Add unsafePartialBecause

[sharkdp]

While updating code to PureScript 0.9, I find myself using unsafePartial quite a lot. This PR is a proposal to add a function unsafePartialBecause which would allow me to describe why using unsafePartial in a particular occasion is "guaranteed" to be safe:

-- only works for strictly positive input:
g :: Partial => Int -> Int
g n | n > 0 = 1

h :: Int -> Int
h x = unsafePartialBecause "the argument to g is strictly positive" $ g (x * x)

In the unexpected case where the usage of unsafePartial was indeed unsafe (I forgot about the x = 0 case ...), this would raise an exception with the custom error message:

> h 2
1

> h 0

Error: unsafePartial failed. The following assumption was incorrect:
'the argument to g is always positive'.

[sharkdp]

I just noticed that I should probably compare this with the crashWith-approach:

g :: Partial => Int -> Int
g n | n > 0 = 1
g _ = crashWith "g only works for strictly positive input"

h :: Int -> Int
h x = unsafePartial $ g (x * x)

The only difference is that unsafePartialBecause would allow to add a custom comment at the call site.

[garyb]

I like this 👍

[paf31]

Sorry for the delay!