Constants and $ names

Names that start with $ are built in. All of them are constants except $e, which holds the error a guard caught.

$t and $f — True and false

$t is true and $f is false. Comparisons give them. Wherever a number is expected, $t is 1 and $f is 0. Booleans has more.

3>1 5                ⍝ $t $f
$t+$t                ⍝ 2ₓ

$n — NaN

$n is the float NaN, the result of an undefined operation such as 0÷0. Under = it equals nothing, but ≡ treats it as one value. Infinities and NaN has more.

0÷0                  ⍝ $n
$n=$n                ⍝ $f

$a — Alphabet

$a is the uppercase Latin alphabet.

3↑$a               ⍝ "ABC"
$a⍳"CAB"           ⍝ [2 0 1]ₓ

$d — Digits

$d is "0123456789".

"a2b"∊$d           ⍝ $f $t $f

$e — Caught error

$e is the error that an error guard’s handler caught, as a record. The error guard page lists its fields. Anywhere else, $e is a VALUE error.