Editing Oz (programming language) (section)

=== Laziness ===
Oz uses [[eager evaluation]] by default, but [[lazy evaluation]]<ref name="Lazy Programming">
{{cite journal
| author = Paul Hudak
| author-link = Paul Hudak
| title = Conception, evolution, and application of functional programming languages
| journal = ACM Computing Surveys
| year = 1989
| volume = 21
| number = 3
| pages = 359–411
| doi=10.1145/72551.72554| s2cid = 207637854
}}
</ref> is possible.  Below, the fact is only computed when value of X is needed to compute the value of Y.
<syntaxhighlight lang="erlang">
fun lazy {Fact N}
   if N =< 0 then 1 else N*{Fact N-1} end
end
local X Y in
  X = {Fact 100} 
  Y = X + 1
end
</syntaxhighlight>

[[Lazy evaluation]] gives the possibility of storing truly infinite data structures in Oz. The power of lazy evaluation can be seen from the following code sample:
<syntaxhighlight lang="erlang">
declare
fun lazy {Merge Xs Ys}
   case Xs#Ys
   of (X|Xr)#(Y|Yr) then
      if X < Y then X|{Merge Xr Ys}
      elseif X>Y then Y|{Merge Xs Yr}
      else X|{Merge Xr Yr}
      end
   end
end

fun lazy {Times N Xs}
   case Xs
   of nil then nil
   [] X|Xr then N*X|{Times N Xr}
   end
end

declare H
H = 1 | {Merge {Times 2 H} {Merge {Times 3 H} {Times 5 H}}}
{Browse {List.take H 6}}
</syntaxhighlight>
The code above elegantly computes all the [[Regular Number]]s<ref name="Hamming Numbers">
{{cite journal
|author1=Rao, AC  |author2=Varada Raju, D
 |name-list-style=amp | title = Application of the Hamming number technique to detect isomorphism among kinematic chains and inversions
| journal = Mechanism and Machine Theory
| volume = 26
| number = 1
| pages = 55–75
| year = 1991
  | doi=10.1016/0094-114x(91)90022-v}}
</ref>  in an infinite list. The actual numbers are computed only when they are needed.