One of the big issues that newcomers feel is most difficult to get their head around in Tcl is when to use square brackets, when to use round brackets, and when to use curly braces.


SQUARE BRACKETS are used to define a block that's run BEFORE the rest of the command on the current line, and the result is substituted into the line.

CURLY BRACES are used to define a block that's deferred - in other words it may be run AFTER the rest of the command on the current line.


ROUND BRACKETS are different - they're used either to indicate an array subscript, or to alter operator precedence in an expr command. They are not used in command precedence.

How do I help people remember this? I often equate the Square brackets with the very irritating Donkey in the Shrek Movies that always saying "Pick me ... pick me" ... an irritating but, as it turns out, very necessary and faithful friend.


This image is reproduced with permission of hobby-box.co.uk where painted horseshoes are offered for sale in support of The Donkey Sanctuary, near Sidmouth, Devon


See also Tcl training course

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