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TypePal Configuration



Configuration options for TypePal


TypePal provides configuration options for

  • Name Resolution & Overloading: configures how names are resolved and which overloading is allowed.
  • Operations on Types: configures how operations like subtype and least-upper-bound (lub) are defined.
  • Retrieval of Types: configures how named and structured types are handled.
  • Extension Points: configures operations before an after solving.
  • Miscellaneous: utility functions that can be configured.
  • Verbosity: configures the verbosity of TypePal.

Here is an overview:



/* Configuration field */ Accept (TModel tm, loc def, Use use) isAcceptableSimple


  • tm is a given TModel
  • def is a proposed definition
  • use is the use (characterized by the Use data type that contains, name, occurrence, scope and identifier roles of the use for which the definition is proposed.

isAcceptableSimple accepts or rejects a proposed definition for the use of a simple name in a particular role. The returned Accept data type is defined as:

data Accept 
= acceptBinding()
| ignoreContinue()
| ignoreSkipPath()

The default isAcceptableSimple returns acceptBinding()`.

Typical concerns addressed by isAcceptableSimple are:

  • enforce definition before use;
  • check access rights, e.g. visibility.

By comparing the offset of the source locations of definition, respectively, the use, we enforce definition before use:

Accept myIsAcceptableSimple(TModel tm, loc def, Use use)
= use.occ.offset > def.offset ? acceptBinding() : ignoreContinue();


/* Configuration field */ Accept (TModel tm, loc def, Use use) isAcceptableQualified


  • tm is a given TModel
  • def is a proposed definition
  • use is the use for which the definition is proposed.

isAcceptableQualified accepts or rejects a proposed definition for the use of a qualified name in a particular role.


/* Configuration field */ 
Accept (TModel tm, loc defScope, loc def, Use use, PathRole pathRole) isAcceptablePath


  • tm is a given TModel;
  • defScope is the scope in which the proposed definition occurs;
  • def is a proposed definition;
  • use is the use for which the definition is proposed;
  • pathRole is the role of the semantic path.

isAcceptablePath accepts or rejects a proposed access path between use and definition.

To illustrate this, assume a language with modules and imports. A module may contain variable definitions but these are not visible from outside the module. This can be enforced as follows:

Accept myIsAcceptablePath(TModel tm, loc def, Use use, PathRole pathRole) {
return variableId() in use.idRoles ? ignoreContinue() : acceptBinding();


/* Configuration field */ bool (set[loc] defs, map[loc, Define] defines) mayOverload 

mayOverload determines whether a set of definitions (defs) are allowed to be overloaded, given their definitions (defines).

For example, Featherweight Java the only allowed overloading is between class names and constructor names.

bool fwjMayOverload (set[loc] defs, map[loc, Define] defines) {
roles = {defines[def].idRole | def <- defs};
return roles ### {classId(), constructorId()};
  • ❶ First collect all the roles in which the overloaded names have been defined.
  • ❷ Only allow the combination of class name and constructor name.

Operations on Types

Various operations on types can be configured by way of user-defined functions.


/* Configuration field */ bool (AType l, AType r) isSubType 

Function that checks whether l is a subtype of r.


/* Configuration field */ AType (AType l, AType r) getLub

Function that computes the least upperbound of two types and l and r.


/* Configuration field */ AType() getMinAType 

Function that returns the smallest type of the type lattice.


/* Configuration field */ AType() getMaxAType

Function that returns the largest type of the type lattice.


/* Configuration field */ AType (Tree current, AType def, AType ins, AType act, Solver s) instantiateTypeParameters

The function instantiateTypeParameters defines instantiation of language-specific type parameters, where:

  • current is a source code fragment for which type act has already been determined, but any language-specific type parameters in act may still need to be instantiated.
  • def is the parameterized type of act.
  • ins is an instantiated version of the type of act (i.e., with bound type parameters).
  • act is the actual type found for current that needs to be instantiated.

instantiateTypeParameters will match def with ins and the resulting bindings will be used to instantiate act. The instantiated version of act is returned.

In the StructParameters demo parameterized structs (records) are defined. The formal type of such a struct is structDef(str name, list[str] formals), i.e., a struct has a name and a list of named formal type parameters. The actual type of a struct is structType(str name, list[AType] actuals), i.e., a struct name followed by actual types for the parameters.

The definition of instantiateTypeParameters for this example is as follows:

AType structParametersInstantiateTypeParameters(Tree current, structDef(str name1, list[str] formals), structType(str name2, list[AType] actuals), AType t, Solver s){
if(size(formals) != size(actuals)) throw checkFailed([]);
bindings = (formals[i] : actuals [i] | int i <- index(formals));

return visit(t) { case typeFormal(str x) => bindings[x] };

default AType structParametersInstantiateTypeParameters(Tree current, AType def, AType ins, AType act, Solver s) = act;


/* Configuration field */  tuple[list[str] typeNames, set[IdRole] idRoles] (AType atype) getTypeNamesAndRole

This function determines whether a given atype is a named type or not. This is needed for the customization of indirect type computations such as useViaType and getTypeInType. When atype is a named type getTypeNamesAndRole returns:

  • A list of names that may be associated with it. In most languages this will contain just a single element, the name of the type. In more sophisticated cases the list may contain a list of named types to be considered.
  • A list of roles in which the type name can be bound.

Here are the definitions for the Struct demo:

tuple[list[str] typeNames, set[IdRole] idRoles] structGetTypeNamesAndRole(structType(str name)){
return <[name], {structId()}>;

default tuple[list[str] typeNames, set[IdRole] idRoles] structGetTypeNamesAndRole(AType t){
return <[], {}>;
  • ❶ A structType(name) has a name that is bound in the role structId(). Return the name and role.
  • ❷ Any other type is unnamed; return an empty list of type names.

Another example is the Rascal type checker, where we need to model the case that all abstract data types are a subtype of Tree. In that case getTypeNamesAndRole will return <["A", "Tree"], roles> for an abstract data type A. The net effect is that when the search for a name in A fails, the search is continued in Tree.


/* Configuration field */  AType (Define containerDef, str selectorName, set[IdRole] idRolesSel, Solver s) getTypeInTypeFromDefine

In some extreme cases (think Rascal) the type of a field selection cannot be determined by considering all the fields defined in a container type and as a last resort one needs to fall back to information that has been associated with the original definition of the container type. getTypeInTypeFromDefine is called as a last resort from getTypeInType.

In the Rascal type checker common keyword parameters of data declarations are handled using getTypeInTypeFromDefine.


/* Configuration field */ AType(AType containerType, Tree selector, loc scope, Solver s) getTypeInNamelessType

getTypeInNamelessType describes field selection on built-types that have not been explicitly declared with a name. A case in point is a length field on a built-in string type.

In the StaticFields demo)) this is done as follows:

AType staticFieldsGetTypeInNamelessType(AType containerType, Tree selector, loc scope, Solver s){
if(containerType ### strType() && "<selector>" ### "length") return intType();, "Undefined field %q on %t", selector, containerType));


/* Configuration field */ TModel(map[str,Tree] namedTrees, TModel tm) preSolver

A function preSolver that can enrich or transform the TModel before the Solver is applied to it.


/* Configuration field */ void (map[str,Tree] namedTrees, Solver s) postSolver

A function postSolver that can enrich or transform the TModel after constraint solving is complete.


/* Configuration field */  str(str) unescapeName  

A function unescapeName to define language-specific escape rules for names. By default, all backslashes are removed from names.


/* Configuration field */ bool validateConstraints = true

When validateConstraints is true, the validity of all constraints is checked before solving starts. For all dependencies (in facts, calculators and requirements) a calculator needs to be present to solve that dependency.


The verbosity of TypePal can be controlled with several configurations settings.


/* Configuration field */ bool showTimes = false

When showTimes is true, the time of the Collector and Solver phases is printed.


/* Configuration field */ bool showSolverSteps = false

When showSolverSteps is true, each step of the Solver is printed.


/* Configuration field */ bool showSolverIterations = false

When showSolverIterations is true, information is printed about each iteration of the Solver.


/* Configuration field */ bool showAttempts = false

When showAttempts is true, the number of evaluation attempts per calculator or requirement is printed when solving is complete.


/* Configuration field */ bool showTModel = false

When showTModel is true, the resulting TModel is printed when solving is complete.