Type Aliases

Guppy provides the ability to define a type alias that gives an existing type another name. They are declared via the guppy.type_alias function like so:

from guppylang import guppy
from guppylang.std.builtins import array

Vec3 = guppy.type_alias("Vec3", "array[float, 3]")

This defines Vec3 as an alias for the type array[float, 3]. Importantly, the type passed to guppy.type_alias must be given as a string.

Now, Vec3 can be used as a type wherever it is in scope:

@guppy
def l1_norm(v: Vec3) -> float:
    return abs(v[0]) + abs(v[1]) + abs(v[2])

l1_norm.check()

Guppy’s type aliases are transparent, meaning that wherever a Vec3 is expected, we are allowed to pass an array[float, 3] and vice versa.

Generic Type Aliases

Type aliases can also refer to type variables which make them generic:

from guppylang.std.quantum import qubit

n = guppy.nat_var("n")

QArray = guppy.type_alias("QArray", "array[qubit, n]")

For example, QArray[10] will refer to an array of 10 qubits. If a type alias refers to multiple type variables, they will be ordered by first appearance in the type. Alternatively, a specific ordering can be chosen via the optional params argument of guppy.type_alias:

T = guppy.type_var("T")
U = guppy.type_var("U")

Pair = guppy.type_alias("Pair", "tuple[T, U]")

# Equivalent to
Pair = guppy.type_alias("Pair", "tuple[T, U]", params=[T, U])

Restrictions

Type aliases are not allowed to refer to themselves:

InfiniteInts = guppy.type_alias("InfiniteInts", "tuple[int, InfiniteInts]")

@guppy
def make_infinite_ints() -> InfiniteInts:
    return (0, make_infinite_ints())

make_infinite_ints.check()

Error: Unsupported (at <In[5]>:1:0)
  | 
1 | InfiniteInts = guppy.type_alias("InfiniteInts", "tuple[int, InfiniteInts]")
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Recursive definitions are not supported

Guppy compilation failed due to 1 previous error