diff --git a/src/Lint.elm b/src/Lint.elm
index e0df9905..799c8d1b 100644
--- a/src/Lint.elm
+++ b/src/Lint.elm
@@ -1,4 +1,4 @@
-module Lint exposing (lintSource, lint, visitExpression, doNothing)
+module Lint exposing (lintSource, lint, visitExpression, doNothing, countErrors)
 
 {-| A linter for Elm.
 
@@ -28,13 +28,16 @@ To run the rules on a source code and get a list of errors:
 
 # Rule creation functions
 @docs lint, doNothing, visitExpression
+
+# Internal
+@docs countErrors
 -}
 
 import Ast
 import Ast.Expression exposing (Expression)
 import Ast.Statement exposing (Statement)
 import Combine
-import Lint.Types exposing (LintRule, LintError, LintImplementation, LintRuleImplementation, Direction, Visitor, Severity, Severity(..))
+import Lint.Types exposing (File, LintRule, LintError, LintImplementation, LintRuleImplementation, Direction, Visitor, Severity, Severity(..))
 import Lint.Visitor exposing (transformStatementsIntoVisitors, expressionToVisitors)
 import Regex
 
@@ -152,3 +155,19 @@ context. This is used to avoid a bit of boilerplate for visitor functions whose
 doNothing : LintImplementation a context
 doNothing ctx _ =
     ( [], ctx )
+
+
+{-| Count the number of errors of a given Severity in the list of errors.
+-}
+countErrors : Severity -> List ( File, List ( Severity, LintError ) ) -> Int
+countErrors severity errors =
+    errors
+        |> List.map
+            (Tuple.second
+                >> List.filter
+                    (Tuple.first
+                        >> (==) severity
+                    )
+                >> List.length
+            )
+        |> List.sum
diff --git a/src/Lint/Types.elm b/src/Lint/Types.elm
index d911d821..a52925e3 100644
--- a/src/Lint/Types.elm
+++ b/src/Lint/Types.elm
@@ -1,4 +1,17 @@
-module Lint.Types exposing (LintError, LintImplementation, Direction(..), LintRule, LintRuleImplementation, LintResult, Visitor, Severity, Severity(..))
+module Lint.Types
+    exposing
+        ( Direction(..)
+        , File
+        , LintError
+        , LintImplementation
+        , LintResult
+        , LintRule
+        , LintRuleImplementation
+        , Reporter
+        , Severity
+        , Severity(..)
+        , Visitor
+        )
 
 {-| This module contains types that are used for writing rules.
 
@@ -6,13 +19,13 @@ module Lint.Types exposing (LintError, LintImplementation, Direction(..), LintRu
 @docs LintError, Direction
 
 # Configuration
-@docs LintRule, Severity
+@docs LintRule, Severity, Reporter
 
 # Writing rules
 @docs LintRuleImplementation, LintImplementation
 
 # Internal types
-@docs Visitor, LintResult
+@docs Visitor, LintResult, File
 -}
 
 import Ast.Expression exposing (..)
@@ -142,3 +155,17 @@ type Severity
     = Disabled
     | Warning
     | Critical
+
+
+{-| Description of an Elm file.
+-}
+type alias File =
+    { filename : String
+    , source : String
+    }
+
+
+{-| Function that summarizes the result of the linting process.
+-}
+type alias Reporter a =
+    List ( File, List ( Severity, LintError ) ) -> a