feat: Build for loops and comprehensions

guppy/ast_util.py

@@ -1,5 +1,7 @@
 import ast
-from typing import TYPE_CHECKING, Any, Generic, Optional, TypeVar
+import textwrap
+from collections.abc import Callable, Mapping, Sequence
+from typing import TYPE_CHECKING, Any, Generic, Optional, TypeVar, cast
 
 if TYPE_CHECKING:
     from guppy.gtypes import GuppyType
@@ -54,51 +56,165 @@ def generic_visit(self, node: Any, *args: Any, **kwargs: Any) -> T:
         raise NotImplementedError(f"visit_{node.__class__.__name__} is not implemented")
 
 
-class NameVisitor(ast.NodeVisitor):
-    """Visitor to collect all `Name` nodes occurring in an AST."""
-
-    names: list[ast.Name]
-
-    def __init__(self) -> None:
-        self.names = []
-
-    def visit_Name(self, node: ast.Name) -> None:
-        self.names.append(node)
+class AstSearcher(ast.NodeVisitor):
+    """Visitor that searches for occurrences of specific nodes in an AST."""
+
+    matcher: Callable[[ast.AST], bool]
+    dont_recurse_into: set[type[ast.AST]]
+    found: list[ast.AST]
+    is_first_node: bool
+
+    def __init__(
+        self,
+        matcher: Callable[[ast.AST], bool],
+        dont_recurse_into: set[type[ast.AST]] | None = None,
+    ) -> None:
+        self.matcher = matcher
+        self.dont_recurse_into = dont_recurse_into or set()
+        self.found = []
+        self.is_first_node = True
+
+    def generic_visit(self, node: ast.AST) -> None:
+        if self.matcher(node):
+            self.found.append(node)
+        if self.is_first_node or type(node) not in self.dont_recurse_into:
+            self.is_first_node = False
+            super().generic_visit(node)
+
+
+def find_nodes(
+    matcher: Callable[[ast.AST], bool],
+    node: ast.AST,
+    dont_recurse_into: set[type[ast.AST]] | None = None,
+) -> list[ast.AST]:
+    """Returns all nodes in the AST that satisfy the matcher."""
+    v = AstSearcher(matcher, dont_recurse_into)
+    v.visit(node)
+    return v.found
 
 
 def name_nodes_in_ast(node: Any) -> list[ast.Name]:
     """Returns all `Name` nodes occurring in an AST."""
-    v = NameVisitor()
-    v.visit(node)
-    return v.names
-
-
-class ReturnVisitor(ast.NodeVisitor):
-    """Visitor to collect all `Return` nodes occurring in an AST."""
+    found = find_nodes(lambda n: isinstance(n, ast.Name), node)
+    return cast(list[ast.Name], found)
 
-    returns: list[ast.Return]
-    inside_func_def: bool
 
-    def __init__(self) -> None:
-        self.returns = []
-        self.inside_func_def = False
-
-    def visit_Return(self, node: ast.Return) -> None:
-        self.returns.append(node)
+def return_nodes_in_ast(node: Any) -> list[ast.Return]:
+    """Returns all `Return` nodes occurring in an AST."""
+    found = find_nodes(lambda n: isinstance(n, ast.Return), node, {ast.FunctionDef})
+    return cast(list[ast.Return], found)
 
-    def visit_FunctionDef(self, node: ast.FunctionDef) -> None:
-        # Don't descend into nested function definitions
-        if not self.inside_func_def:
-            self.inside_func_def = True
-            for n in node.body:
-                self.visit(n)
 
+def breaks_in_loop(node: Any) -> list[ast.Break]:
+    """Returns all `Break` nodes occurring in a loop.
 
-def return_nodes_in_ast(node: Any) -> list[ast.Return]:
-    """Returns all `Return` nodes occurring in an AST."""
-    v = ReturnVisitor()
-    v.visit(node)
-    return v.returns
+    Note that breaks in nested loops are excluded.
+    """
+    found = find_nodes(
+        lambda n: isinstance(n, ast.Break), node, {ast.For, ast.While, ast.FunctionDef}
+    )
+    return cast(list[ast.Break], found)
+
+
+class ContextAdjuster(ast.NodeTransformer):
+    """Updates the `ast.Context` indicating if expressions occur on the LHS or RHS."""
+
+    ctx: ast.expr_context
+
+    def __init__(self, ctx: ast.expr_context) -> None:
+        self.ctx = ctx
+
+    def visit(self, node: ast.AST) -> ast.AST:
+        return cast(ast.AST, super().visit(node))
+
+    def visit_Name(self, node: ast.Name) -> ast.Name:
+        return with_loc(node, ast.Name(id=node.id, ctx=self.ctx))
+
+    def visit_Starred(self, node: ast.Starred) -> ast.Starred:
+        return with_loc(node, ast.Starred(value=self.visit(node.value), ctx=self.ctx))
+
+    def visit_Tuple(self, node: ast.Tuple) -> ast.Tuple:
+        return with_loc(
+            node, ast.Tuple(elts=[self.visit(elt) for elt in node.elts], ctx=self.ctx)
+        )
+
+    def visit_List(self, node: ast.List) -> ast.List:
+        return with_loc(
+            node, ast.List(elts=[self.visit(elt) for elt in node.elts], ctx=self.ctx)
+        )
+
+    def visit_Subscript(self, node: ast.Subscript) -> ast.Subscript:
+        # Don't adjust the slice!
+        return with_loc(
+            node,
+            ast.Subscript(value=self.visit(node.value), slice=node.slice, ctx=self.ctx),
+        )
+
+    def visit_Attribute(self, node: ast.Attribute) -> ast.Attribute:
+        return ast.Attribute(value=self.visit(node.value), attr=node.attr, ctx=self.ctx)
+
+
+class TemplateReplacer(ast.NodeTransformer):
+    """Replaces nodes in a template."""
+
+    replacements: Mapping[str, ast.AST | Sequence[ast.AST]]
+    default_loc: ast.AST
+
+    def __init__(
+        self,
+        replacements: Mapping[str, ast.AST | Sequence[ast.AST]],
+        default_loc: ast.AST,
+    ) -> None:
+        self.replacements = replacements
+        self.default_loc = default_loc
+
+    def _get_replacement(self, x: str) -> ast.AST | Sequence[ast.AST]:
+        if x not in self.replacements:
+            msg = f"No replacement for `{x}` is given"
+            raise ValueError(msg)
+        return self.replacements[x]
+
+    def visit_Name(self, node: ast.Name) -> ast.AST:
+        repl = self._get_replacement(node.id)
+        if not isinstance(repl, ast.expr):
+            msg = f"Replacement for `{node.id}` must be an expression"
+            raise TypeError(msg)
+
+        # Update the context
+        adjuster = ContextAdjuster(node.ctx)
+        return with_loc(repl, adjuster.visit(repl))
+
+    def visit_Expr(self, node: ast.Expr) -> ast.AST | Sequence[ast.AST]:
+        if isinstance(node.value, ast.Name):
+            repl = self._get_replacement(node.value.id)
+            repls = [repl] if not isinstance(repl, Sequence) else repl
+            # Wrap expressions to turn them into statements
+            return [
+                with_loc(r, ast.Expr(value=r)) if isinstance(r, ast.expr) else r
+                for r in repls
+            ]
+        return self.generic_visit(node)
+
+    def generic_visit(self, node: ast.AST) -> ast.AST:
+        # Insert the default location
+        node = super().generic_visit(node)
+        return with_loc(self.default_loc, node)
+
+
+def template_replace(
+    template: str, default_loc: ast.AST, **kwargs: ast.AST | Sequence[ast.AST]
+) -> list[ast.stmt]:
+    """Turns a template into a proper AST by substituting all placeholders."""
+    nodes = ast.parse(textwrap.dedent(template)).body
+    replacer = TemplateReplacer(kwargs, default_loc)
+    new_nodes = []
+    for n in nodes:
+        new = replacer.visit(n)
+        if isinstance(new, list):
+            new_nodes.extend(new)
+        else:
+            new_nodes.append(new)
+    return new_nodes
 
 
 def line_col(node: ast.AST) -> tuple[int, int]:

guppy/cfg/builder.py

@@ -3,13 +3,28 @@
 from collections.abc import Iterator
 from typing import NamedTuple
 
-from guppy.ast_util import AstVisitor, set_location_from, with_loc
+from guppy.ast_util import (
+    AstVisitor,
+    find_nodes,
+    set_location_from,
+    template_replace,
+    with_loc,
+)
 from guppy.cfg.bb import BB, BBStatement
 from guppy.cfg.cfg import CFG
 from guppy.checker.core import Globals
 from guppy.error import GuppyError, InternalGuppyError
 from guppy.gtypes import NoneType
-from guppy.nodes import NestedFunctionDef, PyExpr
+from guppy.nodes import (
+    DesugaredGenerator,
+    DesugaredListComp,
+    IterEnd,
+    IterHasNext,
+    IterNext,
+    MakeIter,
+    NestedFunctionDef,
+    PyExpr,
+)
 
 # In order to build expressions, need an endless stream of unique temporary variables
 # to store intermediate results
@@ -142,6 +157,35 @@ def visit_While(self, node: ast.While, bb: BB, jumps: Jumps) -> BB | None:
         # its own jumps since the body is not guaranteed to execute
         return tail_bb
 
+    def visit_For(self, node: ast.For, bb: BB, jumps: Jumps) -> BB | None:
+        template = """
+            it = make_iter
+            while True:
+                b, it = has_next
+                if b:
+                    x, it = get_next
+                    body
+                else:
+                    break
+            end_iter  # Consume iterator one last time
+        """
+
+        it = make_var(next(tmp_vars), node.iter)
+        b = make_var(next(tmp_vars), node.iter)
+        new_nodes = template_replace(
+            template,
+            node,
+            it=it,
+            b=b,
+            x=node.target,
+            make_iter=with_loc(node.iter, MakeIter(value=node.iter, origin_node=node)),
+            has_next=with_loc(node.iter, IterHasNext(value=it)),
+            get_next=with_loc(node.iter, IterNext(value=it)),
+            end_iter=with_loc(node.iter, IterEnd(value=it)),
+            body=node.body,
+        )
+        return self.visit_stmts(new_nodes, bb, jumps)
+
     def visit_Continue(self, node: ast.Continue, bb: BB, jumps: Jumps) -> BB | None:
         if not jumps.continue_bb:
             raise InternalGuppyError("Continue BB not defined")
@@ -211,20 +255,11 @@ def build(node: ast.expr, cfg: CFG, bb: BB) -> tuple[ast.expr, BB]:
         builder = ExprBuilder(cfg, bb)
         return builder.visit(node), builder.bb
 
-    @classmethod
-    def _make_var(cls, name: str, loc: ast.expr | None = None) -> ast.Name:
-        """Creates an `ast.Name` node."""
-        node = ast.Name(id=name, ctx=ast.Load)
-        if loc is not None:
-            set_location_from(node, loc)
-        return node
-
     @classmethod
     def _tmp_assign(cls, tmp_name: str, value: ast.expr, bb: BB) -> None:
         """Adds a temporary variable assignment to a basic block."""
-        node = ast.Assign(targets=[cls._make_var(tmp_name, value)], value=value)
-        set_location_from(node, value)
-        bb.statements.append(node)
+        lhs = make_var(tmp_name, value)
+        bb.statements.append(make_assign([lhs], value))
 
     def visit_Name(self, node: ast.Name) -> ast.Name:
         return node
@@ -256,7 +291,44 @@ def visit_IfExp(self, node: ast.IfExp) -> ast.Name:
         self.bb = merge_bb
 
         # The final value is stored in the temporary variable
-        return self._make_var(tmp, node)
+        return make_var(tmp, node)
+
+    def visit_ListComp(self, node: ast.ListComp) -> ast.AST:
+        # Check for illegal expressions
+        illegals = find_nodes(is_illegal_in_list_comp, node)
+        if illegals:
+            raise GuppyError(
+                "Expression is not supported inside a list comprehension", illegals[0]
+            )
+
+        # Desugar into statements that create the iterator, check for a next element,
+        # get the next element, and finalise the iterator.
+        gens = []
+        for g in node.generators:
+            if g.is_async:
+                raise GuppyError("Async generators are not supported", g)
+            g.iter = self.visit(g.iter)
+            it = make_var(next(tmp_vars), g.iter)
+            hasnext = make_var(next(tmp_vars), g.iter)
+            desugared = DesugaredGenerator(
+                iter=it,
+                hasnext=hasnext,
+                iter_assign=make_assign(
+                    [it], with_loc(it, MakeIter(value=g.iter, origin_node=node))
+                ),
+                hasnext_assign=make_assign(
+                    [hasnext, it], with_loc(it, IterHasNext(value=it))
+                ),
+                next_assign=make_assign(
+                    [g.target, it], with_loc(it, IterNext(value=it))
+                ),
+                iterend=with_loc(it, IterEnd(value=it)),
+                ifs=g.ifs,
+            )
+            gens.append(desugared)
+
+        node.elt = self.visit(node.elt)
+        return with_loc(node, DesugaredListComp(elt=node.elt, generators=gens))
 
     def visit_Call(self, node: ast.Call) -> ast.AST:
         # Parse compile-time evaluated `py(...)` expression
@@ -291,7 +363,7 @@ def generic_visit(self, node: ast.AST) -> ast.AST:
             self._tmp_assign(tmp, false_const, false_bb)
             merge_bb = self.cfg.new_bb(true_bb, false_bb)
             self.bb = merge_bb
-            return self._make_var(tmp, node)
+            return make_var(tmp, node)
         # For all other expressions, just recurse deeper with the node transformer
         return super().generic_visit(node)
 
@@ -414,3 +486,26 @@ def is_short_circuit_expr(node: ast.AST) -> bool:
     return isinstance(node, ast.BoolOp) or (
         isinstance(node, ast.Compare) and len(node.comparators) > 1
     )
+
+
+def is_illegal_in_list_comp(node: ast.AST) -> bool:
+    """Checks if an expression is illegal to use in a list comprehension."""
+    return isinstance(node, ast.IfExp | ast.NamedExpr) or is_short_circuit_expr(node)
+
+
+def make_var(name: str, loc: ast.AST | None = None) -> ast.Name:
+    """Creates an `ast.Name` node."""
+    node = ast.Name(id=name, ctx=ast.Load)
+    if loc is not None:
+        set_location_from(node, loc)
+    return node
+
+
+def make_assign(lhs: list[ast.AST], value: ast.expr) -> ast.Assign:
+    """Creates an `ast.Assign` node."""
+    assert len(lhs) > 0
+    if len(lhs) == 1:
+        target = lhs[0]
+    else:
+        target = with_loc(value, ast.Tuple(elts=lhs, ctx=ast.Store()))
+    return with_loc(value, ast.Assign(targets=[target], value=value))