peek
Builtin index · Collections & strings
result =push(q, x)/pop(q)/peek(q)
Input: q, x · Output: result
What it is
On a fifo handle q: push(q, x) adds x at the back (returns Nothing); pop(q) removes and returns the front element; peek(q) looks at it without removing. pop on an empty FIFO is an error. On a queue() — the job queue — push(q, "fname") enqueues a function you defined instead (a Str naming it, not a value).
Overloads
One call shape — this function is not overloaded.
| Code | Input | Output | Description |
result = push(q, x) / pop(q) / peek(q) | q —peek(q) looks at it without removing.x —On a fifo handle q: push(q, x) adds x at the back (returns Nothing). | result — | Returns the front element itself from pop/peek, whatever type was pushed; push hands back no value. |
Examples
From the documentation, and run by the build -- the output and the figure below are what the engine actually produced. Simplest first.
f = fifo(3)
f.push(1)
f.push(2)
f.push(3)
f.push(4) # evicts 1 -- buffer is now [2, 3, 4]
print(peek(f)) # 2 -- oldest element, still there
print(f.pop()) # 2
print(len(f)) # 2
2 2 2
from collections-strings
How it works
fifo(capacity) (§ data structures pass, 2026-09-01, Value::Fifo is a guarded arm further up this match (see its own comment there, next to queue()'s own unconditional "push"); every other fifo name has no competing meaning anywhere else, so this arm is unconditional.
From the note above this builtin in the interpreter, where the reasoning was written down as it was built.