Cleaned the code a bit

src/Evaluator.hs

@@ -3,17 +3,19 @@ module Evaluator where
 import Expression
 import Primitives
 
+-- Evals (reduces) an expression
 eval :: Expression -> Either String Expression
 eval n@(Number _) = Right n
 eval s@(Symbol _) = Right s
-eval (SExpr es)   = mapM eval es >>= apply . SExpr
+eval (SExpr es)   = apply . SExpr =<< mapM eval es
 
+-- Applies, in a SExpr, a function to its arguments
 apply :: Expression -> Either String Expression
 apply (SExpr (f:args)) = case f of
   Symbol "+" -> Right $ pAdd args
   Symbol "-" -> Right $ pSub args
   Symbol "*" -> Right $ pMul args
   Symbol "/" -> Right $ pDiv args
-  Symbol _     -> Left "no primitive functions not supported (only +, -, * and /)"
+  Symbol x   -> Left  $ show x ++ ": no primitive functions not supported"
   _          -> Left  "object not applicable"
 apply _                = Left "object not applicable"

src/Expression.hs

@@ -3,6 +3,7 @@ module Expression where
 -- The basic unit of the language are Expressions, so we make an
 -- Expression data with the primitive expression values as the
 -- constructors
+
 data Expression = Number Double
                 | Symbol String
                 | SExpr [Expression]

src/Parser.hs

@@ -1,7 +1,6 @@
 module Parser where
 
 import Text.ParserCombinators.Parsec
-import Data.Char
 
 import Expression
 
@@ -11,11 +10,11 @@ parseExpression s = do
   e <- parse anyExpressionParser "" s
   return e
 
--- Parser any possible expression
+-- Parser for any possible expression
 anyExpressionParser :: GenParser Char st Expression
 anyExpressionParser = numberParser <|> symbolParser <|> sexprParser
 
--- Parses an S-expr
+-- Parser for an S-expr
 sexprParser :: GenParser Char st Expression
 sexprParser = do
   _ <- spaces
@@ -25,7 +24,7 @@ sexprParser = do
   _ <- char ')' >> spaces
   return $ SExpr es
 
--- Parses a symbol
+-- Parser for a symbol
 symbolParser :: GenParser Char st Expression
 symbolParser = Symbol <$> do
   c <- symbolChar
@@ -33,7 +32,7 @@ symbolParser = Symbol <$> do
   return $ c:cs
     where symbolChar = letter <|> oneOf "!#$%&*+-/:<=>?@\\^_`~"
 
--- Parses a number
+-- Parser for a number
 numberParser :: GenParser Char st Expression
 numberParser = do
   num <- read <$> many1 digit

src/Primitives.hs

@@ -1,10 +1,15 @@
-module Primitives where
+module Primitives (pAdd, pSub, pMul, pDiv) where
+
+-- This module exports the primitve expressions for our language
 
 import Expression
 
-pAdd, pSub, pMul, pDiv :: [Expression] -> Expression
+pArith :: (Double -> Double -> Double) -> [Expression] -> Expression
+pArith f es = Number . foldr1 f $ map unnum es
+  where unnum = \(Number x) -> x
 
-pAdd = foldr1 $ \(Number a) (Number b) -> Number (a+b)
-pSub = foldr1 $ \(Number a) (Number b) -> Number (a-b)
-pMul = foldr1 $ \(Number a) (Number b) -> Number (a*b)
-pDiv = foldr1 $ \(Number a) (Number b) -> Number (a/b)
+pAdd, pSub, pMul, pDiv :: [Expression] -> Expression
+pAdd = pArith (+)
+pSub = pArith (-)
+pMul = pArith (*)
+pDiv = pArith (/)