feat: Export the collections extension (#1506)

#1450 embedded the standard extension definitions in hugr-py, including
`collections`, but it didn't add a way to load it as it did with all the
others.

This PR adds a `hugr.std.collections` module that just loads the bundled
json.

drive-by: Implement `__str__` for `FloatVal` and `IntVal`

hugr-py/src/hugr/std/collections.py

@@ -0,0 +1,40 @@
+"""Collection types and operations."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+import hugr.tys as tys
+from hugr import val
+from hugr.std import _load_extension
+from hugr.utils import comma_sep_str
+
+EXTENSION = _load_extension("collections")
+
+
+def list_type(ty: tys.Type) -> tys.ExtType:
+    """Returns a list type with a fixed element type."""
+    arg = tys.TypeTypeArg(ty)
+    return EXTENSION.types["List"].instantiate([arg])
+
+
+@dataclass
+class ListVal(val.ExtensionValue):
+    """Constant value for a list of elements."""
+
+    v: list[val.Value]
+    ty: tys.Type
+
+    def __init__(self, v: list[val.Value], elem_ty: tys.Type) -> None:
+        self.v = v
+        self.ty = list_type(elem_ty)
+
+    def to_value(self) -> val.Extension:
+        name = "ListValue"
+        # The value list must be serialized at this point, otherwise the
+        # `Extension` value would not be serializable.
+        vs = [v._to_serial_root() for v in self.v]
+        return val.Extension(name, typ=self.ty, val=vs, extensions=[EXTENSION.name])
+
+    def __str__(self) -> str:
+        return f"[{comma_sep_str(self.v)}]"

hugr-py/src/hugr/std/float.py

@@ -24,3 +24,6 @@ def to_value(self) -> val.Extension:
         return val.Extension(
             name, typ=FLOAT_T, val=payload, extensions=[EXTENSION.name]
         )
+
+    def __str__(self) -> str:
+        return f"{self.v}"

hugr-py/src/hugr/std/int.py

@@ -14,6 +14,8 @@
 if TYPE_CHECKING:
     from hugr.ops import Command, ComWire
 
+CONVERSIONS_EXTENSION = _load_extension("arithmetic.conversions")
+
 INT_TYPES_EXTENSION = _load_extension("arithmetic.int.types")
 _INT_PARAM = tys.BoundedNatParam(7)
 
@@ -66,6 +68,9 @@ def to_value(self) -> val.Extension:
             extensions=[INT_TYPES_EXTENSION.name],
         )
 
+    def __str__(self) -> str:
+        return f"{self.v}"
+
 
 INT_OPS_EXTENSION = _load_extension("arithmetic.int")