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Java #29

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35 changes: 24 additions & 11 deletions languages/j/java/LambdaCore.java
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,10 @@ interface UnaryBoolOp extends Function<Bool, Bool> {
interface BinaryBoolOp extends Function<Bool, Function<Bool, Bool>> {
}

interface ChurchNumeral extends Function<Function<Integer, Integer>, Function<Integer, Integer>> {
// PRED applies a numeral to functions over functions, so numerals cannot be
// pinned to Function<Integer, Integer>. Term is the untyped lambda calculus'
// single universal type: everything is a function from Term to Term.
interface Term extends Function<Term, Term> {
}

Bool TRUE = x -> y -> x;
Expand All @@ -21,13 +24,13 @@ interface ChurchNumeral extends Function<Function<Integer, Integer>, Function<In
BinaryBoolOp AND = b1 -> b2 -> b1.apply(b2).apply(FALSE);
BinaryBoolOp OR = b1 -> b2 -> b1.apply(TRUE).apply(b2);

ChurchNumeral ZERO = x -> y -> y;
Function<ChurchNumeral, ChurchNumeral> SUCC = w -> y -> x -> y.apply(w.apply(y).apply(x));

// Function<ChurchNumeral, ChurchNumeral> PRED = n -> f -> x ->
// n.apply(g -> h -> h.apply(g.apply(f)))
// .apply(u -> x)
// .apply(u -> u);
Term ZERO = f -> x -> x;
Term SUCC = n -> f -> x -> f.apply(n.apply(f).apply(x));
Term PRED = n -> f -> x ->
n.apply(g -> h -> h.apply(g.apply(f)))
.apply(u -> x)
.apply(u -> u);
Term ONE = SUCC.apply(ZERO);

static void main(String[] args) {
printBool(TRUE); // TRUE
Expand All @@ -47,7 +50,11 @@ static void main(String[] args) {
printBool(OR.apply(TRUE).apply(TRUE)); // TRUE

printChurchNumeral(ZERO); // 0
printChurchNumeral(SUCC.apply(ZERO)); // 1
printChurchNumeral(ONE); // 1
printChurchNumeral(SUCC.apply(ONE)); // 2
printChurchNumeral(PRED.apply(SUCC.apply(ONE))); // 1
printChurchNumeral(PRED.apply(ONE)); // 0
printChurchNumeral(PRED.apply(ZERO)); // 0
}

static void printBool(Bool b) {
Expand All @@ -59,7 +66,13 @@ else if (b == FALSE)
throw new IllegalStateException();
}

static void printChurchNumeral(ChurchNumeral n) {
System.out.println(n.apply(x -> x + 1).apply(0));
static void printChurchNumeral(Term n) {
int[] count = {0};
Term inc = t -> {
count[0]++;
return t;
};
n.apply(inc).apply(t -> t);
System.out.println(count[0]);
}
}