Fun with the recovery feature
Published on February 19, 2016, last updated September 22, 2017
The withRecovery
primitive allows to recover from parse errors
“onthefly” and report several errors after parsing is finished or ignore
them altogether. In this tutorial, we will learn how to use it.
Language that we will parse
For the purposes of this tutorial, we will write parser for a simple
functional language that consists only of equations with a name on the left
hand side and arithmetic expression on the right hand side of =
:
y = 10
x = 3 * (1 + y)
result = x  1 # answer is 32
Here, it can only calculate arithmetic expressions, but if we were to design something more powerful, we could introduce more interesting operators to grab input from console, etc., but since our aim is to explore a new parsing feature, this language will do.
First, we will write a parser that can parse entire program in this language as a list of ASTs representing equations. Then we will make it failuretolerant in a way, so when it cannot parse a particular equation, it does not stop, but continues its work until all input is analyzed.
Parser without recovery
The first version of the parser is very easy to write. We will need the following imports:
{# LANGUAGE FlexibleContexts #}
{# LANGUAGE TypeFamilies #}
module Main where
import Control.Applicative (empty)
import Control.Monad (void)
import Data.Void
import Text.Megaparsec
import Text.Megaparsec.Char
import Text.Megaparsec.Expr
import qualified Text.Megaparsec.Char.Lexer as L
To represent AST of our language we will use these definitions:
type Program = [Equation]
data Equation = Equation String Expr
deriving (Eq, Show)
data Expr
= Value Double
 Reference String
 Negation Expr
 Sum Expr Expr
 Subtraction Expr Expr
 Multiplication Expr Expr
 Division Expr Expr
deriving (Eq, Show)
A program in our language is collection of equations, where every equation gives a name to an expression which in turn can be simply a number, reference to other equation, or some math involving those concepts.
We parse input stream in the form of a String
and we don’t need custom
error messages, so we’ll use the following definition of Parser
:
type Parser = Parsec Void String
As usual, the first thing that we need to handle is white space. We will have two spaceconsuming parsers:

scn
—consumes newlines and white space in general. We will use it for white space between equations, which will start with a newline (since equations are newlinedelimited). 
sc
—this does not consume newlines and is used to define lexemes, i.e. things that automatically eat white space after them.
Here is what I’ve got:
lineComment :: Parser ()
lineComment = L.skipLineComment "#"
scn :: Parser ()
scn = L.space space1 lineComment empty
sc :: Parser ()
sc = L.space (void $ takeWhile1P Nothing f) lineComment empty
where
f x = x == ' '  x == '\t'
lexeme :: Parser a > Parser a
lexeme = L.lexeme sc
symbol :: String > Parser String
symbol = L.symbol sc
Consult Haddocks for description of L.space
, L.lexeme
, and L.symbol
.
In short, L.space
is a helper to quickly put together a generalpurpose
spaceconsuming parser. We will follow this strategy: assume no white space
before lexemes and consume all white space after lexemes. There may be some
white space before the first lexeme, but that will be dealt with specially,
see below.
We also need a parser for equation names (x
, y
, and result
in the
first example). Like in many other programming languages, we will accept
alphanumeric sequences that do not start with a number:
name :: Parser String
name = lexeme ((:) <$> letterChar <*> many alphaNumChar) <?> "name"
All too easy. Parsing of expressions could slow us down, but there is a
solution outofbox in Text.Megaparsec.Expr
module:
expr :: Parser Expr
expr = makeExprParser term table <?> "expression"
term :: Parser Expr
term = parens expr
<> (Reference <$> name)
<> (Value <$> L.float)
table :: [[Operator Parser Expr]]
table =
[ [Prefix (Negation <$ symbol "") ]
, [ InfixL (Multiplication <$ symbol "*")
, InfixL (Subtraction <$ symbol "/") ]
, [ InfixL (Sum <$ symbol "+")
, InfixL (Division <$ symbol "") ]
]
parens :: Parser a > Parser a
parens = between (symbol "(") (symbol ")")
We just wrote a fairly complete parser for expressions in our language! If
you’re new to all this stuff I suggest you load the code into GHCi and play
with it a bit. Use the parseTest'
function to feed input into the parser:
λ> parseTest' expr "5"
Value 5.0
λ> parseTest' expr "5 + foo"
Sum (Value 5.0) (Reference "foo")
λ> parseTest' expr "(x + y) * 5 + 7 * z"
Sum
(Multiplication (Sum (Reference "x") (Reference "y")) (Value 5.0))
(Multiplication (Value 7.0) (Reference "z"))
Power! What remains are the parsers for equations and entire program:
equation :: Parser Equation
equation = Equation <$> (name <* symbol "=") <*> expr
prog :: Parser Program
prog = between scn eof (sepEndBy equation scn)
Note that we need to consume leading whitespace in prog
explicitly. Try
the prog
parser—it’s a complete solution that can parse language we
described in the beginning. Parsing “end of file” eof
explicitly makes the
parser consume all input and fail loudly if it cannot do it, otherwise it
would just stop on the first problematic token and return what it has parsed
so far.
Making use of the recovery feature
Our parser is really dandy, it has nice error messages and does its job well. However, every expression is clearly separated from the others by a newline. This separation makes it possible to analyze many expressions independently, even if one of them is malformed, we have no reason to stop and not to check the others. Reporting multiple parse errors at once may be a more efficient method of communication with the programmer who needs to fix them than when she has to recompile the program every time to get to the next error. In this section we will make our parser failuretolerant and able to report multiple error messages at once.
Let’s add one more type synonym—RawData
:
type RawData t e = [Either (ParseError t e) Equation]
This represents a collection of equations, just like Program
, but every
one of them may be malformed: in that case we get an error message in
Left
, otherwise we have a properly parsed equation in Right
.
You will be amazed just how easy it is to add recovering to an existing parser:
rawData :: Parser (RawData Char Void)
rawData = between scn eof (sepEndBy e scn)
where
e = withRecovery recover (Right <$> equation)
recover err = Left err <$ manyTill anyChar eol
Let try it, here is the input:
foo = (x $ y) * 5 + 7.2 * z
bar = 15
Result:
[ Left
(TrivialError
(SourcePos
{ sourceName = ""
, sourceLine = Pos 1
, sourceColumn = Pos 10 } : [])
(Just (Tokens ('$' : "")))
(fromList [ Tokens (')' : "")
, Label ('o' : "perator")
, Label ('t' : "he rest of expression") ]))
, Right (Equation "bar" (Value 15.0)) ]
How does it work? withRecovery r p
primitive runs the parser p
as usual,
but if it fails, it takes its ParseError
and provides it as an argument to
r
. In r
, we start right were p
failed—no backtracking happens, because
it would make it harder to find position from where to start normal parsing
again. Here we have a chance to consume some input to advance the parser’s
textual position. In our case it’s as simple as eating all input up to the
next newline, but it might be trickier.
You probably want to know now what happens when recovering parser r
fails
as well. The answer is: your parser fails as usual, as if no withRecovery
was used. It’s by design that recovering parser cannot influence error
messages in any way, or it would lead to quite confusing error messages in
some cases, depending on logic of the recovering parser.
Now it’s up to you what to do with RawData
. You can either take all error
messages and print them one by one, or ignore errors altogether and filter
only valid equations to work with.
Conclusion
When you want to use withRecovery
the main thing to remember that parts of
text that you want to allow to fail should be clearly separated from each
other, so recovering parser can reliably skip to the next part if the
current part cannot be parsed. In a language like Python, you could use
indentation levels to tell apart highlevel definitions. In every case you
should use your judgment and creativity to decide how to make use of
withRecovery
.