274 lines
11 KiB
Go
274 lines
11 KiB
Go
/*
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Package pattern implements a simple language for pattern matching Go ASTs.
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Design decisions and trade-offs
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The language is designed specifically for the task of filtering ASTs
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to simplify the implementation of analyses in staticcheck.
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It is also intended to be trivial to parse and execute.
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To that end, we make certain decisions that make the language more
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suited to its task, while making certain queries infeasible.
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Furthermore, it is fully expected that the majority of analyses will still require ordinary Go code
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to further process the filtered AST, to make use of type information and to enforce complex invariants.
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It is not our goal to design a scripting language for writing entire checks in.
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The language
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At its core, patterns are a representation of Go ASTs, allowing for the use of placeholders to enable pattern matching.
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Their syntax is inspired by LISP and Haskell, but unlike LISP, the core unit of patterns isn't the list, but the node.
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There is a fixed set of nodes, identified by name, and with the exception of the Or node, all nodes have a fixed number of arguments.
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In addition to nodes, there are atoms, which represent basic units such as strings or the nil value.
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Pattern matching is implemented via bindings, represented by the Binding node.
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A Binding can match nodes and associate them with names, to later recall the nodes.
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This allows for expressing "this node must be equal to that node" constraints.
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To simplify writing and reading patterns, a small amount of additional syntax exists on top of nodes and atoms.
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This additional syntax doesn't add any new features of its own, it simply provides shortcuts to creating nodes and atoms.
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To show an example of a pattern, first consider this snippet of Go code:
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if x := fn(); x != nil {
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for _, v := range x {
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println(v, x)
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}
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}
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The corresponding AST expressed as an idiomatic pattern would look as follows:
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(IfStmt
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(AssignStmt (Ident "x") ":=" (CallExpr (Ident "fn") []))
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(BinaryExpr (Ident "x") "!=" (Ident "nil"))
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(RangeStmt
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(Ident "_") (Ident "v") ":=" (Ident "x")
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(CallExpr (Ident "println") [(Ident "v") (Ident "x")]))
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nil)
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Two things are worth noting about this representation.
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First, the [el1 el2 ...] syntax is a short-hand for creating lists.
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It is a short-hand for el1:el2:[], which itself is a short-hand for (List el1 (List el2 (List nil nil)).
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Second, note the absence of a lot of lists in places that normally accept lists.
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For example, assignment assigns a number of right-hands to a number of left-hands, yet our AssignStmt is lacking any form of list.
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This is due to the fact that a single node can match a list of exactly one element.
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Thus, the two following forms have identical matching behavior:
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(AssignStmt (Ident "x") ":=" (CallExpr (Ident "fn") []))
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(AssignStmt [(Ident "x")] ":=" [(CallExpr (Ident "fn") [])])
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This section serves as an overview of the language's syntax.
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More in-depth explanations of the matching behavior as well as an exhaustive list of node types follows in the coming sections.
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Pattern matching
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TODO write about pattern matching
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- inspired by haskell syntax, but much, much simpler and naive
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Node types
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The language contains two kinds of nodes: those that map to nodes in the AST, and those that implement additional logic.
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Nodes that map directly to AST nodes are named identically to the types in the go/ast package.
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What follows is an exhaustive list of these nodes:
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(ArrayType len elt)
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(AssignStmt lhs tok rhs)
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(BasicLit kind value)
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(BinaryExpr x op y)
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(BranchStmt tok label)
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(CallExpr fun args)
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(CaseClause list body)
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(ChanType dir value)
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(CommClause comm body)
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(CompositeLit type elts)
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(DeferStmt call)
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(Ellipsis elt)
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(EmptyStmt)
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(Field names type tag)
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(ForStmt init cond post body)
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(FuncDecl recv name type body)
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(FuncLit type body)
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(FuncType params results)
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(GenDecl specs)
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(GoStmt call)
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(Ident name)
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(IfStmt init cond body else)
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(ImportSpec name path)
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(IncDecStmt x tok)
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(IndexExpr x index)
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(InterfaceType methods)
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(KeyValueExpr key value)
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(MapType key value)
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(RangeStmt key value tok x body)
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(ReturnStmt results)
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(SelectStmt body)
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(SelectorExpr x sel)
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(SendStmt chan value)
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(SliceExpr x low high max)
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(StarExpr x)
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(StructType fields)
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(SwitchStmt init tag body)
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(TypeAssertExpr)
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(TypeSpec name type)
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(TypeSwitchStmt init assign body)
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(UnaryExpr op x)
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(ValueSpec names type values)
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Additionally, there are the String, Token and nil atoms.
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Strings are double-quoted string literals, as in (Ident "someName").
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Tokens are also represented as double-quoted string literals, but are converted to token.Token values in contexts that require tokens,
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such as in (BinaryExpr x "<" y), where "<" is transparently converted to token.LSS during matching.
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The keyword 'nil' denotes the nil value, which represents the absence of any value.
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We also defines the (List head tail) node, which is used to represent sequences of elements as a singly linked list.
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The head is a single element, and the tail is the remainder of the list.
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For example,
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(List "foo" (List "bar" (List "baz" (List nil nil))))
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represents a list of three elements, "foo", "bar" and "baz". There is dedicated syntax for writing lists, which looks as follows:
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["foo" "bar" "baz"]
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This syntax is itself syntactic sugar for the following form:
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"foo":"bar":"baz":[]
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This form is of particular interest for pattern matching, as it allows matching on the head and tail. For example,
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"foo":"bar":_
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would match any list with at least two elements, where the first two elements are "foo" and "bar". This is equivalent to writing
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(List "foo" (List "bar" _))
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Note that it is not possible to match from the end of the list.
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That is, there is no way to express a query such as "a list of any length where the last element is foo".
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Note that unlike in LISP, nil and empty lists are distinct from one another.
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In patterns, with respect to lists, nil is akin to Go's untyped nil.
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It will match a nil ast.Node, but it will not match a nil []ast.Expr. Nil will, however, match pointers to named types such as *ast.Ident.
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Similarly, lists are akin to Go's
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slices. An empty list will match both a nil and an empty []ast.Expr, but it will not match a nil ast.Node.
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Due to the difference between nil and empty lists, an empty list is represented as (List nil nil), i.e. a list with no head or tail.
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Similarly, a list of one element is represented as (List el (List nil nil)). Unlike in LISP, it cannot be represented by (List el nil).
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Finally, there are nodes that implement special logic or matching behavior.
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(Any) matches any value. The underscore (_) maps to this node, making the following two forms equivalent:
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(Ident _)
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(Ident (Any))
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(Builtin name) matches a built-in identifier or function by name.
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This is a type-aware variant of (Ident name).
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Instead of only comparing the name, it resolves the object behind the name and makes sure it's a pre-declared identifier.
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For example, in the following piece of code
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func fn() {
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println(true)
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true := false
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println(true)
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}
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the pattern
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(Builtin "true")
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will match exactly once, on the first use of 'true' in the function.
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Subsequent occurrences of 'true' no longer refer to the pre-declared identifier.
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(Object name) matches an identifier by name, but yields the
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types.Object it refers to.
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(Function name) matches ast.Idents and ast.SelectorExprs that refer to a function with a given fully qualified name.
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For example, "net/url.PathEscape" matches the PathEscape function in the net/url package,
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and "(net/url.EscapeError).Error" refers to the Error method on the net/url.EscapeError type,
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either on an instance of the type, or on the type itself.
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For example, the following patterns match the following lines of code:
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(CallExpr (Function "fmt.Println") _) // pattern 1
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(CallExpr (Function "(net/url.EscapeError).Error") _) // pattern 2
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fmt.Println("hello, world") // matches pattern 1
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var x url.EscapeError
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x.Error() // matches pattern 2
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(url.EscapeError).Error(x) // also matches pattern 2
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(Binding name node) creates or uses a binding.
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Bindings work like variable assignments, allowing referring to already matched nodes.
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As an example, bindings are necessary to match self-assignment of the form "x = x",
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since we need to express that the right-hand side is identical to the left-hand side.
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If a binding's node is not nil, the matcher will attempt to match a node according to the pattern.
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If a binding's node is nil, the binding will either recall an existing value, or match the Any node.
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It is an error to provide a non-nil node to a binding that has already been bound.
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Referring back to the earlier example, the following pattern will match self-assignment of idents:
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(AssignStmt (Binding "lhs" (Ident _)) "=" (Binding "lhs" nil))
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Because bindings are a crucial component of pattern matching, there is special syntax for creating and recalling bindings.
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Lower-case names refer to bindings. If standing on its own, the name "foo" will be equivalent to (Binding "foo" nil).
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If a name is followed by an at-sign (@) then it will create a binding for the node that follows.
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Together, this allows us to rewrite the earlier example as follows:
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(AssignStmt lhs@(Ident _) "=" lhs)
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(Or nodes...) is a variadic node that tries matching each node until one succeeds. For example, the following pattern matches all idents of name "foo" or "bar":
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(Ident (Or "foo" "bar"))
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We could also have written
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(Or (Ident "foo") (Ident "bar"))
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and achieved the same result. We can also mix different kinds of nodes:
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(Or (Ident "foo") (CallExpr (Ident "bar") _))
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When using bindings inside of nodes used inside Or, all or none of the bindings will be bound.
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That is, partially matched nodes that ultimately failed to match will not produce any bindings observable outside of the matching attempt.
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We can thus write
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(Or (Ident name) (CallExpr name))
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and 'name' will either be a String if the first option matched, or an Ident or SelectorExpr if the second option matched.
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(Not node)
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The Not node negates a match. For example, (Not (Ident _)) will match all nodes that aren't identifiers.
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ChanDir(0)
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Automatic unnesting of AST nodes
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The Go AST has several types of nodes that wrap other nodes.
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To simplify matching, we automatically unwrap some of these nodes.
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These nodes are ExprStmt (for using expressions in a statement context),
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ParenExpr (for parenthesized expressions),
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DeclStmt (for declarations in a statement context),
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and LabeledStmt (for labeled statements).
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Thus, the query
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(FuncLit _ [(CallExpr _ _)]
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will match a function literal containing a single function call,
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even though in the actual Go AST, the CallExpr is nested inside an ExprStmt,
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as function bodies are made up of sequences of statements.
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On the flip-side, there is no way to specifically match these wrapper nodes.
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For example, there is no way of searching for unnecessary parentheses, like in the following piece of Go code:
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((x)) += 2
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*/
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package pattern
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