[Haskell-cafe] Martin Odersky on "What's wrong with Monads"

[Facundo Domínguez]

> That precedent supports the view that e.g. a command-line flag
> shouldn't affect behavior without the type reflecting it, e.g. by
> doing IO, but the de facto use of the unsafe IO trick means not
> everyone agrees.

For those interested, here's [1] a case where treating command line
arguments as top level constants went wrong.
Look specifically at the section named "Crime Doesn't Pay".

Best,
Facundo

[1] http://www.aosabook.org/en/ghc.html


> Date: Tue, 3 Jul 2012 17:49:48 -0400
> From: Alvaro Gutierrez <radical at google.com>
> Subject: Re: [Haskell-cafe] Martin Odersky on "What's wrong with
>         Monads"
> To: Dominique Devriese <dominique.devriese at cs.kuleuven.be>
> Cc: haskell-cafe at haskell.org
> Message-ID:
>         <CAC6k+tpuiDrtaC_cCFA9ewawDpG1adaht96rgMc_S5wtVJwLVw at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> On Thu, Jun 28, 2012 at 2:53 PM, Dominique Devriese
> <dominique.devriese at cs.kuleuven.be> wrote:
>> 2012/6/27 Tillmann Rendel <rendel at informatik.uni-marburg.de>:
>>> How would you implement this requirement in Haskell without changing the
>>> line "amount (Leaf x) = x"?
>>
>> I may be missing the point here, but having worked on large code bases
>> with a wide variety contributors before, I find it very advantageous
>> that programmers are prevented from writing an amount function whose
>> behaviour depends on command line arguments without at least an
>> indication in the type. The fact that the function can not perform
>> stuff like that is precisely the guarantee that the Haskell type gives
>> me...
>
> I don't think there's an answer that's uniformly right; it depends on
> whether you think of the input to the program, e.g. the environment,
> command-line arguments, etc. as 'constant' and in some sense, pure.
> The latter are constant in the sense that they never change, but they
> are not fixed at compile-time. Other languages effectively treat them
> as pure (by passing them directly to main), whereas Haskell chooses
> not to, which is probably the reason why getArgs has IO in its type
> (something that seems unintuitive at first.)
>
> That precedent supports the view that e.g. a command-line flag
> shouldn't affect behavior without the type reflecting it, e.g. by
> doing IO, but the de facto use of the unsafe IO trick means not
> everyone agrees.
>