--- /dev/null
+# This file is part of NIT ( http://www.nitlanguage.org ).
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Detection of divisions by zero in obvious cases
+# This module is only an example of how to implements a simple phase in the nit tools
+module div_by_zero
+
+import phase
+import literal # Because we need the real value of literal in the AST
+import typing # Because we need the static type of receivers
+
+# Note, `ToolContext` is the main overlord class
+# Among its jobs, it orchestrates and executes the various phases
+redef class ToolContext
+ # Instantiate a new specific phase `div_by_zero_phase`
+ # It must be executed after the two phases `literal_phase` and `typing_phase`
+ # Those two phases are statically known since they are introduced by the modules `literal` and `typing`
+ # Note that the constructor automatically register the phase to the ToolContext; this explains why there is `self`
+ var div_by_zero_phase: Phase = new DivByZeroPhase(self, [literal_phase, typing_phase])
+end
+
+# OK, let we create a specific phase
+private class DivByZeroPhase
+ super Phase
+
+ # Specific phases just have to implement the `process_nmodule` method.
+ redef fun process_nmodule(nmodule)
+ do
+ # The AST node is not enough, we need also the associated model element
+ var mmodule = nmodule.mmodule.as(not null)
+ # For the specific job we have, the simpler it to launch a visitor on
+ # all elements of the AST.
+ var visitor = new DivByZeroVisitor(toolcontext, mmodule)
+ visitor.enter_visit(nmodule)
+ end
+end
+
+# The real visitor that does the real job
+private class DivByZeroVisitor
+ super Visitor
+
+ # The toolcontext is our entree point to most services
+ var toolcontext: ToolContext
+
+ # The mmodule is the current module
+ var mmodule: MModule
+
+ init(toolcontext: ToolContext, mmodule: MModule)
+ do
+ self.toolcontext = toolcontext
+ self.mmodule = mmodule
+ end
+
+ redef fun visit(node)
+ do
+ # Recursively visit all sub-nodes
+ node.visit_all(self)
+
+ # Now just filter out until we get what we want...
+
+ # 1. We need a `/` operation
+ if not node isa ASlashExpr then return
+
+ # 2. The second operand must be a integer literal
+ var op2 = node.n_expr2
+ if not op2 isa AIntExpr then return
+
+ # 3. Its value must be 0
+ # Note: because of `literal_phase` the `value` method exists
+ if op2.value != 0 then return
+
+ # 4. The receiver (first operand) must be an Int
+ var op1 = node.n_expr
+ var int_type = mmodule.get_primitive_class("Int").mclass_type
+ if not op1.mtype.is_subtype(mmodule, null, int_type) then return
+
+ # Error detected
+ toolcontext.error(node.location, "Error: Definitely division by zero")
+ end
+end