UpValues

UpValues[f] gives a list of transformation rules corresponding to all upvalues (definitions for expressions containing f) defined for the symbol f.

Examples

Define an upvalue:

g /: h[g] = 42;
UpValues[g]
(* {HoldPattern[h[g]] :> 42} *)

Upvalues allow a symbol to define how other functions handle it:

myType /: Length[myType[args___]] := Length[{args}];
Length[myType[a, b, c]]
(* 3 *)

Please visit the official Wolfram Language Reference for more details.