• ← TagSetDelayed
• → UpSetDelayed

UpSet (^=)

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f[x] ^= expression

evaluates expression and assigns it to the value of f[x], associating the value with x.

UpSet creates an upvalue:

• • a[b] ^= 3;

• • DownValues[a]

• • UpValues[b]

You can use UpSet to specify special values like format values.
However, these values will not be saved in UpValues:

• • Format[r] ^= "custom";

• • r

• • UpValues[r]