Hi;
There is a Rosetta Code task for which the Ada solution fails to compile for me.
I have not added any pragma to the code nor have I used any GNAT flags.
I think that this topic was discussed here previously, but I can’t seem to find it…
Here is the result from gnatmake:
test_amb.adb:18:04: warning: type "Amb" is frozen by body [enabled by default]
test_amb.adb:18:04: warning: an equality operator cannot be declared after this point [enabled by default]
test_amb.adb:39:13: error: equality operator must be declared before type "Amb" is frozen (RM 4.5.2 (9.8)) (Ada 2012)
So, is the code as written on the web site correct and I have add the appropriate GNAT flag(s) and/or an Ada pragma; or is the code incorrect?
Here’s the code as it currently stands on the Rosetta Code web site:
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;
with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Amb is
type Alternatives is array (Positive range <>) of Unbounded_String;
type Amb (Count : Positive) is record
This : Positive := 1;
Left : access Amb;
List : Alternatives (1..Count);
end record;
function Image (L : Amb) return String is
begin
return To_String (L.List (L.This));
end Image;
function "/" (L, R : String) return Amb is
Result : Amb (2);
begin
Append (Result.List (1), L);
Append (Result.List (2), R);
return Result;
end "/";
function "/" (L : Amb; R : String) return Amb is
Result : Amb (L.Count + 1);
begin
Result.List (1..L.Count) := L.List ;
Append (Result.List (Result.Count), R);
return Result;
end "/";
function "=" (L, R : Amb) return Boolean is
Left : Unbounded_String renames L.List (L.This);
begin
return Element (Left, Length (Left)) = Element (R.List (R.This), 1);
end "=";
procedure Failure (L : in out Amb) is
begin
loop
if L.This < L.Count then
L.This := L.This + 1;
else
L.This := 1;
Failure (L.Left.all);
end if;
exit when L.Left = null or else L.Left.all = L;
end loop;
end Failure;
procedure Join (L : access Amb; R : in out Amb) is
begin
R.Left := L;
while L.all /= R loop
Failure (R);
end loop;
end Join;
W_1 : aliased Amb := "the" / "that" / "a";
W_2 : aliased Amb := "frog" / "elephant" / "thing";
W_3 : aliased Amb := "walked" / "treaded" / "grows";
W_4 : aliased Amb := "slowly" / "quickly";
begin
Join (W_1'Access, W_2);
Join (W_2'Access, W_3);
Join (W_3'Access, W_4);
Put_Line (Image (W_1) & ' ' & Image (W_2) & ' ' & Image (W_3) & ' ' & Image (W_4));
end Test_Amb;
I’m on MacOS (Apple Silicon).
/opt/gnat-aarch64-darwin-15.1.0-2/bin/gnatmake
gnat --version
GNAT 15.0.1 20250418 (prerelease)
Copyright (C) 1996-2025, Free Software Foundation, Inc.
Thanks,
RBE