Re: [Haskell-cafe] What are side effects in Haskell?

Granting the "everything is a function" idea the best intent, maybe what it's trying to express is the related but different notion that everything *can be* a function -- i.e. that any value can be replaced by a function which yields a value of the appropriate type. Alternately, it could be a confusion between the untyped lambda calculus, in which everything really is a function, and the typed lambda calculus, in which things are more complicated? --S. On Jan 2, 2009, at 2:53 PM, Conal Elliott wrote:

Hi Cristiano,

Similarly, any value can be thought of as a list (perhaps singleton), a Maybe (Just), a pair ((,) undefined or (,) mempty), an IO (return), ... (any Applicative).

And yet I've heard "everything is a function" on several occasions, but not these others. Hence my (continuing) puzzlement about the source of that idea. I have some speculations:

* In pure "OO" programming, everything is an object, so in pure "functional" programming, one might assume everything is a function. I find the term "value-oriented programming" a more accurate label than "functional programming".

* C has definitions for functions but assignments for other types. Since pure functional languages eliminate assignment, one might assume that only functions remain. (I also hear people refer to top-level definitions in a Haskell module as "functions", whether they're of function type or not.)

Are there other thoughts & insights about the source of the idea that "everything is a function"?

Thanks,

- Conal

On Fri, Jan 2, 2009 at 2:52 AM, Cristiano Paris wrote: On Tue, Dec 30, 2008 at 8:35 AM, Conal Elliott wrote:

...

...

Everything in Haskell is a function [...]

Where did this idea come from?

I'd say every expression in Haskell denotes a pure value, only some of which are functions (have type a->b for some types a & b).

Maybe more formally correct, but my statement still holds true as any values can be tought as constant functions, even those representing functions themselves.

Cristiano

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