[Hugs-users] Excerpt from "Yet another Haskell Tutorial"

[luc.duponcheel at accenture.com]

Hi,

The very idea of 'pure functional programming'
is that it is 'declarative'
(i.e. works with constants (nothing ever changes)).
As a consequence, given a function 'f' and an input 'x',
evaluating the expression 'f x' should always behave in the same way.

Input/Output is an example of a programming aspect that is not pure in the sense above.
The idea of I/O is (for example) to change information on your file system.

So, a function that appends the string "Foo" to a file 'Foo' and then shows the contents of 'Foo',
will NOT always behave in the same way. If the contents of the file was originally "someString",
then, the first time the function is invoked, it will show "someStringFoo", and
the second time the function is invoked, it will show "someStringFooFoo".

One of the goals of monads is to encapsulate non-pure aspects (side effects)
behind a convenient interface.


Luc


-----Original Message-----
From: hugs-users-bounces at haskell.org on behalf of aldirithms at gmx.net
Sent: Sat 8/5/2006 3:11 PM
To: Hugs-Users at haskell.org
Subject: [Hugs-users] Excerpt from "Yet another Haskell Tutorial"
 
Hello, (and sorry for the preceding accidental empty message),


I'd like to ask a question concerning a following passage in
"Yet another Haskell Tutorial" that ushers in the topic of monads:

<<After all, suppose you have a function that reads a string from the keyboard. Ifyou call this function twice, and the user types something the first time and something else the second time, then you no longer have a function, since it would return two different values.>>

How do I have to interpret this? I cannot make sense of it for I do not see where the definition of a function that I know is violated. I view it that way: The string is just a kind of argument to the string-reading function and that different strings (i.e. different arguments) yield different return values is a commonplace phenomenon with functions, isn't it? How do I have to alter this (over?)simple interpretation to see the point the author wants to make?

Thank you very much in advance.

Christian
-- 


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