What Quotes What When

This chapter is not really needed as all of its contents can be found elsewhere in this manual, but as people manage to become confused anyway, here's a summary. There are three ways in which characters can alter SM's behaviour: they can affect the way that characters and keywords are interpreted, they can be special to the grammar, and they can perform both functions. If you are confused, you might find a verbosity of four or five helpful.

"..."

Turn off the expansion of variables, the significance of mathematical operators such as / or :, and the recognition of keywords. For example, after DEFINE rhl Patricia, /data/$rhl would be interpreted as four tokens (/, data, /, and Patricia), while "/data/$rhl" is only one (/data/$rhl). Note that in the former case, the data will be taken to be part of a DATA command, and may well lead to a syntax error. If you need to force a variable to be expanded within double quotes, use an exclamation mark: "$!rhl". Double quotes have no syntactical significance.

'...'

Single quotes surround strings which may contain white space (spaces or tabs), characters such as + or /, or keywords. They don't affect variable expansion, so you have to enclose them in double quotes if you want to protect them. Of course, you must make sure that you are not quoting the single quotes -- use '"$abcd"' rather than "'$abcd'" . Strings are significant to the grammar so only use single quotes where they are needed. For example DATA 'my_file' is a syntax error, and SET s='abc' and SET s=abc mean quite different things.

{...}

Turn off all interpretation of special characters and keywords, just like double quotes. In fact, braces even turn off the expansion of $!var, to expand a variable within braces you need to say $!!var but you only very seldom need this. Braces have syntactical significance, delimiting lists. Wherever you can use braces you can use angle brackets.

<...>

Almost identical to {...}, but don't turn off variable expansions. In fact, <> don't always turn off keyword interpretation -- specifically they only do so in str_list's (if you want to know what this means you'll have to read SM's grammar in file `control.y'). In practice you'll probably only meet this if you try saying Foreach f < quit when ahead > { echo $f }.

(...)

Parentheses don't turn off any sort of expansion, but can have syntactical significance. You are most likely to see this in DEFINE var ( 1 + 2 ) where the parentheses tell SM to evaluate the expression before defining the variable var as 3. Note that DEFINE var < 1 + 2 > defines var as 1 + 2 .

$name

Expand name as a variable. Expansion is turned off within single and double quotes and within braces. Because the expansion happens before the parser sees the input, variables cannot have any syntactical significance as the parser never knows about them. This means that variables could be used to make SM look like something quite different; for example after DEFINE begin "{ " DEFINE end "} ", you can say

MACRO hi $begin echo Hello World $!!end

(This is a little tricky. The spaces in DEFINE begin "{ " are needed so that SM is still in quote mode when it processes the {. You can probably figure out why I need to say $!!end -- the !! should be a Helpful Hint.)

$(expr)

Evaluate the expression expr and substitute the resulting value. This is identical to DEFINE temp ( expr ) $temp, but much more convenient.

As a reasonably complex example, try to guess the output from:

DEFINE hi {<"$!('Hello')" World>} DEFINE hello $hi echo :$hi:$hello: