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From: Frank Poettgen <Frank.Poett...@Post.RxWyTxH-AyAzCyHzEyN.de>
Newsgroups: comp.lang.haskell
Subject: Re: reads
Date: Fri, 12 Jun 2009 13:34:26 +0200
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Dirk Thierbach schrieb:
> Mark T.B. Carroll <Mark.Carr...@aetion.com> wrote:
>> Frank Poettgen <Frank.Poett...@Post.RxWyTxH-AyAzCyHzEyN.de> writes:
>>
>>> I would expect
>>>
>>> reads "1.2" :: [(Int,String)]
>>>
>>> to be evaluated to
>>>
>>> [(1,".2")]
>>>
>>> but using Hugs (Version Sep 2006) it is just
>>>
>>> []
>>>
>>> Bug or feature?
> 
>> Good question. GHCi, version 6.8.2, agrees with Hugs.
> 
> The reason at least in GHC is that all numeric instances use the same
> function to parse the string. The string is then converted, unless
> conversion fails.

Thanks for your explanation.

> Because of that, one also gets e.g.
>> reads "1e2" :: [(Int,String)]
> [(100,"")]

Really? -Hugs says [].

:type 1e2
100.0 :: Fractional a => a

> AFAIK the report doesn't specify exactly how to parse a number, so I 
> strongly suspect "feature". But I never noticed that before, too.

It can be a problem doing parsing with polymorphism. Consider '.' to be 
a binary infix symbol describing the branching of a tree of arbitrary 
type. Then one gets e.g.

parseTree "'a'.'b'.'c'.'d'" :: (Tree Char)
   /\
  /\d
/\c
ab

parseTree "1.2.3.4" :: (Tree Float)
    /\
   /  \
1.2  3.4

but

parseTree "1.2.3.4" :: (Tree Int)
no parse

You can't parse a tree of type String as well (without escape codes), 
but that's obvious.