Multiple base numbers parser
do
   digits &lt;- someParser
   return $ someFunction digits


is just

someFunction &lt;$&gt; someParser


We can make parseNumberBase a lot shorter with that information:

-- | Parses a number at a specific base
parseNumberBase :: Char -&gt; Parser LispVal
parseNumberBase 'b' = Number . fromJust . readBinary &lt;$&gt; many1 (oneOf "01")
parseNumberBase 'o' = Number . fst . head . readOct  &lt;$&gt; many1 octDigit     -- see below
parseNumberBase 'd' = Number . read                  &lt;$&gt; many1 digit
parseNumberBase 'h' = Number . fst . head . readHex  &lt;$&gt; many1 hexDigit     -- see below
parseNumberBase _   = error "Wrong number base"


readFunc digits !! 0 is head (readFunc digits), so we were able to get rid of !! too.

That being said, parseNumberBase has a lot of responsibility. Split it into multiple parsers, and it's suddenly a lot easier to grasp:

parseBinary :: Parser LispVal
parseBinary = Number . fromJust . readBinary &lt;$&gt; many1 (oneOf "01")

parseDecimal :: Parser LispVal
parseDecimal = Number . read &lt;$&gt; many1 digit

parseHexadecimal :: Parser LispVal
parseHexadecimal = Number . fst . head . readHex &lt;$&gt; many1 hexDigit

parseOct :: Parser LispVal
parseOct = Number . fst . head . readOct  &lt;$&gt; many1 octDigit


We can now test all those functions in isolation. We can now write parseNumberBase as another parser:

parseNumber :: Parser LispVal
parseNumber = parseDecimal &lt;|&gt; char '#' *&gt; parseNumberBase

parseNumberBase :: Parser LispVal
parseNumberBase =  char 'h' *&gt; parseHexadecimal 
               &lt;|&gt; char 'd' *&gt; parseDecimal 
               &lt;|&gt; char 'o' *&gt; parseOct
               &lt;|&gt; char 'b' *&gt; parseBinary