# Snapshots, incrementality and reacting to changes

FCS is an incremental execution engine. The aim is to make it Roslyn-like. We're not quite there.

There are two dimensions of incrementality:

• The inputs change, e.g. the source files are edited or a referenced assembly changes, appears or disappears
• The results of analysis on the inputs (e.g. a parse tree) are further enriched with information (e.g. symbol uses are requested) and this information is held, i.e. not re-computed, perhaps by a returned object.

The logical results of all "Check" routines (ParseAndCheckFileInProject, GetBackgroundCheckResultsForFileInProject, TryGetRecentTypeCheckResultsForFile, ParseAndCheckProject) depend on results reported by the file system, especially the IFileSystem implementation described in the tutorial on project wide analysis. Logically speaking, these results would be different if file system changes occur. For example, referenced DLLs may change on disk, or referenced files may change.

There is work-in-progress to make all parsing and checking deliver results based on immutable snapshots of inputs.