Dyalog APL - Towers of Hanoi - does not loop properly - recursion

Question

I cannot make it work in Dyalog APL
solve←{
n a c b←⍵
n≤0:⍬
solve(n-1)a b c
⎕←'Move disk from' a 'to' c
solve(n-1)b c a
}
solve 4 'A' 'C' 'B'
It loops from the first solve (n-1) a b c but never goes to line 4.
The same code works in JavaSCript:
solve = (n, a, c, b) => {
if (n <= 0) return
solve(n-1, a, b, c)
console.log(`Move disk from ${a} to ${c}`)
solve(n-1, b, c, a)
}
solve(4, 'A', 'C', 'B')
When I print the input params it shows:
solve←{
n a c b←⍵
⎕←n a c b
n≤0:⍬
solve(n-1)a b c
⎕←'Move disk from' a 'to' c
solve(n-1)b c a
}
4 ACB
3 ABC
2 ACB
1 ABC
0 ACB
Any ideas?

Answer 1

A dfn will return on the first non-assignment. Both your recursive calls are non-assignments, so the function doesn't return where you think it does. You can tweak it slightly like so:
solve←{
(n a c b)←⍵
n≤0:⍬
_←∇(n-1)a b c ⍝ Capture and ignore result
⎕←'Move disk from' a 'to' c
_←∇(n-1)b c a ⍝ Capture and ignore result
⍬ ⍝ Return empty vector
}
Note the use of ∇ for recursion -- this is a shorthand for the current dfn. If I run this modified version, I get:
solve 4 'A' 'C' 'B'
Move disk from A to B
Move disk from A to C
Move disk from B to C
Move disk from A to B
Move disk from C to A
Move disk from C to B
Move disk from A to B
Move disk from A to C
Move disk from B to C
Move disk from B to A
Move disk from C to A
Move disk from B to C
Move disk from A to B
Move disk from A to C
Move disk from B to C
Roger Hui wrote a paper on this problem that's well worth a read: https://www.jsoftware.com/papers/50/50_38.htm