SWI-Prolog 9.2.9 Reference Manual: Section 9.8

## 9.8 CHR Compiler Errors and Warnings

In this section we summarize the most important error and warning messages of the CHR compiler.

### 9.8.1 CHR Compiler Errors

Type clash
  for variable ... in rule ...

  This error indicates an inconsistency between declared types; a variable can not belong to two types. See static type checking.

Invalid functor
  in head ... of rule ...

  This error indicates an inconsistency between a declared type and the use of a functor in a rule. See static type checking.

Cyclic alias
  definition: ... == ...

  You have defined a type alias in terms of itself, either directly or indirectly.

Ambiguous type aliases

  You have defined two overlapping type aliases.

Multiple definitions
  for type

  You have defined the same type multiple times.

Non-ground type
  in constraint definition: ...

  You have declared a non-ground type for a constraint argument.

Could not find type definition
  for ...

  You have used an undefined type in a type declaration.

Illegal mode/type declaration

  You have used invalid syntax in a constraint declaration.

Constraint multiply defined

  There is more than one declaration for the same constraint.

Undeclared constraint
  ... in head of ...

  You have used an undeclared constraint in the head of a rule. This often indicates a misspelled constraint name or wrong number of arguments.

Invalid pragma
  ... in ... Pragma should not be a variable.

  You have used a variable as a pragma in a rule. This is not allowed.

Invalid identifier
  ... in pragma passive in ...

  You have used an identifier in a passive pragma that does not correspond to an identifier in the head of the rule. Likely the identifier name is misspelled.

Unknown pragma
  ... in ...

  You have used an unknown pragma in a rule. Likely the pragma is misspelled or not supported.

Something unexpected
  happened in the CHR compiler

  You have most likely bumped into a bug in the CHR compiler. Please contact Tom Schrijvers to notify him of this error.