Local scope lambdas

[coconParticipant]

Monkey

Function Main() Local add := Lambda:Int(a:Int, b:Int) Return a + b End Local str:String Local append := Lambda(s:String) ' Error : Value 'str' is not assignable 'str += ", " + s End Print(add(10, 10)) End

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Function Main() Local add := Lambda:Int(a:Int, b:Int) Return a + b End Local str:String Local append := Lambda(s:String) ' Error : Value 'str' is not assignable 'str += ", " + s End   Print(add(10, 10)) End

I would like to know how I can reference str variable inside the append lambda.

[Mark SiblyKeymaster]

You need to declare ‘str’ before the lamba, not after.

[MikeHartParticipant]

Mark, I think he did, or? He got the error inside the append function.

[DruggedBunnyParticipant]

Yeah, the relevant code is:

Monkey

Function Main() Local str:String Local append := Lambda(s:String) str += ", " + s End End

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Function Main()   Local str:String Local append := Lambda(s:String) str += ", " + s End   End

Don’t know much about lambdas in practice, but that to me looks like it ‘should’ work.

[peterigzParticipant]

I’m not sure what the variable scope is when it comes to lambdas in monkey 2, I know that would work in javascript, but it’s probably out of scope here, unless it’s a bug…

[Mark SiblyKeymaster]

Forgot to also mention, you cannot modify ‘captured’ variables inside a lambda, ie: this should give an error:

 

Monkey

Function Test() Local t:=0 Local f:=Lambda() t=1 'Error! 't' cannot be modified here. End End

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Function Test()    Local t:=0    Local f:=Lambda()       t=1   'Error! 't' cannot be modified here.    End End

A lambda effectively gets a ‘copy’ of outer locals at the time the lambda is evaluated. So this..

Monkey

Function Test() Local t:=0 Local f:=Lambda() Print t End t=1 f() End

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Function Test()    Local t:=0    Local f:=Lambda()       Print t    End    t=1    f() End

…will print ‘0’.

[coconParticipant]

The ideal lambdas have the function purity concept (that no data modifications are allowed). I don’t know if Monkey uses that design indeed.

What I had more in mind was the C# concept of Action and Func that modify variables. Perhaps C++ as well by using the ampersand operator.

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