Changed synopsis from

A railway oriented mini-DSL for unified error handling.

Railway-oriented error handling: unwrap or throw in one short operator.

Changed description from

A railway oriented mini-DSL for handling and interpreting error states 
and cardinality checks across diverse failure types in a unified way.

Railway-oriented error handling for Haskell: one short operator per
step, and the failure paths stop taking up lines.

Each operator peels one layer of a result (@Maybe@, @Either@, @Bool@,
@Validation@, or a collection of them) and says what to throw if it
failed. Where you would write

> rows <- runQuery (userById uid) >>= either (const (throwError err503)) pure
> user <- case rows of
>   []  -> throwError err404
>   [u] -> pure u
>   _   -> throwError err500

you write

> user <- runQuery (userById uid) ? err503 ?! cardinalityErr err404 (const err500)

Layers are peeled left to right: @? err503@ unwraps the database
@Either@, then @?!@ demands exactly one row (none is a 404, several a 500).

* Works in any @MonadError@ ("Railroad.MonadError": @mtl@, @ExceptT@,
  servant's @Handler@) and in @effectful@ with the @Error@ effect
  ("Railroad").

* Unwrap or throw: @?@ (constant error) and @??@ (error computed from
  the failure). Recover instead of throwing: @?~@ and @??~@.

* Tag by a predicate with @?>@, or fall back to another source with @?|@
  and @?|<>@.

* @Validation@ failures accumulate: validate a whole request and report
  every problem at once.

* All operators are @infixl 1@ like @>>=@ and @\<&\>@, so they chain without
  parentheses.

Collections are checked with quantifiers: @?@ is ∀ ("every element
succeeds", so an empty collection passes), @?+@ is ∃ ("at least one"),
@?!@ is ∃! ("exactly one") and @?∅@ is ¬∃ ("none"). They chain:
@xs ?+ e1 ? e2@ means "at least one, and all succeed". The readme has the
details and a longer servant example.