${} arg slice idea

[Li Wei]

Hello All,

    I have an idea for the new ${} syntax for m4 2.0.  If we were to allow for arguments in ${1,2,3} like macro(a,b,c), then we could have argument list splicing. (Like array splices in other languages.)  My hope is that we can use this opportunity to both make the things that were impossible or difficult in m4 easy and fast. The format would be:

${FROM, TO, ALT-TEXT}

FROM, and TO are evaluated with `eval' syntax

Only arguments in the inclusive range FROM..TO are expanded.  `${3,5}' -> `$3,$4,$5'

If either FROM, or TO are negative, then they would be relative to the last argument `${1,-1}' -> `$@'

If TO < FROM then the expanded order is reversed (e.g.  `${4,2}' -> `$4',`$3',`$2' )

When TO is missing, it is set to FROM, so that `${3}' -> `${3, 3}' -> `$3'.

As a special rule if ALT-TEXT is missing, and either FROM or TO is 0 then a macro call syntax is expanded.

`${0}' -> `${0,0}' -> `$0'

`${0,N}' -> `$0($N,...,$-1)'

`${N,0}' -> `$0($1,...,$N)'

As a bonus feature, if ALT-TEXT is given and $# is within the range START,STOP then ALT-TEXT is expanded.

If ALT-TEXT is given then 0 looses its earlier special meaning.

This feature is somewhat like bash's ${VAR:+alt text} syntax.

`${2,2,`arg 2 was given'}'

`${0,4,`4 or fewer args were given'}'

The special values `#', `*', and `@' are also allowed with the following meanings (I may have gone off the deep end here):

 

Examples:

${1} -> "The first argument"

${-1} -> "The last argument"

${0} -> "Macro name"

${#} -> "Number of arguments"

${*} -> "unquoted arguments separated by `,' "

address@hidden -> "quoted arguments separated by `,' "

 

${N,N} -> as above

${N,0} -> "call syntax `$0($1,...,$N)' "

${N,#} -> RESERVED

${N,*} -> " $1, ..., $N unquoted like $*"

${N,@} -> " `$1',...,`$N' quoted like $@"

 

${0, N} -> " `$0($N,...,$-1)'  "

${0, 0} -> "same as `$0' "

${0,#, `f'} -> "Map: f(`$1'),...,f(`$N')"

${0,*} -> "same as `$0($*)' "

${0,@} -> "same as `$0($@)' "

 

${#, N} -> "return eval _expression_ N"

${#,0} -> " return `0' "

${#, #} -> "same as $#"

${#, *} -> "concatenate $1$2...$N "

${#, @} -> "concatenate `$1'`$2'...`$N' "

 

${*, N} -> " $N,...,$-1  unquoted like $*"

${*,0, `:'} -> "join $1:,...,:$N "

${*, #, `PRE-', `-POST'} -> " `PRE-$1-POST,...,PRE-$N-POST' "

${*, *} -> "same as $*"

${*, @} -> " ``$1'',...``$N'' "

 

${@, N} -> " `$N',...,`$-1' quoted like $@"

${@,0, `-'} -> "join `$1'-,...,-`$N' "

${@, #, `PRE-', `-POST'} -> "`PRE-'`$1'`-POST',...,`PRE-'`$N'`-POST' "

${@, *} -> " ``$1',...,`$N''  "

${@, @} -> "same as $@"

 

-=Li Wei=-