make the file and string stream type-compatible

src/som/compiler/sourcecode_compiler.py

 import os
 
-from rpython.rlib.streamio import open_file_as_stream
-
-from StringIO import StringIO
+from rpython.rlib.streamio import open_file_as_stream, Stream, StreamError
 
 from som.compiler.parser                   import Parser
 from som.compiler.class_generation_context import ClassGenerationContext
 
+
+class _StringStream(Stream):
+    def __init__(self, string):
+        self._string = string
+        self.pos     = 0
+        self.max     = len(string) - 1
+
+    def write(self, data):
+        raise StreamError("StringStream is not writable")
+    def truncate(self, size):
+        raise StreamError("StringStream is immutable")
+
+    def peek(self):
+        if self.pos < self.max:
+            return self._string[self.pos:]
+        else:
+            return ''
+
+    def tell(self):
+        return self.pos
+
+    def seek(self, offset, whence):
+        if whence == 0:
+            self.pos = max(0, offset)
+        elif whence == 1:
+            self.pos = max(0, self.pos + offset)
+        elif whence == 2:
+            self.pos = max(0, self.max + offset)
+        else:
+            raise StreamError("seek(): whence must be 0, 1 or 2")
+
+    def read(self, n):
+        assert isinstance(n, int)
+        end = self.pos + n
+        data = self._string[self.pos:end]
+        self.pos += len(data)
+        return data
+
 def compile_class_from_file(path, filename, system_class, universe):
     return _SourcecodeCompiler().compile(path, filename, system_class, universe)
 
@@ -40,7 +76,7 @@ class _SourcecodeCompiler(object):
         return result
 
     def compile_class_string(self, stream, system_class, universe):
-        self._parser = Parser(StringIO(stream), universe)
+        self._parser = Parser(_StringStream(stream), universe)
 
         result = self._compile(system_class, universe)
         return result