When implementing a circuit, we could use the features of the chips we've selected directly. Typically, though, we will use them via gadgets. This indirection is useful because, for reasons of efficiency and limitations imposed by PLONKish circuits, the chip interfaces will often be dependent on low-level implementation details. The gadget interface can provide a more convenient and stable API that abstracts away from extraneous detail.
For example, consider a hash function such as SHA-256. The interface of a chip supporting
SHA-256 might be dependent on internals of the hash function design such as the separation
between message schedule and compression function. The corresponding gadget interface can
provide a more convenient and familiar
finalize API, and can also handle parts
of the hash function that do not need chip support, such as padding. This is similar to how
for cryptographic primitives on CPUs are typically accessed via software libraries, rather
Gadgets can also provide modular and reusable abstractions for circuit programming at a higher level, similar to their use in libraries such as libsnark and bellman. As well as abstracting functions, they can also abstract types, such as elliptic curve points or integers of specific sizes.