[Haskell-cafe] Why does this blow the stack?

Don Stewart dons at galois.com
Wed Dec 26 16:54:40 EST 2007

> Since it's possible to support laziness for Integer (while still
> avoiding any stack overflow), I think it makes sense to do so.  What
> if you have some big complicated program like the following:
> x = some big slow computation
> y = [x..]
> lots of code
> z = length $ take 10 y
> Why bother computing x if you don't have to?  And as an added benefit,
> if you keep laziness, you don't have to worry about possibly breaking
> any programs that depend on the current lazy behavior.
> Laziness would NOT make sense for Int, since Int is bounded.  You
> can't tell how long [(x::Int) ..] is without evaluating x.
> On 12/22/07, Don Stewart <dons at galois.com> wrote:
> > People shouldn't be writing code that depends on this!
> One of the main features of Haskell is that it's lazy.  I don't think
> it's wrong to write code that depends on laziness.

Depending on laziness if fine, but depending on undefined strictness semantics
for particular types is more fragile.  Whether Int or Bool or whatever
type has a strict or lazy accumulator in enumFromTo is entirely
unspecified -- you can't look up the report to check.

-- Don

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