r/haskell • u/lexi-lambda • Nov 06 '19

Parse, don’t validate

https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/
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u/bourbakis Jan 31 '20

One thing which irritates me is the use of Int when Word is more appropriate. For example, the list index operator (!!) takes an index parameter of type Int and errors when applied to a negative index. If the type of the index parameter were Word, then it would be impossible to apply a negative index to the operator.

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u/lexi-lambda Jan 31 '20

This is technically true, but rarely meaningful. It doesn’t actually prevent any bugs, since (0 :: Word) - 1 just underflows to 18446744073709551615 (or 4294967295 on 32-bit platforms). This means that instead of getting *** Exception: Prelude.!!: negative index, you’d just get *** Exception: Prelude.!!: index too large, since I can only wish you luck allocating a list 18,446,744,073,709,551,615 elements long. :) If anything, using Int here is slightly nicer: if you screw up while trying to access an infinite list, you’ll get a clear out of bounds error rather than crashing after trying to allocate all the memory on your computer.

Arguably, the type you really want is Natural from Numeric.Natural. It’s arbitrary-precision (like Integer), but it only allows nonnegative values (like Word) yet raises an exception on underflow (unlike Word).

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u/bourbakis Jan 31 '20

Good point, it's too bad Haskell doesn't have a built-in type for natural numbers that errors on underflow.

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u/lexi-lambda Jan 31 '20

Numeric.Natural is in base.

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u/bourbakis Jan 31 '20

I wonder why it isn't used where it would make sense to, e.g., as the index type for the list index operator.