Suppose you want to do a complicated task with an Array, like, say, convert all of its strings to lowercase, or check if its datamap have “health” data equal to 0, or join all of its strings together into a single string. You want to be able to tell Harlowe to search for “each string where the string's 1st letter is A”. You want to write a “function” for how the search is to be conducted.
There are several types of lambdas.
_item where _item's 1st is "A"tells the macro to searches for items whose
1stis the string “A”.
_item via _item + "s"tells the macro to add the string “s” to the end of each item.
_item making _total via _total + (max: _item, 0)tells the macro to add each item to the total, but only if the item is greater than 0. (Incidentally, you can also use “where” inside a “making” lambda - you could rewrite that lambda as
_item making _total via _total + _item where _item > 0.)
_item where true, for instance, will include every item. There is a special, more readable shorthand for this type of “where” lambda: writing just
each _itemis equivalent.
Lambdas use temp variables as “placeholders” for the actual values. For instance, in
(find: _num where _num > 2, 5,6,0),
the temp variable
_num is used to mean each individual value given to the macro, in turn. It will be 5, 6 and 0, respectively.
Importantly, this will not alter any existing temp variable called
_num - the inside of a lambda can be thought
of as a hook, so just as the inner
(set: _x to 1) |a>[ (set:_x to 2) ] is different from the outer
_num in the
lambda will not affect any other
An important feature is that you can save lambdas into variables, and reuse them in your story easily. You
could, for instance,
(set: $statsReadout to (_stat making _readout via _readout + "|" + _stat's name + ":" + _stat's value)),
and then use $printStats with the (folded:) macro in different places, such as
(folded: $statsReadout, ...(dataentries: $playerStats)) for displaying the player's stats,
(folded: $statsReadout, ...(dataentries: $monsterStats)) for a monster's stats, etc.
Lambdas are named after the lambda calculus, and the “lambda” keyword used in many popular programming languages. They may seem complicated, but as long as you think of them as just a special way of writing a repeating instruction, and understand how their macros work, you may find that they are very convenient.