In this example all data fields have the same type. This simplifies encoding work as all fields will be encoded the same way. We use IP address since all fields are single byte size.

Simplified Part header

Simplified Email header

This section shows a type safe processing of emails.

SimplifiedEmailF is an over-simplified email type, it has parts that can be either

• binary and have to be Base 64 encoded or
• are text that have either UTF8 or ASCII character set

The text parts can be optionally can be Base 64 encoded but do not have to be.

For simplicity, the layout of simplified headers is assumed the same as encoding annotations in this library.

tstEmail contains some simple data to play with

This example encodes fields in SimplifiedEmailF into an untyped version of Enc which stores verified encoded data and encoding information is stored at the value level: CheckedEnc () B.ByteString.

Part of email are first converted to UncheckedEnc (that stores encoding information at the value level as well). UncheckedEnc that can easily represent parts of the email

>>> let part = parts tstEmail L.!! 2
>>> part
(["enc-B64","r-UTF8"],"U29tZSBVVEY4IFRleHQ=")
>>> let unchecked = toUncheckedEnc (fst part) () (snd part)
>>> unchecked
MkUncheckedEnc ["enc-B64","r-UTF8"] () "U29tZSBVVEY4IFRleHQ="

We can play Alternative (<|>) game (we acually use Maybe) with final option being a RecreateEx error:

>>> check @'["enc-B64","r-ASCII"] @(Either RecreateEx) $ unchecked
Nothing
>>> check @'["enc-B64","r-UTF8"] @(Either RecreateEx) $ unchecked
Just (Right (UnsafeMkEnc Proxy () "U29tZSBVVEY4IFRleHQ="))

Since the data is heterogeneous (each piece has a different encoding annotation), we need wrap the result in another plain ADT: CheckedEnc.

CheckedEnc is similar to UncheckedEnc with the difference that the only (safe) way to get values of this type is from properly encoded Enc values.

Using unsafeCheckedEnc would break type safety here.

It is important to handle all cases during encoding so decoding errors become impossible.

Again, use of dependently typed variant types that could enumerate all possible encodings would made this code nicer.

Example decodes parts of email that are base 64 encoded text and nothing else.

This provides a type safety assurance that we do not decode certain parts of email (like trying to decode base 64 on a plain text part).

>>> decodeB64ForTextOnly <$> recreateEncoding tstEmail
Right (SimplifiedEmailF {emailHeader = "Some Header", parts = [UnsafeMkCheckedEnc ["enc-B64"] () "U29tZSBBU0NJSSBUZXh0",UnsafeMkCheckedEnc ["r-ASCII"] () "Some ASCII Text",UnsafeMkCheckedEnc ["r-UTF8"] () "Some UTF8 Text",UnsafeMkCheckedEnc ["r-ASCII"] () "Some ASCII plain text"]})

Combinator fromCheckedEnc @'["enc-B64", "r-UTF8"] acts as a selector and picks only the ["enc-B64", "r-UTF8"] values from our Traversable type.

We play the (<|>) game on all the selectors we want picking and decoding right pieces only.

Imagine this is one of the pieces:

>>> let piece = unsafeCheckedEnc ["enc-B64","r-ASCII"] () ("U29tZSBBU0NJSSBUZXh0" :: B.ByteString)
>>> displ piece
"UnsafeMkCheckedEnc [enc-B64,r-ASCII] () (ByteString U29tZSBBU0NJSSBUZXh0)"

This code will not pick it up:

>>> fromCheckedEnc @ '["enc-B64", "r-UTF8"] $ piece
Nothing

But this one will:

>>> fromCheckedEnc @ '["enc-B64", "r-ASCII"]  $ piece
Just (UnsafeMkEnc Proxy () "U29tZSBBU0NJSSBUZXh0")

so we can apply the decoding on the selected piece

>>> fmap (toCheckedEnc . decodePart @'["enc-B64"]) . fromCheckedEnc @ '["enc-B64", "r-ASCII"] $ piece
Just (UnsafeMkCheckedEnc ["r-ASCII"] () "Some ASCII Text")