39 lines
873 B
Markdown
39 lines
873 B
Markdown
# 7. Paxos
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## CHOOSE
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`CHOOSE x \in S: P` selects one particular value
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in S such that P holds.
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Example: how to write a maximum function:
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```tla+ code fragment
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Maximum(S) ==
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IF S = {}
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THEN -1
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ELSE CHOOSE n \in S: \A m \in S: n >= m
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```
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Elements of a `SYMMETRY` set can not appear in a `CHOOSE`
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expression.
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## Set constructors
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- `{ v \in S: P }` the set of all `v` in `S` for which `P` holds
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- `{e: v \in S}` the set of `e` for all `v` in `S`
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- Example: `{ n^2: n \in Nat }`: the set of squares of all natural numbers
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## `EXCEPT` drilling
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```tla+ code fragment
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[aState EXCEPT ![m.ins][acc].mbal = m.bal]
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\* means
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[aState EXCEPT ![m.ins] =
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[aState[m.int] EXCEPT ![acc] =
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[aState[m.ins][acc] EXCEPT !.mbal = m.bal]
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]
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]
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\* just like this programming statement:
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\* aState[m.ins][acc].mbal = mbal
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```
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