Racket – tipi di dati built-in – 5

FairchildPheonixContinuando da qui oggi sono qui: [doc]/guide/datatypes.html.

Hash tables

Sì, non so tradurre 😥
A hash table implements a mapping from keys to values, where both keys and values can be arbitrary Racket values, and access and update to the table are normally constant-time operations. Keys are compared using equal?, eqv?, or eq?, depending on whether the hash table is created with make-hash, make-hasheqv, or make-hasheq.

d37

The hash, hasheqv, and hasheq functions create immutable hash tables from an initial set of keys and values, in which each value is provided as an argument after its key. Immutable hash tables can be extended with hash-set, which produces a new immutable hash table in constant time.

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A literal immutable hash table can be written as an expression by using #hash (for an equal?-based table), #hasheqv (for an eqv?-based table), or #hasheq (for an eq?-based table). A parenthesized sequence must immediately follow #hash, #hasheq, or #hasheqv, where each element is a dotted key–value pair. The #hash, etc. forms implicitly quote their key and value sub-forms.

d39

Both mutable and immutable hash tables print like immutable hash tables, using a quoted #hash, #hasheqv, or #hasheq form if all keys and values can be expressed with quote or using hash, hasheq, or hasheqv otherwise:

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A mutable hash table can optionally retain its keys weakly, so each mapping is retained only so long as the key is retained elsewhere.

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Beware that even a weak hash table retains its values strongly, as long as the corresponding key is accessible. This creates a catch-22 dependency when a value refers back to its key, so that the mapping is retained permanently. To break the cycle, map the key to an ephemeron that pairs the value with its key (in addition to the implicit pairing of the hash table).

Dove l’ephemeron è una cosa mysteryousa ma c’è l’help:

LOL :wink:

oops! 😉

OOPS! no, occorre andare alla Reference, Ephemerons, qui: [doc]/reference/ephemerons.html.

d43

ecco l’ephemeron:

d44

Boxes

OK, fatto il plurale, tanto sto copiando 😉

A box is like a single-element vector. It can print as a quoted #& followed by the printed form of the boxed value. A #& form can also be used as an expression, but since the resulting box is constant, it has practically no use.

d45

void e undefined

Some procedures or expression forms have no need for a result value. For example, the display procedure is called only for the side-effect of writing output. In such cases the result value is normally a special constant that prints as #<void>. When the result of an expression is simply #<void>, the REPL does not print anything.

The void procedure takes any number of arguments and returns #<void>. (That is, the identifier void is bound to a procedure that returns #<void>, instead of being bound directly to #<void>.)

d46

The undefined constant, which prints as #<undefined>, is sometimes used as the result of a reference whose value is not yet available. In previous versions of Racket (before version 6.1), referencing a local binding too early produced #<undefined>; too-early references now raise an exception, instead.

d47

OK, finito con i tipi di dati prossimamente espressioni e definizioni :mrgreen:

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