What's the difference between Text.ParserCombinators.Parsec and Text.Parsec

At the moment there are two widely used major versions of Parsec: Parsec 2 and Parsec 3.

My advice is simply to use the latest release of Parsec 3. But if you want to make a conscientious choice, read on.

New in Parsec 3

Monad Transformer

Parsec 3 introduces a monad transformer, ParsecT, which can be used to combine parsing with other monadic effects.

Streams

Although Parsec 2 lets you to choose the token type (which is useful when you want to separate lexical analysis from the parsing), the tokens are always arranged into lists. List may be not the most efficient data structure in which to store large texts.

Parsec 3 can work with arbitrary streams -- data structures with a list-like interface. You can define your own streams, but Parsec 3 also includes a popular and efficient Stream implementation based on ByteString (for Char-based parsing), exposed through the modules Text.Parsec.ByteString and Text.Parsec.ByteString.Lazy.

Reasons to prefer Parsec 2

Fewer extensions required

Advanced features provided by Parsec 3 do not come for free; to implement them several language extensions are required.

Neither of the two versions is Haskell-2010 (i.e. both use extensions), but Parsec 2 uses fewer extensions than Parsec 3, so chances that any given compiler can compile Parsec 2 are higher than those for Parsec 3.

By this time both versions work with GHC, while Parsec 2 is also reported to build with JHC and is included as one of the JHC's standard libraries.

Performance

Originally (i.e. as of 3.0 version) Parsec 3 was considerably slower than Parsec 2. However, work on improving Parsec 3 performance has been done, and as of version 3.1 Parsec 3 is only slightly slower than Parsec 2 (benchmarks: 1, 2).

Compatibility layer

It has been possible to "reimplement" all of the Parsec 2 API in Parsec 3. This compatibility layer is provided by the Parsec 3 package under the module hierarchy Text.ParserCombinators.Parsec (the same hierarchy which is used by Parsec 2), while the new Parsec 3 API is available under the Text.Parsec hierarchy.

This means that you can use Parsec 3 as a drop-in replacement for Parsec 2.

I believe the latter is a backwards-compatible layer for Parsec 2, implemented in terms of the newer API.