I have a datatype:

>    data Tree a = Empty
>            | Leaf a
>            | Node (Tree a) a (Tree a)
>            deriving (Eq, Show, Read)

For results of expressions that were `Node`s, the built-in printer just
ignored the first field. (`show`/print of those values works fine.)

>        FoldableTree>  myTree <> fdmap toUpper myTree
>        Node 'c' Empty

>        FoldableTree> print $ myTree <> fdmap toUpper myTree
>        Node (Node Empty 'A' (Node Empty 'C' (Leaf 'a'))) 'c' Empty


Most disconcerting until I realised what was going on. (Of course I don't
expect the built-in printer to produce pretty output).

AntC

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<div dir=3D"ltr"><font face=3D"arial, sans-serif">I usually have &#39;Use S=
how instance&#39; switched off (aka use &#39;built-in printer&#39;), so I c=
an see (at least in crude form) imported or &#39;system&#39; datatypes that=
 don&#39;t have show instances/also it&#39;s a bit more revealing if I get =
exceptions.</font><div><font face=3D"arial, sans-serif"><br></font></div><d=
iv><font face=3D"arial, sans-serif">I have a datatype:</font><div><font fac=
e=3D"arial, sans-serif"><br></font></div><div><font face=3D"arial, sans-ser=
if">&gt;=C2=A0 =C2=A0 data Tree a =3D Empty<br>&gt;=C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0 | Leaf a<br>&gt;=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 | Node (Tree a) a (Tree a) =C2=A0<br>&gt;=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 deriving (Eq, Show, Read)<br></font></div></div><div><font face=
=3D"arial, sans-serif"><br></font></div><div><font face=3D"arial, sans-seri=
f">For results of expressions that were `Node`s, the built-in printer just =
ignored the first field. (`show`/print of those values works fine.)</font><=
/div><div><font face=3D"arial, sans-serif"><br></font></div><div><font face=
=3D"arial, sans-serif">&gt;=C2=A0 =C2=A0 =C2=A0 =C2=A0 FoldableTree&gt; =C2=
=A0myTree &lt;&gt; fdmap toUpper myTree<br>&gt;=C2=A0 =C2=A0 =C2=A0 =C2=A0 =
Node &#39;c&#39; Empty<br></font></div><div><font face=3D"arial, sans-serif=
"><br></font></div><div><font face=3D"arial, sans-serif">&gt;=C2=A0 =C2=A0 =
=C2=A0 =C2=A0 FoldableTree&gt; print $ myTree &lt;&gt; fdmap toUpper myTree=
<br>&gt;=C2=A0 =C2=A0 =C2=A0 =C2=A0 Node (Node Empty &#39;A&#39; (Node Empt=
y &#39;C&#39; (Leaf &#39;a&#39;))) &#39;c&#39; Empty<br><br></font></div><d=
iv><font face=3D"arial, sans-serif"><br></font></div><div><font face=3D"ari=
al, sans-serif">Most disconcerting until I realised what was going on. (Of =
course I don&#39;t expect the built-in printer to produce pretty output).</=
font></div><div><font face=3D"arial, sans-serif"><br></font></div><div><fon=
t face=3D"arial, sans-serif">AntC</font></div></div>

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