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

Conal Elliott conal at conal.net
Fri Jan 2 14:53:42 EST 2009

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

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

* 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"?


  - Conal

On Fri, Jan 2, 2009 at 2:52 AM, Cristiano Paris <frodo at theshire.org> wrote:

> On Tue, Dec 30, 2008 at 8:35 AM, Conal Elliott <conal at conal.net> 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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