Deprecating fromIntegral

Andrew Martin andrew.thaddeus at gmail.com
Tue Sep 26 12:06:14 UTC 2017


I'm -1 on deprecating this, but I would like to see the docs warn about its
behavior.

On Thu, Sep 21, 2017 at 8:24 AM, Niklas Hamb├╝chen <mail at nh2.me> wrote:

> This is not a joke.
>
> I just found a bug in base (https://ghc.haskell.org/trac/ghc/ticket/14262
> ):
>
>   import System.IO
>   hWaitForInput stdin 4294968296
>
> This code should wait for 49.something days, but it waits for 1 second.
>
> It is caused by a quick use of `fromIntegral` to "convert" `Int -> CInt`
> (where CInt = Int32), causing an overflow.
>
> I argue that `fromIntegral` causes unnoticed data corruption without any
> warning, and thus are worse than partial functions.
>
> In this case, this data corruption occurs in the most fundamental code
> that we all use and have to rely upon (`base`).
>
> So I propose that we add a deprecation pragma to `fromIntegral`
> (however, keeping it forever, like Java does, as there's no benefit to
> removing it, we just want that people use safer functions over time),
> and instead provide a `safeFromIntegral` that can only widen ranges, and
> one or more `unsafeFromIntegral` or appropriately named functions that
> can error, wrap, return Maybe, or have whatever desired behaviour when
> the input value doesn't fit into the output type.
>
> For some inspiration, `foundation` provides ideas in that direction:
>
> http://hackage.haskell.org/package/basement-0.0.2/docs/Basement-From.html
> http://hackage.haskell.org/package/basement-0.0.2/docs/
> Basement-IntegralConv.html
>
> It is a bit funny how we argue that Haskell is good for high assurance
> programming, and we build all sorts of fancy constructs to eliminate
> more run-time errors, when in fact we do worse than C in detecting
> errors in the most basic types a computer can use, such as converting
> between 64-bit and 32-bit integers.
>
> Let's fix that.
>
> It avoids real problems in real production systems that are difficult to
> debug when they happen (who writes a unit test checking that a timeout
> of 50 days still works? This is the kind of stuff where you have to rely
> on -- very basic -- types).
>
> I'd like to probe support for such a change before doing any work on it.
>
> Niklas
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-- 
-Andrew Thaddeus Martin
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