# Grammar::PrettyErrors
[](
https://travis-ci.org/bduggan/p6-grammar-prettyerrors)
Generate pretty errors when your parse fails.
# Synopsis
Input:
```perl6
use Grammar::PrettyErrors;
grammar G does Grammar::PrettyErrors {
rule TOP {
'orange'+ % ' '
}
}
G.parse('orange orange orange banana');
```
Output:
```
--errors--
1 │▶orange orange orange banana
^
Uh oh, something went wrong around line 1.
Unable to parse TOP.
```
# Description
`Grammar::PrettyErrors` provides a role that can
be applied to a grammar to provide pretty error
messages when a parse fails. It works by wrapping
the `<ws>` token and keeping track of a highwater
mark (the position), and the name of the most
recent rule that was encountered.
This technique is described by moritz in his
excellent book [0] (see below).
# Classes, Roles, Methods
## Grammar::PrettyErrors (Role)
### ATTRIBUTES
* `quiet` -- Bool, default false: save errors, don't throw them.
* `colors` -- Bool, default true: make output colorful.
* `error` -- a `PrettyError` object (below).
### METHODS
* `new` -- wraps the `<ws>` token as described above, it also takes
additional named arguments (to set the ATTRIBUTEs above).
## PrettyError (class)
### METHODS
* `report` -- returns the text of a report with the context for an error.
# EXAMPLES
```
grammar G does Grammar::PrettyErrors { ... }
# Throw an exception with a message when a parse fails.
G.parse('orange orange orange banana');
# Same thing
G.new.parse('orange orange orange banana');
# Pass options to the constructor
my $g = G.new(:quiet, :!colors);
$g.parse('orange orange orange banana');
# Report a parse error
say .report with $g.error;
```
# SEE ALSO
* [0] [Parsing with Perl 6 Regexes and Grammars](
https://www.apress.com/us/book/9781484232279) and the accompanying [code](
https://github.com/Apress/perl-6-regexes-and-grammars/blob/master/chapter-11-error-reporting/03-high-water-mark.p6)
* Grammar::ErrorReporting
# AUTHOR
Brian Duggan
