1 files changed, 174 insertions, 0 deletions
diff --git a/bsfs/query/ast/fetch.py b/bsfs/query/ast/fetch.py
new file mode 100644
index 0000000..d653a8a
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+++ b/bsfs/query/ast/fetch.py
@@ -0,0 +1,174 @@
+"""
+
+Part of the BlackStar filesystem (bsfs) module.
+A copy of the license is provided with the project.
+Author: Matthias Baumgartner, 2022
+"""
+# imports
+from collections import abc
+import typing
+
+# bsfs imports
+from bsfs.utils import URI, typename, normalize_args
+
+# exports
+__all__ : typing.Sequence[str] = (
+    'All',
+    'Fetch',
+    'FetchExpression',
+    'Node',
+    'This',
+    'Value',
+    )
+
+
+## code ##
+
+class FetchExpression(abc.Hashable):
+    """Generic Fetch expression."""
+
+    def __repr__(self) -> str:
+        """Return the expressions's string representation."""
+        return f'{typename(self)}()'
+
+    def __hash__(self) -> int:
+        """Return the expression's integer representation."""
+        return hash(type(self))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        """Return True if *self* and *other* are equivalent."""
+        return isinstance(other, type(self))
+
+
+class All(FetchExpression):
+    """Fetch all child expressions."""
+
+    # child expressions.
+    expr: typing.Set[FetchExpression]
+
+    def __init__(self, *expr):
+        # unpack child expressions
+        unfolded = set(normalize_args(*expr))
+        # check child expressions
+        if len(unfolded) == 0:
+            raise AttributeError('expected at least one expression, found none')
+        if not all(isinstance(itm, FetchExpression) for itm in unfolded):
+            raise TypeError(expr)
+        # initialize
+        super().__init__()
+        # assign members
+        self.expr = unfolded
+
+    def __iter__(self) -> typing.Iterator[FetchExpression]:
+        return iter(self.expr)
+
+    def __len__(self) -> int:
+        return len(self.expr)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), tuple(sorted(self.expr, key=repr))))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.expr == other.expr
+
+
+class _Branch(FetchExpression):
+    """Branch along a predicate."""
+
+    # FIXME: Use a Predicate (like in ast.filter) so that we can also reverse them!
+
+    # predicate to follow.
+    predicate: URI
+
+    def __init__(self, predicate: URI):
+        if not isinstance(predicate, URI):
+            raise TypeError(predicate)
+        self.predicate = predicate
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.predicate))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.predicate == other.predicate
+
+
+class Fetch(_Branch):
+    """Follow a predicate before evaluating a child epxression."""
+
+    # child expression.
+    expr: FetchExpression
+
+    def __init__(self, predicate: URI, expr: FetchExpression):
+        # check child expressions
+        if not isinstance(expr, FetchExpression):
+            raise TypeError(expr)
+        # initialize
+        super().__init__(predicate)
+        # assign members
+        self.expr = expr
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate}, {self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.expr))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.expr == other.expr
+
+
+class _Named(_Branch):
+    """Fetch a (named) symbol at a predicate."""
+
+    # symbol name.
+    name: str
+
+    def __init__(self, predicate: URI, name: str):
+        super().__init__(predicate)
+        self.name = str(name)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate}, {self.name})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.name))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.name == other.name
+
+
+class Node(_Named): # pylint: disable=too-few-public-methods
+    """Fetch a Node at a predicate."""
+    # FIXME: Is this actually needed?
+
+
+class Value(_Named): # pylint: disable=too-few-public-methods
+    """Fetch a Literal at a predicate."""
+
+
+class This(FetchExpression):
+    """Fetch the current Node."""
+
+    # symbol name.
+    name: str
+
+    def __init__(self, name: str):
+        super().__init__()
+        self.name = str(name)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.name})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.name))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.name == other.name
+
+## EOF ##