[Haskell-cafe] Newbie vs. laziness

Jules Bean jules at jellybean.co.uk
Tue Mar 20 10:18:06 EDT 2007

Alex Queiroz wrote:
>> However that's a bit clumsy. What kind of error are you seeing when you
>> say 'it crashes'?
>     I must fetch all rows of the result, otherwise the database will
> be in an inconsistent state and all subsequent queries crash the
> application. Fetching all rows automatically finishes the previous
> statement, so I must evaluate the spine of the list.

Odd. That sounds like a suprising library design to me. What's the point 
in returning a lazy list if you then leave everything in an inconsistent 
state until the list is fully evaluated? In that circumstance I'd rather 
have a strict list.

I was intrigued and went and read the library documentation. It says

"...Please note that the careless use of this function can lead to some 
unpleasant behavior...But then, similar caveats apply with hGetContents..."

I don't know if I agree; these caveats seem definably worse than those 
which apply with hGetContents, if they can in fact cause crashes. The 
worse problem with hGetContents is that if someone else writes to that 
file while you're reading, you can see a mixture of old and new data. 
This seems to be far worse: you can't even make two queries unless you 
guarantee you finish the first completely first.

Personally I'd inclined to suggest use of the strict FetchRow which the 
library provides than struggle with explicit uses of seq to fight an 
over-lazy IO action.


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