From: Samir Benmendil <me@rmz.io>
Date: Sat, 13 Jan 2018 23:24:30 +0000 (+0000)
Subject: Ignore spaces
X-Git-Url: https://git.rmz.io/my-scheme.git/commitdiff_plain/429ac1845f1b93f54bca8511a4efbf8cbcbb9bf3?ds=sidebyside

Ignore spaces

We add the spaces `Parser` which returns nothing `()` and skips any
spaces.

`spaces` is already defined in Parsec, but does not do exactly what we
want, we can hide it from the import.

`skipMany a` matches 0 or more `a` and skips them.

`>>` is the **bind** operator. It may have completely different meanings
depending on the monad you use. In the Parser monad in means "try the
first parameter, then try the second with anything that is left", i.e.
"first skip all the spaces, then match the symbol".
---

diff --git a/app/Main.hs b/app/Main.hs
index a94dc2c..8520891 100644
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -1,12 +1,15 @@
 module Main where
-import Text.ParserCombinators.Parsec
+import Text.ParserCombinators.Parsec hiding (spaces)
 import System.Environment
 
 symbol :: Parser Char
 symbol = oneOf "!#$%&|*+-/:<=>?@^_~"
 
+spaces :: Parser ()
+spaces = skipMany space
+
 readExpr :: String -> String
-readExpr input = case parse symbol "lisp" input of
+readExpr input = case parse (spaces >> symbol) "lisp" input of
     Left err -> "No match: " ++ show err
     Right val -> "Found value"