Neil wrote:
Laziness. It is very difficult to have a lazy language which is not
pure.
Exactly.
Laziness and purity together help with equational reasoning, compiler
transformations, less obscure bugs, better compositionality etc.
Related, but nevertheless a shameless plug: Jurriaan Hage and myself
did some thinking about this lately and put some stuff on paper that I
presented two days ago at PEPM [1].
Here's a quote, taken from the introduction:
"Functional programming languages can be classified along several
axes: we can distinguish between pure and impure langauges as well as
between langauges with strict and nonstrict semantics. In practice,
not all combinations make sense. Nonstrict languages better be pure,
because reasoning about unrestricted side-effects becomes more
complicated when the order of evaluation gets less predictable."
"Purity has some clear advantages. For example, it enables
equational reasoning and it opens the road to memoization, common
subexpression elimination, and parallel evaluation strategies. The
driving force that enables these opportunities is referential
transparency: in a pure language, each of a program's terms can, at
any time, be replaced by its value without changing the meaning of the
program as a whole."
Cheers,
Stefan
-----
[1] Jurriaan Hage and Stefan Holdermans. Heap recycling for lazy
languages. In John Hatcliff, Robert Glück, and Oege de Moor, editors,
_Proceedings of the 2008 ACM SIGPLAN Symposium on Partial Evaluation
and Semantics-Based Program Manipulation_, PEPM'08, San Francisco,
California, USA, January 7--8, 2008, pages 189--197. ACM Press, 2008. http://doi.acm.org/10.1145/1328408.1328436
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