title	author
Correct parsers for bounded integral values	Stefan Klinger <https://www.stefan-klinger.de>

Correct parsers for bounded integral values

Integer parsers for Parsec (on Text streams) and Attoparsec (on ByteString and Text streams), that fail instead of wrapping around.

Assume a program needs to parse the decimal representation of a bounded integral type. You have chosen Word8, and use Attoparsec on Text streams. The following examples can be replicated by loading GHCi with the appropriate set of modules. Run stack ghci robust-int:demo.

Current situation

The base package provides read:

> read "298" :: Word8
42

Attoparsec provides decimal:

> parseOnly (decimal :: Parser Word8) $ pack "298"
Right 42

Both are wrong. The user did not say "42". The chosen type not big enough for 298.

Improvement

This library gives you bounded (plus some more low-level parsers), which correctly reports the mistake:

> parseOnly (bounded :: Parser Word8) $ pack "298"
Left "Failed reading: out of bounds"

This can be resolved by providing valid input:

> parseOnly (bounded :: Parser Word8) $ pack "198"
Right 198

Or by choosing an adequate type for the desired value:

> parseOnly (bounded :: Parser Word16) $ pack "298"
Right 298

Build instructions

$ stack build --test

$ stack haddock
$ firefox "$(stack path --local-doc-root)/index.html"