How to generate a sequence from another sequence ?

[Ashish Negi]

Hi

I have a sequence of records with field `id`. I want to get a sequence of all ids from original sequence.

My first attempt was to try this :

[r \in some_sequence_of_records |-> r.id]

"The exception was a java.lang.RuntimeException
: util.Assert$TLCRuntimeException: The domains of formal parameters must be enumerable.
The error occurred when TLC was evaluating the nested
expressions at the following positions:
    The error call stack is empty."

Q1. Is "some_sequence_of_records" a set with DOMAIN 1..Len(sequence) ?

Q2. What is wrong with my first try ?

Currently, i am using custom map and filter functions for the sequences.

RECURSIVE mapSeq(_,_,_)
mapSeq(seqs, M(_), result) ==
    IF seqs = <<>>
    THEN result
    ELSE mapSeq(Tail(seqs),
                   M,
                   Append(result, M(Head(seqs))))

GetAllIds(files) ==
    LET getId(r) == r.id
    IN mapSeq(files, getId, <<>>)

Q3. Is there a better way ?

Thanks

Ashish

[Stephan Merz]

As far as I can tell, your attempt failed because a sequence is a function whereas the construct [x \in S |-> ...] expects a set S. Why not simply

[i \in 1 .. Len(files) |-> files[i].id]

where `files' is your input sequence?

Stephan

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[Ashish Negi]

Thanks Stephan for providing alternate method.

Is sequence a set or primitive type ?

Stephan

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[Ashish Negi]

Q. does `[i \in 1 .. Len(files) |-> files[i].id]` gives back a sequence ?

Q. Is there something like a REPL where i can try different statements of TLA+ to check the output ?

On Tuesday, 10 November 2020 23:17:42 UTC-8, Stephan Merz wrote:

Stephan

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[Stephan Merz]

On 12 Nov 2020, at 04:26, Ashish Negi <thisismy...@gmail.com> wrote:

Q. does `[i \in 1 .. Len(files) |-> files[i].id]` gives back a sequence ?

Yes: in TLA+ a sequence is a function with domain 1..n, for some n \in Nat.

Q. Is there something like a REPL where i can try different statements of TLA+ to check the output ?

You can use TLC to evaluate constant expressions.

Is sequence a set or primitive type ?

Semantically, all TLA+ values are sets, but there are many values for which it is not specified which sets represent them. These include numbers and functions, and therefore also sequences and records. It is more useful to think of these classes of values as being primitive.

Stephan

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[Willy Schultz]

Note that the latest versions of TLC also include a simple built-in REPL for evaluating TLA+ expressions. You can run it with:

java -cp tla2tools.jar tlc2.REPL

You can see a demo here and you can download the latest (nightly) build of the TLC tools from here: https://nightly.tlapl.us/dist/tla2tools.jar.

Will