[Haskell-cafe] efficient operations on immutable structures

Tobias Dammers tdammers at gmail.com
Thu Jul 14 07:29:42 UTC 2016

Another trick is lazy evaluation. Rather than building the actual data
structure fully, Haskell creates a thunk, that is, it keeps a reference to
a recipe that, when evaluated, will produce the desired value. And it does
this recursively. So when you say let foo = Foo bar baz, neither bar nor
baz are necessarily evaluated at all, you just get a recipe for a Foo based
on two other recipes for its fields. Creating another Foo based on this
one, but with one field swapped out for a different value, merely created a
new recipe, and evaluating the new data structure will not descend into the
field values that are no longer used in the recipe.

This stuff takes a while to settle in, but it's actually quite neat.
On Jul 14, 2016 8:27 AM, "Bardur Arantsson" <spam at scientician.net> wrote:

> On 07/14/2016 08:07 AM, Christopher Howard wrote:
> > Hi again all. From some online research, I understand that operations on
> > complex immutable structures (for example, a "setter" function a ->
> > (Int, Int) -> Matrix -> Matrix which alters one element in the Matrix)
> > is not necessarily inefficient in Haskell, because the (compiler?
> > runtime?) has some means of sharing unchanged values between the input
> > and output structure. What I am not clear on, however, is how this
> > works, and how you ensure that this happens. Could someone perhaps
> > elaborate, or point me to a really good read on the subject?
> >
> https://en.wikipedia.org/wiki/Persistent_data_structure
> See in particular the section "Trees". (It basically works the same  for
> records; just imagine that a record is a 2-level tree, where each
> field/label in the record is an edge from the "field label" to the
> "field value". Multiple levels of records work essentially work the same
> as multi-level trees do.)
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