Add the :hy pragma

NEWS.rst

@@ -5,6 +5,10 @@ Unreleased
 
 Supports Python 3.9 – Python 3.x
 
+New Features
+------------------------------
+* New pragma `hy`.
+
 1.0.0 ("Afternoon Review", released 2024-09-22)
 ======================================================================
 

docs/api.rst

@@ -169,9 +169,17 @@ Fundamentals
   The effect of each pragma is locally scoped to its containing function,
   class, or comprehension form (other than ``for``), if there is one.
 
-  Only one pragma is currently implemented:
+  These pragmata are currently implemented:
 
-.. _warn-on-core-shadow:
+  - ``:hy``: Set this to a string giving a Hy version number or prefix thereof,
+    such as "1.1.0" or "1", to raise a compile-time error if the currently
+    executing version of Hy isn't at least this new. If you're writing a
+    package, you should still declare the required version of Hy in ``setup.py``
+    or ``pyproject.toml`` or whatever, because ``pip`` won't look for ``(pragma
+    :hy …)`` calls. In the future, this pragma may also switch on features of Hy
+    that were introduced in or before the given version.
+
+    .. _warn-on-core-shadow:
 
   - ``:warn-on-core-shadow``: If true (the default), :hy:func:`defmacro` and
     :hy:func:`require` will raise a warning at compile-time if you define a macro

hy/__init__.py

@@ -1,5 +1,9 @@
 __version__ = 'unreleased'
 nickname = None
+last_version = '1.0.0'
+  # This is used by `(pragma :hy …)` to guess whether an unreleased
+  # version of Hy is new enough. In a released version, it's simply
+  # equal to `__version__`.
 
 
 def _initialize_env_var(env_var, default_val):

hy/core/result_macros.py

@@ -11,10 +11,11 @@
 import ast
 import textwrap
 from contextlib import nullcontext
-from itertools import dropwhile
+from itertools import dropwhile, zip_longest
 
 from funcparserlib.parser import finished, forward_decl, many, maybe, oneplus, some
 
+from hy import last_version
 from hy.compat import PY3_11, PY3_12
 from hy.compiler import Result, asty, mkexpr
 from hy.errors import HyEvalError, HyInternalError, HyTypeError
@@ -165,11 +166,31 @@ def compile_inline_python(compiler, expr, root, code):
 @pattern_macro("pragma", [many(KEYWORD + FORM)])
 def compile_pragma(compiler, expr, root, kwargs):
     for kw, value in kwargs:
-        if kw == Keyword("warn-on-core-shadow"):
+
+        if kw == Keyword("hy"):
+            min_version = compiler.eval(value)
+            if not isinstance(min_version, str):
+                raise compiler._syntax_error(value, "The version given to the pragma `:hy` must be a string")
+            parts = min_version.split('.')
+            if not all(p.isdigit() for p in parts):
+                raise compiler._syntax_error(value, "The string given to the pragma `:hy` must be a dot-separated sequence of integers")
+            for have, need in zip_longest(
+                    map(int, last_version.split('.')),
+                    map(int, parts)):
+                if need is None:
+                    break
+                if have is None or have < need:
+                    raise compiler._syntax_error(kw, f"Hy version {min_version} or later required")
+                if have > need:
+                    break
+
+        elif kw == Keyword("warn-on-core-shadow"):
             compiler.local_state_stack[-1]['warn_on_core_shadow'] = (
                 bool(compiler.eval(value)))
+
         else:
             raise compiler._syntax_error(kw, f"Unknown pragma `{kw}`. Perhaps it's implemented by a newer version of Hy.")
+
     return Result()
 
 

tests/native_tests/other.hy

@@ -6,7 +6,27 @@
   importlib
   pydoc
   pytest
-  hy.errors [HyLanguageError])
+  hy.errors [HyLanguageError HySyntaxError])
+
+
+(defn test-pragma-hy []
+
+  (pragma :hy "1")
+  (pragma :hy "1.0")
+  (pragma :hy "1.0.0")
+  (pragma :hy "0.28.0")
+
+  (eval-when-compile (setv a-version "1.0"))
+  (pragma :hy a-version)
+
+  (defn bad [v msg]
+    (with [e (pytest.raises HySyntaxError)]
+      (hy.eval `(pragma :hy ~v)))
+    (assert (in msg e.value.msg)))
+  (bad "5" "version 5 or later required")
+  (bad "1.99.1" "version 1.99.1 or later required")
+  (bad 5 "must be a string")
+  (bad "Afternoon Review" "must be a dot-separated sequence of integers"))
 
 
 (defn test-illegal-assignments []