Digest #102 2020-06-23
Contributions
This line: T{ -7 S>D 3 SM/REM -> 1 -2 }T
should be: T{ -7 S>D 3 SM/REM -> -1 -2 }T
Replies
proposal - F>R and FR> to support dynamically-scoped floating point variables
I like this idea. The F>R and FR> words can be useful to implement the DESCRIPTOR-COMPOSITE ( td*i i -- td-new ) word.
Terms definitions
to create a token descriptor (informally): to create an implementation dependent token descriptor object producing its identifier or producing a named definition that returns this identifier. (see other terms at GitHub/ForthHub/fep-recognizer)
An approach to create any token descriptor
If we will add one special predefined descriptor, we will be able to express any possible semantics for the token descriptors, without providing a reproducing or a postponing action.
This descriptor should describe a token that is a pair of xt: ( xt-interpret xt-compile ). Let's call it TD-DUAL for a while.
And let's consider an example: the string literals (see the parse-slit-end definition there). The old recognizer: rec-sliteral ( c-addr1 u1 -- c-addr2 u2 rectype-slit|rectype-slit-parsing | 0 )
The new one:
\ Create the token descriptor for a token ( c-addr u xt-interp xt-compil )
td-slit td-dual 2 descriptor constant td-slit-parsing
\ A helper that returns the second part of the fully qualified token
: token-slit-parsing ( -- xt-interp xt-compil td-slit-parsing ) ['] parse-slit-end [: parse-slit-end slit, ;] td-slit-parsing ;
\ The recognizer for a string literal: a lexeme that starts with '"' (quote)
: recognize-sliteral ( c-addr1 u1 -- c-addr2 u2 td-slit | c-addr2 u2 xt-interp xt-compil td-slit-parsing | 0 )
dup 0= if nip exit then
over c@ '"' <> if 2drop 0 exit then
1 /string dup 0= if token-slit-parsing exit then
\ todo: fail if '"' is found in the middle of the string (c-addr1 u1)
2dup + char- c@ '"' = if char- td-slit exit then
token-slit-parsing
;
NB: we have to provide neither a reproducing action nor a postponing action. POSTPONE "abc" works as expected.
POSTPONE "x can work in the same way as POSTPONE S" — i.e., to append the compilation semantics for the corresponding lexeme. And the compilation semantics for "x is to parse characters up to ", prepend the result string with "x " and compile this united string. Although, I don't sure it has a practical application. If supported, it should work in this way from conceptual point of view, but it is not obligated to be supported.
Automation of POSTPONE action
StephenPelc wrote:
We cannot predict how recognisers will be used, so attempting to automate the POSTPONE actions is doomed to failure.
I know only one case when POSTPONE action cannot be automated: a special behavior that Anton suggested when POSTPONE is applied to a local variable, namely that POSTPONE local-x should be equivalent to local-x POSTPONE LITERAL, that is local-x LIT,. Actually, this behavior violates the specification for POSTPONE, that should append the compilation semantics for local-x in this case. And the only reason for this violation is to seamlessly use of local variables inside ]] ... [[ construct (in the simplest implementation).
Could somebody provide a correct POSTPONE action that cannot be automated?
confirmed