IF Statement

if branches the parse — on a test, or on whether something parses at all.

if (cond) [then | else]     # both branches
if (cond) [then]            # then only
if (cond) [| else]          # else only

A logic condition

numvar x = 0;

A := (char::to_num => x) . if ({x == 1}) [T | F];

The block is a test. It reads no input and moves nothing, it only answers.

A grammar condition

The condition may be grammar instead, and then the question is simply “did it match?”:

E := G . if (M) [T | F];   # if M parses, parse T, else F
H := I . if (M) [T];       # if M parses, parse T
J := K . if (M) [| F];     # if M does not parse, parse F

Important

A grammar condition consumes what it matched. M is not a lookahead — it really parses, and its nodes are as real as any other unit’s.

With no branch to take, an if behaves like E?: the cursor sits where it was and the next unit is read.