feat: Add StringVal to hugr-py (#1818)

Closes #1817

Enables strings in Guppy, see CQCL/guppylang#695

hugr-py/src/hugr/std/prelude.py

@@ -0,0 +1,34 @@
+"""HUGR prelude values."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from hugr import val
+from hugr.std import _load_extension
+
+PRELUDE_EXTENSION = _load_extension("prelude")
+
+STRING_T_DEF = PRELUDE_EXTENSION.types["string"]
+
+STRING_T = STRING_T_DEF.instantiate([])
+
+
+@dataclass
+class StringVal(val.ExtensionValue):
+    """Custom value for a string."""
+
+    v: str
+
+    def to_value(self) -> val.Extension:
+        name = "ConstString"
+        payload = {"value": self.v}
+        return val.Extension(
+            name,
+            typ=STRING_T,
+            val=payload,
+            extensions=[PRELUDE_EXTENSION.name],
+        )
+
+    def __str__(self) -> str:
+        return f"{self.v}"

hugr-py/tests/test_prelude.py

@@ -0,0 +1,8 @@
+from hugr.std.prelude import STRING_T, StringVal
+
+
+def test_string_val():
+    ext_val = StringVal("test").to_value()
+    assert ext_val.name == "ConstString"
+    assert ext_val.typ == STRING_T
+    assert ext_val.val == {"value": "test"}