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Dynamic Rules

Plain rewrite rules are context-free, i.e. do not take their context into account. Context-sensitive transformations can be defined by passing context information using additional arguments to rules and strategies. Alternatively, Stratego provides linguistic support to dynamically define rewrite rules based on context information1.

Defining Dynamic Rules

rules( $Id : $Rule ... )

A dynamic rule definition is a regular (conditional) rewrite rule that is defined as part of a strategy rather than at top-level.

The difference is that any variables that are bound in the context of the rule, take their binding from the context, rather then being universally quantified. Thus, a dynamic rule instance can be thought of as having the context variables replaced by the corresponding terms from the context.

Example. The following strategy DefineInlineCall defines the dynamic rule InlineCall:

DefineInlineCall =
  ?FunDef(f, args, e)
  ; rules(
      InlineCall :
        Call(f, es) -> Let(dec*, e)
        where <zip(\(x, e) -> VarDec(x, e)\)> (args, es) => dec*

The variables f, args, and e in the dynamic rule are bound in context, while the variables es and dec* are universally quantified. The variables x and e in the embedded lambda rule are local to that rule. Thus, the application

   FunDef("inc", ["x"], Add(Var("x"), Int("1")))

can be thought of to give rise to the definition

InlineCall :
  Call("inc", es) -> Let(dec*, Add(Var("x"), Int("1")))
  where <zip(\(x, e) -> VarDec(x, e)\)> (["x"], es) => dec*

Invoking Dynamic Rules

When $Id is defined as a dynamic rule it can be invoke like a regular rule or strategies. The invocation only succeeds when applied to a term that coincide with the left-hand side pattern variables bindings inherited from the context.

Thus, in the example above InlineCall can be called to invoke a previously defined dynamic rule. For example, the following are calls to DefineInlineCall and InlineCall

   FunDef("inc", ["x"], Add(Var("x"), Int("1")))

   Call("inc", [Mul(Var("y"), Int("3"))]) =>
   Let([VarDec("x", Mul(Var("y"), Int("3")))],
       Add(Var("x"), Int("1")))

   Call("foo", []) // fails

Note that the application to Call("foo", []) fails since it does not match the dynamically defined rule.

Parameterized Dynamic Rules

Dynamic rules can be parameterized like regular rewrite rules and strategies.

Multiple Definitions

Dynamic rules can be defined for multiple contexts simultaneously.

For example, the following applications of DefineInlineCall

   FunDef("inc", ["x"], Add(Var("x"), Int("1")))

   FunDef("twice", ["y"], Mul(Var("x"), Int("2")))

can be thought of defining multiple top-level rewrite rules

InlineCall :
  Call("inc", es) -> Let(dec*, Add(Var("x"), Int("1")))
  where <zip(\(x, e) -> VarDec(x, e)\)> (["x"], es) => dec*

InlineCall :
  Call("twice", es) -> Let(dec*, Mul(Var("y"), Int("2")))
  where <zip(\(x, e) -> VarDec(x, e)\)> (["y"], es) => dec*

Overriding Dynamic Rules

A definition of a dynamic rule with the same left-hand side as a previous definition, overrides that previous definition.

Thus, if after the applications of DefineInlineCall above, we apply

   FunDef("twice", ["z"], Add(Var("z"), Var("z")))

Then the dynamic rule for twice above is undefined, and instead the rule

InlineCall :
  Call("twice", es) -> Let(dec*, Add(Var("z"), Var("z")))
  where <zip(\(x, e) -> VarDec(x, e)\)> (["z"], es) => dec*

is added to the collection of rules.

Dynamic Rule Scope

It is possible to limit the scope in which dynamic rule definitions are available. The dynamic rule scope {| $Id ... : $Strategy |} limits the availability of dynamic rules named $Id .. defined within the brackets to that scope. After exiting the scope, the state of the dynamic rule definitions before the scope is restored.

For example, the following strategy defines inlining rules that are only available during the visit of the body of the Let:

  inline :
    Let(dec1*, e1) -> Let(dec2*, e2)
    with <inline> dec1* => dec2*
    with {| InlineCall
          : <map(try(DefineInlineCall))> dec2*
          ; <inline> e1 => e2

Application of this strategy to the program term

Let([FunDef("inc", [..], ..)
    , FunDef("twice", [..], ..)]
  , Add(
      Let([FunDef("twice", [..], [..])]
        , Call("twice", [..])) // inline second def of twice
      , Call("twice", [..]) // inline first def of twice

will result in locally overriding the first dynamic rule for "twice", but undoing that override at the end of the dynamic rule scope, such that it is available again at the second call to "twice".

While dynamic rule scopes can deal with lexical scope systems, the preferred way to deal with scope in programming languages is to perform name (and type) analysis using the Statix meta-language and perform a uniquify transformation to guarantee unique names.

Multiple Right-Hand Sides

In order to collect multiple ways to rewrite a term use rules( $Id :+ $Rule).

For example, the following is a small API for for emitting nodes in a control-flow graph consisting of blocks.

add-cfg-node  :: CBlock -> CBlock
all-cfg-nodes :: List(CBlock) -> List(CBlock)

add-cfg-node =
  ; rules( CFGNode :+ _ -> block )

all-cfg-nodes =
  <bagof-CFGNode <+ ![]>()

The bagof-$Id strategy is generated automatically and produces all right-hand sides corresponding to a left-hand side.

Other Dynamic Rule Extensions

The papers by Olmos and Visser2 and Bravenboer et. al1 describe more advanced features of dynamic rules, primarily inspired by data-flow transformations. For defining data-flow analyses, Spoofax now provides the FlowSpec meta-language.


  1. Martin Bravenboer, Arthur van Dam, Karina Olmos, and Eelco Visser. Program transformation with scoped dynamic rewrite rules. Fundamenta Informaticae, 69\(1\-2\):123–178, 2006. URL:

  2. Karina Olmos and Eelco Visser. Composing source-to-source data-flow transformations with rewriting strategies and dependent dynamic rewrite rules. In Rastislav Bodík, editor, Compiler Construction, 14th International Conference, CC 2005, Held as Part of the Joint European Conferences on Theory and Practice of Software, ETAPS 2005, Edinburgh, UK, April 4-8, 2005, Proceedings, volume 3443 of Lecture Notes in Computer Science, 204–220. Springer, 2005. URL:, doi:10.1007/978-3-540-31985-6_14

Last update: 2021-11-15
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