> Hi
> I am new to Haskell and have a question about types.
>
> I have defined a type Type
>
> data Type = D1 [Type]
> | D2 String String [Type]
> | D3 String String
> | D4 ........
> | ........
> | D14
> deriving (Show)
>
> Where D4 - D14 each have 2-5 String
>
> (I am using it together with the parser combinators from ParseLib
> if that makes any difference.)
>
> Hugs98 complains that it can not derive Show after 40 iterations.
> I have to change the cutoff limit to 100 before it can derive Show.
I can confirm this behaviour
> In the Hugs manual it says that
> 1, The default is 16
> 2, "We have not yet seen any examples of valid Hugs programs that
> are rejected with this setting. It is possible to construct
> artificial programs thatdo require higher values, but such examples
> are pathological and they do not seem to appear in practice.)"
>
> So
> 1, If 16 is enough for all "normal" cases, why is the default on
> my system 40?
> 2, What makes my, (simple), Type pathologcal?
I cant answer these questions, but the following
works:
data DTag = D1
| D2
| D3
| D4
| D5
| D6
| D7
| D8
| D9
| D10
| D11
| D12
| D13
| D14
deriving (Show)
data Dtype = Dt1 DTag String
| Dt2 DTag String String
| Dt3 DTag String String String
| Dt4 DTag String String String String
| Dt5 DTag String String String String String
deriving (Show)
.But this will be much clumsier than the desired
type declaration.
Strange. I have built several toy parsers using
ParseLib, and I have not come across this problem.
Incidentally, you should check out Doatse Swierstra's
parsing combinators, which are *very* impressive:
http://www.cs.uu.nl/groups/ST/Software/Parse/
(It took me a few days to work out how to use them,
but once you have done that, they are very quick to
use)
Regards,
Rob MacAulay
Rob MacAulay Vulcan Asic
email : [EMAIL PROTECTED]
http : www.vulcanasic.com
Tel +[44] 1763 247624 (direct)
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