improve Unsafe

[brainrake]

• add char as discussed in Char constructor is unsafe #24, but stricter, so char "c" can be eg. safely replaced with character literals (eventually), since it only accepts strings of length 1
• re Data.String.Unsafe is very unsafe #25 make all unsafe functions throw an exception right away on bad input, instead of returning invalid values and maybe causing hard-to-debug errors later on

[davidchambers]

You have a superfluous full stop (.) at the end of this sentence.

[garyb]

I see what you mean now, I thought you were talking about using purescript-exceptions before. I like this, we should probably do it anywhere unsafe functions have the possibility to result in error occurring away from the call site.

[davidchambers]

I like this, we should probably do it anywhere unsafe functions have the possibility to result in error occurring away from the call site.

Sounds like a good idea to me.

[brainrake]

Exactly, I was going to suggest that but first I went looking for such occurences.
Throwing new Error will show a backtrace, and including the function name in the exception string will make it even easier to debug.
As far as I can tell, the use case for unsafe functions is the situation when you want to forego proving safety or handling failure. We should make sure that even unsafe functions are unsafe in a controlled manner, and won't crash the code anywhere else but when they're called, and crash with an exception.

[brainrake]

eg Prelude.unsafeIndex just returns undefined
doc:

Note: this function can cause unpredictable failure at runtime if the index is out-of-bounds.

It should throw an exception and the documentation should say so. It will be somewhat slower though.

[brainrake]

I sort of see how unpredictable failure can be a good deterrent from using unsafe functions though.

[brainrake]

Array.Unsafe uses unsafeIndex, an incomplete pattern match, and Data.Maybe.Unsafe.fromJust

[brainrake]

We should also think about the interaction with Exceptions, ie catchException will probably catch exceptions inside unsafe pure code it calls.

[brainrake]

Good to merge though?

[garyb]

👍 thanks!