A collection of information for pretty printing an error.

Creates a simple error that has a single block, with an optional header or body.

Information about a block in the source code, such as pointers and messages.

Each block has a style associated with it.

A simple block that points to only one line and optionally has a label, header, or body message.

A variant of blockSimple that only points at one column.

A block that points to two parts of the source that are visually connected together.

A variant of blockConnected where the pointers point at only one column.

A block that points to two parts of the source that are visually connected together.

If the two parts of the source happen to be on the same line, the pointers are merged into one.

A variant of blockMerged where the pointers point at only one column.

A pointer is the span of the source code at a line, from one column to another. Each of the positions start at 1.

A pointer may also have a label that will display inline.

A pointer may also be connected to all the other pointers within the same block.

Stylization options for a block, e.g. characters to use.

A basic style using only ASCII characters.

Errors should look like so:

error header message
--> file.ext:1:16
block header message
  |
1 |   line 1 foo bar do
  |  ________________^^ start label
2 | | line 2
  | |      ^ unconnected label
3 | | line 3
. | |______^ middle label
6 | | line 6
7 | | line 7 baz end
  | |______^_____^^^ end label
  |        |
  |        | inner label
block body message
error body message

A fancy style using Unicode characters.

Errors should look like so:

error header message
→ file.ext:1:16
block header message
  │
1 │   line 1 foo bar do
  │ ┌────────────────^^ start label
2 │ │ line 2
  │ │      ^ unconnected label
3 │ │ line 3
. │ ├──────^ middle label
6 │ │ line 6
7 │ │ line 7 baz end
  │ └──────^─────^^^ end label
  │        │
  │        └ inner label
block body message
error body message

A fancy style using Unicode characters and ANSI colors, similar to fancyStyle. Most things are colored red.

A fancy style using Unicode characters and ANSI colors, similar to fancyStyle. Most things are colored yellow.

Pretty prints errors. The original source is required. Returns Text (lazy). If the list is empty, an empty string is returned.

Suppose we had an error of this type:

data ParseError = ParseError
    { peFile       :: FilePath
    , peLine       :: Int
    , peCol        :: Int
    , peUnexpected :: T.Text
    , peExpected   :: [T.Text]
    }

Then we can create a simple pretty printer like so:

import qualified Data.List.NonEmpty as N
import qualified Data.Text as T
import qualified Data.Text.Lazy.IO as TL
import           Errata

toErrata :: ParseError -> Errata
toErrata (ParseError fp l c unexpected expected) =
    errataSimple
        (Just "an error occured!")
        (blockSimple basicStyle fp
            (Just "error: invalid syntax")
            (l, c, c + T.length unexpected, Just "this one")
            (Just $ "unexpected " <> unexpected <> "\nexpected " <> T.intercalate ", " expected))
        Nothing

printErrors :: T.Text -> [ParseError] -> IO ()
printErrors source es = TL.putStrLn $ prettyErrors source (toErrata <$> es)

Note that in the above example, we have OverloadedStrings enabled to reduce uses of pack.

An example error message from this might be:

an error occured!
--> ./comma.json:2:18
error: invalid syntax
  |
2 |     "bad": [1, 2,]
  |                  ^ this one
unexpected ]
expected null, true, false, ", -, digit, [, {

A variant of prettyErrors for non-empty lists. You can ensure the output is never an empty string.