hof.md

hof

Higher order functions

Index

Operation	Description
apply	Apply a function using arguments on stack
eval	Evaluate top of stack
filter	Filter items in the stack
fold, reduce	Reduce items to a single value
map	Apply an operation
repeat	Repeat the execution of an operation

Operations

apply

Evaluates the expression in fn by first popping nargs off the stack and pushing them back as a single argument. This is useful for higher order functions, like map, where some of the arguments are from existing results found on the stack.

Stack effects:

( args:Any* expr:Text n:Int/u32 -- Any* )

Example:

Input	Stack
1 2 3 4	1 | 2 | 3 | 4
n	1 | 2 | 3 | 4 | 4
[swap sub] [map] 2 apply	3 | 2 | 1 | 0

eval

Evaluate expr as if it was input to the calculator.

Stack effects:

( expr:Text Any* -- Any* )

Example:

Input	Stack
'1 2 add' eval	3

filter

Filter the stack by keeping items that are true when evaluated by expression expr.

Stack effects:

( Any* expr:Text -- Any* )

Example:

Input	Stack
1 2 3 4 5 6	1 | 2 | 3 | 4 | 5 | 6
[2 rem 0 eq] filter	2 | 4 | 6

fold

Reduce the stack to a single value using the expression expr. An invalid argument error is raised if expr does not reduce.

Alias: reduce

Stack effects:

( Any* expr:Text -- Any* )

Example:

Input	Stack
1 2 3 4 5	1 | 2 | 3 | 4 | 5
/add fold	15

map

Apply the expression, expr, to each item on the stack.

Stack effects:

( expr:Text Any* -- Any* )

Example:

Input	Stack
1 2 3 4 5	1 | 2 | 3 | 4 | 5
[2 mul] map	2 | 4 | 6 | 8 | 10

repeat

Repeat execution of expression expr for n times.

Stack effects:

( Any* expr:Text n:Int/u --  )

Example:

Input	Stack
1	1
[2 mul] 8 repeat	256