Re: [Haskell-cafe] There is no null; Maybe/Option types

On Sat, Apr 23, 2011 at 5:57 PM, Ketil Malde wrote:

Evan Laforge writes:

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Most of the (non-IO) runtime errors I get using Haskell software is due to head or fromJust, it's actually quite annoying.

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Just singling this one out because I've heard it before.  I've never gotten a runtime error from these.  It seems quite easy to just not use these functions.

Unfortunately, it is quite easy to just use them, too :-)  Sometimes I just *know* that this list will never be empty, or that that Maybe will never be nothing - but often I'm wrong.

That's the thing I was commenting on... I do a lot of things wrong, but maybe somewhere I'm doing something right, because for whatever reason this never comes up for me. The only times I can think of are 'maximum' that I know will complete because I wrote 'maximum (0 : stuff)' and "split" functions, which are guaranteed to return at least one element. As Henning points out, (a, [a]) is an explicit type for those, but usually I just trust the postcondition of the function and match right away on its output. I have lots of 'maybe [] id' or whatever, but it's always cases where I expect a Nothing so I have an escape route planned.