diff --git a/src/file.lua b/src/file.lua
index 2ac840f..372d49b 100644
--- a/src/file.lua
+++ b/src/file.lua
@@ -15,6 +15,50 @@ function start_reading(fs, filename)
   }
 end
 
+-- fake file object with the same 'shape' as that returned by start_reading
+function fake_file_stream(s)
+  local i = 1
+  local max = string.len(s)
+  return {
+    read = function(format)
+      if i > max then
+        return nil
+      end
+      if type(format) == 'number' then
+        local result = s:sub(i, i+format-1)
+        i = i+format
+        return result
+      elseif format == '*a' then
+        local result = s:sub(i)
+        i = max+1
+        return result
+      elseif format == '*l' then
+        local start = i
+        while i <= max do
+          if s:sub(i, i) == '\n' then
+            break
+          end
+          i = i+1
+        end
+        local result = s:sub(start, i)
+        i = i+1
+        return result
+      elseif format == '*n' then
+        error('fake file streams: *n not yet supported')
+      end
+    end,
+  }
+end
+
+function test_fake_file_system()
+  local s = fake_file_stream('abcdefgh\nijk\nlmn')
+  check_eq(s.read(1), 'a', 'fake_file_system: 1 char')
+  check_eq(s.read(1), 'b', 'fake_file_system: 1 more char')
+  check_eq(s.read(3), 'cde', 'fake_file_system: multiple chars')
+  check_eq(s.read('*l'), 'fgh\n', 'fake_file_system: line')
+  check_eq(s.read('*a'), 'ijk\nlmn', 'fake_file_system: all')
+end
+
 -- primitive for writing files to a file system (or, later, network)
 -- returns an object or nil on error
 -- write to the object using .write()