[Haskell-beginners] Why getArgs is in the IO monad
michael at orlitzky.com
Mon Apr 14 19:41:03 UTC 2014
On 04/14/2014 03:20 PM, Mike Meyer wrote:
> From: "John M. Dlugosz" <ngnr63q02 at sneakemail.com
> <mailto:ngnr63q02 at sneakemail.com>>
> (And that begs the question of why getArgs needs to be monadic in
> the first place. It
> doesn't change its value; it's a strict constant at run-time, and
> not knowing it at
> compile time is my problem how?)
> Actually, the value of the arguments can be changed - at least on some
> platforms. They are writable from C, if nothing else. What should
> getArgs do if some ffi changes the arguments before it's called?
You can do it right from within Haskell:
Prelude> import System.Environment
Prelude System.Environment> getArgs
Prelude System.Environment> withArgs ["foo"] $ getArgs
It's useful if you have a command-line interface and the user mistypes
something; all you have to do is continue as if they passed "--help"
instead of a separate code path.
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