Re: [Haskell-beginners] mayBe stuck

On Fri, Aug 06, 2010 at 09:40:05AM +0100, Colin Paul Adams wrote:

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"Brent" == Brent Yorgey writes:

Brent> On Thu, Aug 05, 2010 at 06:20:06PM -0500, aditya siram wrote: >> Normally yes, but here we are guaranteed to get a 'Just ...' >> value because of the 'isNothing' guard. -deech

Brent> You are correct, but that's not the point. Every time you Brent> use fromJust (or head, or unsafePerformIO...) you shift the Brent> burden of proving that it is safe from the compiler onto Brent> yourself.

But the compiler could indeed prove that it's safe, if the typing system reflected the precondition.

Sure. But it doesn't. My concern is a pragmatic one rather than theoretical. Maybe for the benefit of other beginners reading this, I should spell it out a bit more clearly rather than trying to be too cute: if you ever find yourself using fromJust, (or head, or any other functions that can sometimes cause your program to crash), you have to convince yourself that such a use is safe -- but this probably means you are doing too much work yourself, rather than letting the compiler do the work for you. You should see if there is a way to refactor your code to use pattern matching, or 'maybe', or 'fromMaybe'. In deech's particular case there was a way to pattern-match on the Maybe value (and also make the code a lot shorter at the same time) rather than using isNothing and fromJust.

Since Haskell allows programming with partial functions, you always have this burden at present.

Yes. But that doesn't mean there isn't value in the discipline of avoiding them. The burden of making sure you never use fromJust (or head, or ...) is much lighter than the burden of proving that every such use is safe. -Brent