[Haskell] Re: Why is getArgs in the IO monad?

Jules Bean jules at jellybean.co.uk
Tue Jan 18 03:10:45 EST 2005

On 18 Jan 2005, at 06:31, Jim Apple wrote:

> Tomasz Zielonka wrote:
>> I like to think that pure functions don't change between executions.
> I'd like to think they wouldn't change within executions. Is there a 
> pure haskell way to check the value of a function between exections?

In principle, a haskell compiler might notice that a function is always 
called with a particular argument, and inline the result of the 
function at compile-time.

For example, you might well expect a program containing the subterm 
"3+4" to be compiled into machine code which directly uses the constant 
value 7. Certainly most decent C compilers will perform this 

Such an optimisation, depends on the knowledge that "3+4" is always the 
same value: i.e. that a pure function, given the same arguments, always 
returns the same value (even between executions).

More sophisticated optimisation transforms might also depend on the 
same property.

Whether GHC actually performs an optimisation along these lines I have 
no idea.


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