semantic: add semantic checks for variables

Check read before first assignment
Check unused variables

compiler/semantic.py

@@ -5,10 +5,11 @@ from typing import Literal
 
 from . import nodes
 from .errors import SemanticAnalysisError
-from .logger import Logger
+from .logger import Logger, Tracer, LogLevel
 from .typechecking import typecheck
 
 logger = Logger(__name__)
+tracer = Tracer(logger=logger, level=LogLevel.Debug)
 
 
 class SymbolABC(abc.ABC):
@@ -181,6 +182,33 @@ class Context:
     def __repr__(self) -> str:
         return self._pprint()
 
+    @tracer.trace_method
+    def check(self) -> None:
+        for variable in self.variables.values():
+
+            # Check for reads before assignments
+            for read in variable.reads:
+                previous_write = None
+                for write in variable.writes:
+                    if write.location() < read.location():
+                        previous_write = write
+                        break
+                if previous_write is None:
+                    message = f"Reading undefined variable {variable.name}"
+                    if len(variable.writes) > 0:
+                        first_write = min(variable.writes, key=lambda w: w.location())
+                        message += f" (first assignment is at {first_write.location().begin})"
+                    raise SemanticAnalysisError(location=read.location(),
+                                                message=message)
+
+            # Check for unused variables
+            if len(variable.reads) == 0 and variable.definition is not None:
+                raise SemanticAnalysisError(variable.definition.location(),
+                                            message=f"Variable '{variable.name}' is unused")
+
+        for context in self.child_contexts.values():
+            context.check()
+
 
 class BuiltinContext(Context):
     def __init__(self):