Interfaces and queries

An interface describes another contract as function headers. view methods are read-only and can be called immediately with query, which compiles to STATICCALL.

interfaces.lithe

(* An interface types another contract. `view` methods are read-only and run
   now, through STATICCALL. Other methods change state and can only be deferred. *)
 
interface Oracle is {
  view function price(asset : address) : uint;
  view function decimals() : uint8;
}
 
error StalePrice of uint;
 
storage is record {
  oracle : Oracle;
  lastPrice : uint;
}
 
(* An interface value is an address in the ABI. *)
external function setOracle(const oracle : Oracle) is {
  storage.oracle := oracle;
}
 
external function refresh(const asset : address)
  raises StalePrice
is {
  const price : uint := query storage.oracle.price(asset);
  assert(price > 0, StalePrice(price));
  storage.lastPrice := price;
}
 
external view function quote(const asset : address; const amount : uint) : uint is
  amount * query storage.oracle.price(asset)

query storage.oracle.price(asset) returns a uint right away. The EVM guarantees the oracle can't change state during the call.
quote is an external view that queries. Queries are allowed in views, but not in pure functions.