[Haskell-beginners] explaining effects

[Alexander Berntsen]

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Rein explains effects well enough. Now for side-effects: a side-effect
is what is allowed to happen in the absence of a weak equivalence
between call-by-name (including memoised versions) and call-by-value[0].

Let's play "spot the side-effect!"

- -- Haskell function for adding two numbers.
f x y = x + y

- -- function in a language that happens to be syntactically like Haskell.
f x y = print "Hallo!" >> x + y

- -- program to evaluate the function.
- -- it just happens to be equal in Haskell and the Haskell-like language.
main = let a = f 4 2 in print a >> print a

evaluation-strategy     | call-by-need | call-by-value
- -------------------------------------------------------------
x + y                   | 6\n6         | 6\n6
print "Hallo" >> x + y  | Hallo!\n6\n6 | Hallo!\n6\nHallo!\n6

See how the latter function differs in call-by-need and call-by-value?
This is because it has a *side-effect* of printing stuff. This is not
allowed in Haskell. And in fact, the latter function is impossible in
Haskell.

As Rein was getting at by abstraction leaks -- side-effects have an
observable interaction with the outside world.

[0]  <https://www.cs.indiana.edu/~sabry/papers/purelyFunctional.ps>
- -- 
Alexander
alexander at plaimi.net
https://secure.plaimi.net/~alexander
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