Y combinator: Difference between revisions
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given Args satisfies Anything[]
=> flatten((Args args) => f(y3(f))(*args));</lang>
=={{header|Chapel}}==
Strict (non-lazy = non-deferred execution) languages will race with the usually defined Y combinator (call-by-name) so most implementations are the Z combinator which lack one Beta Reduction from the true Y combinator (they are call-by-value). Although one can inject laziness so as to make the true Y combinator work with strict languages, the following code implements the usual Z call-by-value combinator using records to represent closures as Chapel does not have First Class Functions that can capture bindings from outside their scope other than from global scope:
{{works with|Chapel version 1.24.1}}
<lang chapel>proc fixz(f) {
record InnerFunc {
const xi;
proc this(a) { return xi(xi)(a); }
}
record XFunc {
const fi;
proc this(x) { return fi(new InnerFunc(x)); }
}
const g = new XFunc(f);
return g(g);
}
record Facz {
record FacFunc {
const fi;
proc this(n: int): int {
return if n <= 1 then 1 else n * fi(n - 1); }
}
proc this(f) { return new FacFunc(f); }
}
record Fibz {
record FibFunc {
const fi;
proc this(n: int): int {
return if n <= 1 then n else fi(n - 2) + fi(n - 1); }
}
proc this(f) { return new FibFunc(f); }
}
const facz = fixz(new Facz());
const fibz = fixz(new Fibz());
writeln(facz(10));
writeln(fibz(10));</lang>
{{out}}
<pre>3628800
55</pre>
One can write a true call-by-name Y combinator by injecting one level of laziness or deferred execution at the defining function level as per the following code:
{{works with|Chapel version 1.24.1}}
<lang chapel>// this is the longer version...
/*
proc fixy(f) {
record InnerFunc {
const xi;
proc this() { return xi(xi); }
}
record XFunc {
const fi;
proc this(x) { return fi(new InnerFunc(x)); }
}
const g = new XFunc(f);
return g(g);
}
// */
// short version using direct recursion as Chapel has...
// note that this version of fix uses function recursion in its own definition;
// thus its use just means that the recursion has been "pulled" into the "fix" function,
// instead of the function that uses it...
proc fixy(f) {
record InnerFunc { const fi; proc this() { return fixy(fi); } }
return f(new InnerFunc(f));
}
record Facy {
record FacFunc {
const fi;
proc this(n: int): int {
return if n <= 1 then 1 else n * fi()(n - 1); }
}
proc this(f) { return new FacFunc(f); }
}
record Fiby {
record FibFunc {
const fi;
proc this(n: int): int {
return if n <= 1 then n else fi()(n - 2) + fi()(n - 1); }
}
proc this(f) { return new FibFunc(f); }
}
const facy = fixy(new Facy());
const fibz = fixy(new Fiby());
writeln(facy(10));
writeln(fibz(10));</lang>
The output is the same as the above.
=={{header|Clojure}}==
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