Read without copying with view
A view lets you borrow a string or list for reading. It does not copy the data or take ownership. It is usable only while the original data remains available.
def main() -> Result[unit, Error]:
text = "Nagi language"
borrowed = view(text)
first = try slice(borrowed, 0, 4)
print(first)
saved = copy(first)
print(saved)
return ok(print(text))
Save this as view.nagi and run nagic run view.nagi. It prints Nagi, Nagi, and Nagi language. first borrows part of the original string. saved is a separate owned string created with copy.
On this page
Borrowing a range
slice(view(data), start, end) borrows from start up to, but not including, end. String positions are UTF-8 byte offsets. An out-of-range position or an offset inside a character returns a Result error.
To store a view, first put the original value in a variable, as above. Moving or changing the original value is restricted while it is borrowed. See Ownership for examples.
saved = view("Nagi") fails because it stores a borrow of a temporary string. An immediate call such as print(view("Nagi")) is valid because it finishes using the borrow within that call.
Copying a list requires its elements to implement Rust's Clone. Generated classes receive Clone only when they are Copy classes. To copy a list of classes that own strings, for example, provide a Clone implementation in a Rust adapter. This implementation is verified by build.
Returning a view
A function can return its input view or part of it.
def identity(data: view[str]) -> view[str]:
return data
It cannot return a view into a string created inside that function: the original data disappears when the function ends. Return an owned value when the caller needs to retain the data. Storing views in class fields or passing them to another task is not supported.
An HTTP body: view[bytes] also borrows received data. Creating that view adds no copy, but receiving data and decoding JSON still involve other work. See Reading benchmarks for measurement details.