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Parse Error

rascal-0.34.0

Synopsis​

Parse of a syntactically incorrect string.

Types​

data RuntimeException = ParseError(loc parseloc) | ParseError(loc parseloc, str nt, str s);

Usage​

import Exception; (only needed when ParseError is used in catch)

Description​

This error is generated when during the execution of a Rascal program the parse function is applied to a syntactically incorrect input sentence.

Remedies:

  • Correct the input sentence.
  • Adapt the grammar so that it accepts the input sentence.
  • Catch the ParseError yourself, see try catch.

Examples​

Define the non-terminal As that accepts one or more letters a:

rascal>syntax As = "a"+;
ok

Then import ParseTree so that we can use the parse function:

rascal>import ParseTree;
ok

Now we can parse sentences consisting of letters a:

rascal>parse(#As, "aaaaaaaa");
As: (As) `aaaaaaaa`

But we get an error when parsing syntactically incorrect input (i.e., that does not consists of letters a only):

rascal>parse(#As, "aaaabaaa");
|TODO:///|: ParseError(|unknown:///|(4,1,<1,4>,<1,5>))
ok

We can also catch the ParseError but first import the Rascal modules Exception and IO:

rascal>import Exception;
ok
rascal>import IO;
ok
rascal>try 
>>>>>>>  parse(#As, "aaaabaaa"); 
>>>>>>>catch ParseError(e): 
>>>>>>>  println("Your input cannot be parsed: <e>");
Your input cannot be parsed: |unknown:///|(4,1,<1,4>,<1,5>)
ok