Higher order functions

The Wikipedia definition of a higher-order function is:

A function that does at least one of the following:

• takes one or more functions as arguments, which is a parameter of a procedure that is itself a procedure,
• returns a function as its result.

Meld's supported higher-order functions are shown in the table below.

Higher-order function	Product sequence mapping	User operation
Transform (map)	$(\boldsymbol{a})_n \xrightarrow{f} (\boldsymbol{b})_n$	$f(\boldsymbol{a}) \rightarrow \boldsymbol{b}$
Filter	$(\boldsymbol{a})_n \xrightarrow{f} (\boldsymbol{a})_m$	$f(\boldsymbol{a}) \rightarrow \text{Bool}$
Monitor	$(\boldsymbol{a})_n \xrightarrow{f} (\ \ )_0$	$f(\boldsymbol{a}) \rightarrow \text{Void}$
Reduction (fold)	$(\boldsymbol{a})_n \xrightarrow{f} (\boldsymbol{b})_1$	$f(\boldsymbol{a}, \boldsymbol{b}) \rightarrow \boldsymbol{b}$
Splitter (unfold)	$(\boldsymbol{a})_1 \xrightarrow{f} (\boldsymbol{b})_n$	$f(\boldsymbol{a}) \rightarrow (\boldsymbol{b})_p$