# Migrating to the Concurrent Solver¶

In this how-to guide, we explain what changes should be made to enable the concurrent solver for a Statix project.

## Enabling the Concurrent solver for a Language¶

To enable the concurrent solver for a language, set the language.statix.concurrent property in the metaborg.yaml file to true. This ensures that the concurrent solver is used for all sources in the language.

id: org.example:mylang:0.1.0-SNAPSHOT
name: mylang
language:
statix:
mode: concurrent


## Enabling the Concurrent solver for an Example Project only¶

To enable the concurrent solver for a particular project only, set the runtime.statix.modes property in the metaborg.yaml file to a map that contains all names of the languages for which you want to use the concurrent solver, and their corresponding modes. The name of the language should correspond to the name property in the metaborg.yaml of the language definition project.

id: org.example:mylang.example:0.1.0-SNAPSHOT
runtime:
statix:
modes:
- mylang: concurrent


## Indirect Type Declaration¶

Type checking with the concurrent solver might result in deadlock when type-checkers have mutual dependencies on their declarations. This problem can be solved by adding an intermediate declaration that splits the part of the declaration data that is filtered on (usually the declaration name), and the part that is processed further by the querying unit (usually the type). This pattern is best explained with an example:

signature
relations
type : ID -> TYPE

rules
declareType : scope * ID * TYPE
resolveType : scope * ID -> TYPE

declareType(s, x, T) :-
!type[x, T] in s.

resolveType(s, x) = T :-
query type
filter P* I* and { x' :- x' == x }
in s |-> [(_, (_, T))].


This specification needs to be changed in the following:

signature
relations
type   : ID -> scope
typeOf : TYPE

rules
declareType : scope * ID * TYPE
resolveType : scope * ID -> TYPE

declareType(s, x, T) :-
!type[x, withType(T)] in s.

resolveType(s, x) = typeOf(T) :-
query type
filter P* I* and { x' :- x' == x }
in s |-> [(_, (_, T))].

rules
withType : TYPE -> scope
typeOf   : scope -> TYPE

withType(T) = s :-
new s, !typeOf[T] in s.

typeOf(s) = T :-
query typeOf filter e in s |-> [(_, T)].


We now discuss the changes one-by-one. First, the signature of relation type is be changed to ID -> scope. In this scope, we store the type using the newly introduced typeOf relation. This relation only carries a single TYPE term. In this way, the original term is still indirectly present in the outer declaration.

The withType and typeOf rules allow to convert between these representations. The withType rule creates a scope with a typeOf declaration that contains the type. In the adapted declareType rule, this constraint is used to convert the T argument to the representation that the type relation accepts. Likewise, the typeOf rule queries the typeOf declaration to extract the type from a scope. This rule is used in the resolveType rule to convert back to the term representation of a type.

Performing this change should resolve potential deadlocks when executing your specifications. Because the signatures of the rules in the original specification did not change, and the new specification should have identical semantics, the remainder of the specification should not be affected.

## Using Grouping¶

The traditional solver uses two special constraints as entry points: the project constraint (usually named projectOk) was solved once for each project, while the file constraint (usually called fileOk) was solved for each file in that project. The concurrent solver adds the concepts of groups in between these concepts. Files can be organized in groups, while groups can in addition contain subgroups. This gives rise to a tree-shaped hierarchy, where the project is the root node, the files are the leaf nodes, and all nodes in between are groups. Most often, this hierarchy follows the directory structure of a project. In order to use grouping, two steps need to be performed.

First, a group constraint must be defined. The group constraint must have the signature scope * string * scope. For each group, this constraint will be instantiated with the parent group scope, group name and own group scope as arguments (in that order).

A simple example of a group constraint can look as follows:

signature

sorts MODULE constructors
MODULE: scope -> MODULE

relations
mod: string * MODULE

rules

groupOk: scope * string * scope
groupOk(s_prnt, name, s_grp) :-
!mod[name, MODULE(s_grp)] in s_prnt.


In this fragment, we define the groupOk constraint, which has the appropriate signature. The body of the rule for this constraint simply declares that a module with the appropriate name exists in the parent scope.

Second, the builder needs to be adapted in two ways: the name of the group constraint must be passed to the solver, and a strategy that determines the group of a file must be provided to the solver. Such a strategy should have type (String * AST) -> List(String). The input arguments represent the file path and its AST, respectively. The output list should contain all group identifiers from project root to the respective file. For example, a Java specification should return ["java", "lang", "Object"] for the Object class in the java.lang package. Note that the file must be assigned a name, and that that name should be included in the output list.

A project that uses the directory structure as its grouping structure could call the stx-editor-analyze as follows:

rules

editor-analyze = stx-editor-analyze(pre-analyze,group-key,post-analyze|"statics", "projectOk", "groupOk", "fileOk")

group-key: (resource, ast) -> key
with rel-path := <current-language-relative-source-or-include-path> resource
; key := <string-tokenize> (['/','\'], rel-path)


In this snippet, the "groupOk" argument between the file and project constraint names points to our newly defined group constraint. The group-key strategy, passed between the pre and post transformation strategies, is the strategy that performs the grouping. It first makes the path relative to the project root, and then splits it on each '/' or '\' character.

## Using Libraries¶

Secondly, the concurrent solver allows to export the scope graph of a project in a library. These libraries can be linked with other projects, potentially decreasing analysis times significantly. Statix libraries can be generated and linked by following three steps.

As a first step, use the Spoofax ‣ Statix ‣ Make project library menu on a file in your library project to export its scope graph. A project.stxlib file will now appear in the root directory of that project.

Project Scope Configuration

Currently, the project scope in the project.stxlib file must still be configured manually. Some understanding of the Statix library format is helpful for that. The signature for Statix libraries looks roughly as follows:

sorts Library constructors
Library : list(Scope) * list(Scope) * ScopeGraph -> Library

sorts ScopeGraph constructors
ScopeGraph: list((Scope * Datum? * list(Edge))) -> ScopeGraph


The top-level Library term contains (in this order): the list of shared scopes, the list of all scopes of the library, and the actual scope graph. A scope graph consists of scope entries, which are defined as three-tuples of: scope, datum associated to that scope, and the list of outgoing edges for that scope.

In order to configure the root scopes correctly, perform the following steps:

1. Identify the project scope. Names of the project scope are: Scope("/.", "s_prj-0") for project scopes generated by the concurrent solver, and Scope("", "s_1-1") for project scopes generated by the traditional solver.
2. Add the project scope to the list of shared scopes in the exported library. Instead of Library([], ...), this term should now look like Library([Scope("/.", "s_prj-0")], ...).
3. If it is present, remove the project root scope from the second list in the Library term.
4. Remove the datum on the project scope. To do this, find the project scope entry in the scope graph, and change its second argument from Some(...) to None(). The entry should now roughly look like (Scope("/.", "s_prj-0"), None(), [...]).

The second step is to copy this file in the lib/ directory of the project that will use the library. Additionally, you might rename the file to something more descriptive (e.g. stdlib), but ensure that the .stxlib extension is preserved.

Thirdly, the library must be enabled. To enable the library, create a lib/stxlibs file (no extension) that contains a list of enabled library names. Continuing our previous example, the content of that file would be ["stdlib"].

Project-level Declarations

When the project constraint asserts declarations, these will be duplicated, because the project constraint is solved both when analyzing the library and when analyzing the project using the library. As a general principle, project constraints should not make declarations. Instead, libraries are meant as a replacement for the project constraint to declare built-in types.

Multiple Libraries

Using multiple libraries is supported by adding multple *.stxlib files in the lib/ directory, and having multiple entries in the stxlibs file. An example of such a file could look like ["lib1", "lib2"]. Note however that libraries will not link properly when different exports are used, due to the fact that scope identities are not deterministic.

## Incremental Solver¶

Thirdly, there is experimental support for incremental analysis. To enable this, the following options for the mode/modes settings are available:

• In language projects: incremental-scope-graph-diff.
• In example projects: incremental-deadlock or incremental-scope-graph-diff.

Last update: 2022-05-27