Parsing – How to Implement Actions in Raku Based on Parse Success

parsingraku

Is there a way to configure Raku actions to only execute after a parse has completed successfully? For example, the program below outputs "Jane is telling us her age….", even with malformed input.

grammar Foo {
  rule TOP { <name> is \d+ }
  token name { [Anne | Jane] { say "$/ is telling us her age...."; } }
}

say so Foo.parse('Anne is 100');  # True
say so Foo.parse('Jane is Leaf'); # False

Best Answer

You could move the code block to the rule TOP;

grammar Foo {
    rule TOP { <name> is \d+ { say  "$/<name> is telling us her age...."  } }
    token name { [Anne | Jane]  }
}

say so Foo.parse('Anne is 100');
say so Foo.parse('Jane is Leaf');

Anne is telling us her age....
True
False

using the routines make and made;

grammar Foo {
    rule TOP { <name> is \d+ }
    token name { [Anne | Jane]  { make  "$/ is telling us her age...."  } }
}

say Foo.parse('Anne is 100').<name>.made;
say Foo.parse('Jane is Leaf').<name>.made;

Anne is telling us her age....
Nil

or their combination.

grammar Foo {
    rule TOP { <name> is \d+  { say  $/<name>.made } }
    token name { [Anne | Jane]  { make  "$/ is telling us her age...."  } }
}

say so Foo.parse('Anne is 100');
say so Foo.parse('Jane is Leaf');

Anne is telling us her age....
True
False

Remember that for more complex problems, you can explore using Actions.

Related Solutions

Python – How to Parse a String to a Float or Int

>>> a = "545.2222"
>>> float(a)
545.22220000000004
>>> int(float(a))
545

C++ String Parsing – How to Parse a String in C++ Using String Delimiter

You can use the std::string::find() function to find the position of your string delimiter, then use std::string::substr() to get a token.

Example:

std::string s = "scott>=tiger";
std::string delimiter = ">=";
std::string token = s.substr(0, s.find(delimiter)); // token is "scott"

The find(const string& str, size_t pos = 0) function returns the position of the first occurrence of str in the string, or npos if the string is not found.
The substr(size_t pos = 0, size_t n = npos) function returns a substring of the object, starting at position pos and of length npos.

If you have multiple delimiters, after you have extracted one token, you can remove it (delimiter included) to proceed with subsequent extractions (if you want to preserve the original string, just use s = s.substr(pos + delimiter.length());):

s.erase(0, s.find(delimiter) + delimiter.length());

This way you can easily loop to get each token.

Complete Example

std::string s = "scott>=tiger>=mushroom";
std::string delimiter = ">=";

size_t pos = 0;
std::string token;
while ((pos = s.find(delimiter)) != std::string::npos) {
    token = s.substr(0, pos);
    std::cout << token << std::endl;
    s.erase(0, pos + delimiter.length());
}
std::cout << s << std::endl;

Output:

scott
tiger
mushroom

Best Answer

Related Solutions

Python – How to Parse a String to a Float or Int

C++ String Parsing – How to Parse a String in C++ Using String Delimiter

Complete Example

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