I had this same problem... There was one more thing.... I think it needed a type annotation somewhere. I'll take a look.

Edit: found it. I also needed to make this change to get it to compile in psci...

diff --git a/src/Language/PureScript/Parser/Lexer.hs b/src/Language/PureScript/Parser/Lexer.hs
index 20ddf8b..0510b91 100644
--- a/src/Language/PureScript/Parser/Lexer.hs
+++ b/src/Language/PureScript/Parser/Lexer.hs
@@ -517,4 +517,4 @@ reservedTypeNames = [ "forall", "where" ]
 -- The characters allowed for use in operators
 --
 isSymbolChar :: Char -> Bool
-isSymbolChar c = (c `elem` ":!#$%&*+./<=>?@\\^|-~") || (not (isAscii c) && isSymbol c)
+isSymbolChar c = (c `elem` (":!#$%&*+./<=>?@\\^|-~" :: String)) || (not (isAscii c) && isSymbol c)

I've just seen the same thing that @kritzcreek had, except with cabal repl (but not cabal build) using GHC 7.10.3. I've been using 7.10.3 for a while so I have no idea how I haven't noticed this before.

Thanks!

Do either of you have a global .ghci file, in $HOME/.ghci? I just realised that in my case, a global .ghci file was the cause of the IsString problem that @kritzcreek mentioned in his comment. My $HOME/.ghci was enabling the OverloadedStrings extension. Removing that extension fixed it.