[Haskell-beginners] Parse error in pattern

Daniel Fischer daniel.is.fischer at web.de
Sat Feb 27 10:41:17 EST 2010

Am Samstag 27 Februar 2010 15:41:29 schrieb Tom Davie:
> While the advice here is sound 90% of the time, it's evidence of falling
> into the trap of underestimating lazyness.  If 0 and length list are
> both of type Nat, the first will be exactly as fast as the second.
> Bob

As it is, we have

length :: [a] -> Int

Of course, an implementation with a lazy Int type is thinkable, but for the 
time being, it's a pretty safe bet that Int comparisons need complete 
evaluation of the arguments. (In GHC, you can use rewrite rules to avoid 
it, but if you think of that, you wouldn't write "length xs == k" unless 
you really mean it in the first place.)

Most of the uses of "length xs == k" are due to the fact that people are 
used to strict list-like structures carrying their length with them and not 
to singly linked lazy lists of unknown length, I believe.

So I'd say the advice is sound 100% of the time, though often the lists are 
so short that it's not a big deal. Thus the advice may be important less 
than 25% of the time.

Now, genericLength is a different case. "genericLength xs == k" is a 
dangerous thing if used at strict types, but perfectly fine for lazy 

P.S.: Tom, I'm curious, why do you sign as Bob?

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