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Empty Map

rascal-0.41.2

Synopsis​

Illegal operation on an empty map.

Types​

data RuntimeException = EmptyMap();

Usage​

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

Description​

Rascal provides many operations and functions on maps, see map values and map functions. This error is generated when a function or operations cannot handle the empty map case.

Remedies:

  • Guard the function or operation with a test on the empty map (isEmpty) and take alternative action in that case.
  • Catch the EmptyMap yourself, see try catch.

Examples​

Import the Map library and introduce M with an empty map as value:

rascal>import Map;
ok
rascal>M = ();
map[void, void]: ()

Trying to get an arbitrary value from it gives an error:

rascal>getOneFrom(M);
|std:///Map.rsc|(1872,385,<78,0>,<91,41>): EmptyMap()
    at *** somewhere ***(|std:///Map.rsc|(1872,385,<78,0>,<91,41>))
    at getOneFrom(|prompt:///|(11,1,<1,11>,<1,12>))

We can also catch the EmptyMap error. First import the Rascal exceptions (which are also included in Prelude) and IO:

rascal>import Exception;
ok
rascal>import IO;
ok
rascal>try 
|1 >>>>  println(getOneFrom(M)); 
|2 >>>>catch EmptyMap(): 
|3 >>>>  println("Cannot use getOneFrom on empty map");
Cannot use getOneFrom on empty map
ok