the built-in function enumFrom
"Paqui Lucio" <[email protected]> Thu, 29 Oct 2009 11:06:31 +0100
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Hi,=20
In Prelude.hs you can find the definition=20
enumFrom x =3D map toEnum [ fromEnum x ..]=20
Load a file.hs of just one line:=20
data TBool =3D Falsity | Undefined |Truth deriving (Enum, Show)=20
Then, I evaluate=20
enumFrom Falsity=20
and, as expected, it gives=20
[Falsity,Undefined,Truth]=20
However, the expression=20
map toEnum [ fromEnum Falsity ..]=20
gives first an ambiguity problem and then the problem I expected=20
Main> map toEnum [ fromEnum Falsity ..]=20
ERROR - Unresolved overloading=20
*** Type : Enum a =3D> [a]=20
*** Expression : map toEnum (enumFrom (fromEnum Falsity))=20
Main> map toEnum [ fromEnum Falsity ..]::[TBool]=20
[Falsity,Undefined,Truth,=20
Program error: {_toEnum TBool_Falsity 3}=20
(98 reductions, 301 cells)=20
Why the function enumFrom works perfectly in spite of its erroneous
definition?=20
Thanks in advance,=20
Paqui=20
P.D.=20
I am using the=20
|| || || || || || ||__ Hugs 98: Based on the Haskell 98 standard=20
||___|| ||__|| ||__|| __|| Copyright (c) 1994-2002=20
||---|| ___|| World Wide Web: http://haskell.org/hugs=20
|| || Report bugs to: [email protected]=20
|| || Version: Nov 2002=20
---------------------------------=20
Paqui Lucio =20
Dpto de LSI =20
Facultad de Inform=E1tica
Paseo Manuel de Lardizabal, 1
20080-San Sebasti=E1n
SPAIN
---------------------------------
e-mail: [email protected]
Tfn: (+34) (9)43 015049 =20
Fax: (+34) (9)43 015590
Web: http://www.sc.ehu.es/paqui
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<P><SPAN LANG=3D"es"><FONT FACE=3D"Times New Roman">Hi,<BR>
In Prelude.hs you can find the definition<BR>
enumFrom x =3D map toEnum [ fromEnum x ..]<BR>
Load a file.hs of just one line:<BR>
data TBool =3D Falsity | Undefined |Truth deriving (Enum, Show)<BR>
Then, I evaluate<BR>
enumFrom Falsity<BR>
and, as expected, it gives<BR>
[Falsity,Undefined,Truth]<BR>
However, the expression<BR>
map toEnum [ fromEnum Falsity ..]<BR>
gives first an ambiguity problem and then the problem I expected =
</FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT FACE=3D"Times New Roman">Main> map toEnum =
[ fromEnum Falsity ..]<BR>
ERROR - Unresolved overloading<BR>
*** Type : Enum a =3D> [a]<BR>
*** Expression : map toEnum (enumFrom (fromEnum Falsity)) </FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT FACE=3D"Times New Roman">Main> map toEnum =
[ fromEnum Falsity ..]::[TBool]<BR>
[Falsity,Undefined,Truth,<BR>
Program error: {_toEnum TBool_Falsity 3}<BR>
(98 reductions, 301 cells) </FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT FACE=3D"Times New Roman">Why the function =
enumFrom works perfectly in spite of its erroneous definition?<BR>
Thanks in advance,<BR>
Paqui </FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT COLOR=3D"#0000FF" FACE=3D"Times New =
Roman">P.D. </FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT COLOR=3D"#000000" FACE=3D"Times New Roman">I =
am using the<BR>
|| || || || || || ||__ Hugs 98: Based on the Haskell 98 standard<BR>
||___|| ||__|| ||__|| __|| Copyright (c) 1994-2002<BR>
||---|| ___|| World Wide Web:</FONT> </SPAN><A =
HREF=3D"http://haskell.org/hugs"><SPAN LANG=3D"es"><U></U><U><FONT =
COLOR=3D"#0000FF" FACE=3D"Times New =
Roman">http://haskell.org/hugs</FONT></U></SPAN></A><SPAN =
LANG=3D"es"><FONT COLOR=3D"#000000" FACE=3D"Times New Roman"><BR>
|| || Report bugs to: [email protected]<BR>
|| || Version: Nov 2002</FONT> </SPAN>
</P>
<BR>
<P><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier =
New">--------------------------------- </FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier New">Paqui =
Lucio &n=
bsp; &nb=
sp; </FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier New">Dpto de =
LSI &nbs=
p; </FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier New">Facultad de =
Inform=E1tica</FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier New">Paseo Manuel =
de Lardizabal, 1</FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier New">20080-San =
Sebasti=E1n</FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier =
New">SPAIN</FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier =
New">---------------------------------</FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier New">e-mail: =
[email protected]</FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier New">Tfn: (+34) =
(9)43 015049 </FONT></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier New">Fax: (+34) =
(9)43 015590</FONT></SPAN>
<BR><SPAN LANG=3D"en-gb"><FONT SIZE=3D2 FACE=3D"Courier New">Web: =
</FONT></SPAN><A HREF=3D"http://www.sc.ehu.es/paqui"><SPAN =
LANG=3D"en-gb"><U><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Courier =
New">http://www.sc.ehu.es/paqui</FONT></U></SPAN></A><SPAN =
LANG=3D"en-gb"></SPAN>
<BR><SPAN LANG=3D"es"><FONT SIZE=3D2 FACE=3D"Courier =
New">---------------------------------</FONT></SPAN><SPAN =
LANG=3D"es"></SPAN>
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