Map should have a method to retrieve a value but throws if the value is not present.

[alan-knight]

The current implementation of putIfAbsent in ImmutableMap is

  V putIfAbsent(K key, V ifAbsent()) {     throw new UnsupportedError("Cannot set value in unmodifiable Map");   }

I'd prefer it if it actually called the ifAbsent function before that. My use case is that I have a method   externalObjectNamed(key) => externalObjects.putIfAbsent(key, () =>       throw 'Cannot find named object to link to: $key');

which is conveniently easy to write compared to doing an indexing operation and checking the result for null. But it cannot be used if the map is immutable.

[lrhn]

I don't think we'd want this. It's clever use of putIfAbsent, but not its intended use. If you use a put operation on an unmodifiable collection or map, you are doing something wrong. In a less dynamic language it would be a type error, and we want to fail early on this.

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cc @floitschG.

[floitschG]

I agree with Lasse, invoking a mutating method on an immutable map should throw as early as possible.

What you actually want is a method that retrieves a value, but throws if the value is not there. This is not something I'm immediately opposed to. Changing the summary accordingly.

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Changed the title to: "Map should have a method to retrieve a value but throws if the value is not present.".

[DartBot]

This comment was originally written by @alan-knight

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Yes, some kind of general ifAbsent method would be better in this case. Rather than having one that throws if the value isn't there, it would be nice if, like putIfAbsent, it could evaluate an arbitrary function. That could be used to throw if the value wasn't there, but also to return a default value if the value wasn't there, or other variations.

[dgrove]

Removed Library-Collections label. Added Library-Collection label.

[lrhn]

The analogue to putIfAbsent would be "getOrElse" (really bad name, but I like the vaguely menacing sound of it :).

  V getOrElse(K key, V fallback()) {     .. do internal lookup ..;     if (...not there...) return fallback();     return ... value of lookup ...;   }    

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Removed Type-Defect label. Added Type-Enhancement label.