fix: Fix printing of generic function parameters (#516)

Fixes #482

guppylang/tys/param.py

@@ -110,7 +110,7 @@ def to_existential(self) -> tuple[Argument, ExistentialVar]:
         var = ExistentialTypeVar.fresh(self.name, self.can_be_linear)
         return TypeArg(var), var
 
-    def to_bound(self, idx: int | None = None) -> Argument:
+    def to_bound(self, idx: int | None = None) -> TypeArg:
         """Creates a bound variable with a given index that can be instantiated for this
         parameter.
         """
@@ -169,7 +169,7 @@ def to_existential(self) -> tuple[Argument, ExistentialVar]:
         var = ExistentialConstVar.fresh(self.name, self.ty)
         return ConstArg(var), var
 
-    def to_bound(self, idx: int | None = None) -> Argument:
+    def to_bound(self, idx: int | None = None) -> ConstArg:
         """Creates a bound variable with a given index that can be instantiated for this
         parameter.
         """

guppylang/tys/printing.py

@@ -128,12 +128,12 @@ def _visit_NumericType(self, ty: NumericType, inside_row: bool) -> str:
     @_visit.register
     def _visit_TypeParam(self, param: TypeParam, inside_row: bool) -> str:
         # TODO: Print linearity?
-        return self.bound_names[-param.idx - 1]
+        return self.bound_names[param.idx]
 
     @_visit.register
     def _visit_ConstParam(self, param: ConstParam, inside_row: bool) -> str:
         kind = self._visit(param.ty, True)
-        name = self.bound_names[-param.idx - 1]
+        name = self.bound_names[param.idx]
         return f"{name}: {kind}"
 
     @_visit.register

tests/error/array_errors/linear_len.err

@@ -4,4 +4,4 @@ Guppy compilation failed. Error in file $FILE:14
 13:    def main(qs: array[qubit, 42]) -> int:
 14:        return len(qs)
                   ^^^^^^^
-GuppyTypeError: Cannot instantiate non-linear type variable `T` in type `forall n, T: nat. array[T, n] -> int` with linear type `qubit`
+GuppyTypeError: Cannot instantiate non-linear type variable `T` in type `forall T, n: nat. array[T, n] -> int` with linear type `qubit`

tests/test_type_printing.py

@@ -0,0 +1,21 @@
+from guppylang.tys.builtin import array_type_def
+from guppylang.tys.param import ConstParam, TypeParam
+from guppylang.tys.ty import (
+    FunctionType,
+    FuncInput,
+    NumericType,
+    OpaqueType,
+    InputFlags,
+)
+
+
+def test_generic_function_type():
+    ty_param = TypeParam(0, "T", can_be_linear=True)
+    len_param = ConstParam(1, "n", NumericType(NumericType.Kind.Nat))
+    array_ty = OpaqueType([ty_param.to_bound(0), len_param.to_bound(1)], array_type_def)
+    ty = FunctionType(
+        params=[ty_param, len_param],
+        inputs=[FuncInput(array_ty, InputFlags.Inout)],
+        output=ty_param.to_bound(0).ty,
+    )
+    assert str(ty) == "forall T, n: nat. array[T, n] -> T"