diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,9 @@
 # Changelog for hpc-threshold
 
+## 0.1.0.3
+
+Add documentation to HPCThreshold.
+
 ## 0.1.0.2
 
 Change the module name to HPCThreshold.
diff --git a/hpc-threshold.cabal b/hpc-threshold.cabal
--- a/hpc-threshold.cabal
+++ b/hpc-threshold.cabal
@@ -2,10 +2,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: da7496f9be2296b4dff3d2e626512cf322514acefdbf3e55fa9cb287d4322641
+-- hash: 253eeafac618ebe072d4acd3afa728e381503d01f7b2c00473f8e3a6ee720ec7
 
 name:           hpc-threshold
-version:        0.1.0.2
+version:        0.1.0.3
 synopsis:       Ensure the code coverage is above configured thresholds
 description:    Please see the README on Github at <https://github.com/eckyputrady/hpc-threshold#readme>
 category:       Development
diff --git a/src/HPCThreshold.hs b/src/HPCThreshold.hs
--- a/src/HPCThreshold.hs
+++ b/src/HPCThreshold.hs
@@ -4,6 +4,7 @@
 import Data.ByteString (ByteString)
 import Data.String.Interpolate
 
+-- | Data structure for a single threshold configuration.
 data Threshold = Threshold
   { thresholdName :: String
   , thresholdRegex :: ByteString
@@ -11,9 +12,15 @@
   } deriving (Read, Show, Eq)
 
 type Coverage = Double
+
+-- | The result of evaluation. The _3 indicates whether the coverage passes the threshold or not.
 type ThresholdEvaluationResult = (Threshold, Coverage, Bool)
 
-extractCoverage :: String -> ByteString -> Coverage
+-- | Extract the coverage from input string using a regex string
+extractCoverage 
+  :: String -- ^ The input string
+  -> ByteString -- ^ The regex to extract the coverage. The regex should contain `(\\d+)` capture otherwise the coverage is always be 0.
+  -> Coverage -- ^ The extracted coverage
 extractCoverage src regexStr =
   case compileM regexStr [] of
     Left e ->
@@ -25,6 +32,7 @@
         _ ->
           0
 
+-- | Evaluate the given string against the given threshold, producing an evaluation result
 evaluate :: String -> Threshold -> ThresholdEvaluationResult
 evaluate src threshold =
   let 
@@ -35,6 +43,7 @@
   in
     (threshold, coverage, isPass)
 
+-- | Produce a human-friendly output of the evaluation result
 reportThreshold :: ThresholdEvaluationResult -> String
 reportThreshold (Threshold tName _ tValue, coverage, isPass) =
   let
@@ -43,7 +52,11 @@
   in
     [i|#{remark} #{tName}: #{coverage}% (#{sign} #{tValue}%)|]
 
-evaluateAndReport :: String -> [Threshold] -> (String, Bool)
+-- | Evaluate a string against a list of thresholds configuration and produce a report
+evaluateAndReport
+  :: String -- ^ The input string
+  -> [Threshold] -- ^ The list of thresholds
+  -> (String, Bool) -- ^ Pair of report and whether there is any coverage under thresholds
 evaluateAndReport src thresholds =
   let
     evalResults = map (evaluate src) thresholds
@@ -55,4 +68,3 @@
     report = unlines $ reportSummary : map reportThreshold evalResults
   in
     (report, isAllThresholdPass)
-