Very simple anonymous lambda generator in Rebol

simple-lambda-example.reb

>> double: lambda [? * 2]
>> double 2
== 4

>> double: lambda [? + ?]
>> double 4
== 8

>> add2: lambda [?1 + ?2]
>> add2 2 4
== 6

>> add3: lambda [?a + ?b + ?c]
>> add3 1 2 4
== 7

>> invalid-func: lambda [? + ?1]
** User error: "cannot match ? with ?name placeholders"

simple-lambda.reb

Rebol [
    title: {Very simple anonymous lambda generator}
]

lambda: function [block] [
    spec: make block! 0

    parse block [
        any [
            set word word! (if #"?" == first to-string word [append spec word])
            | skip
        ]
    ]

    spec: unique sort spec
    
    if all [
        (length? spec) > 1
        found? find spec '?
    ][do make error! {cannot match ? with ?name placeholders}]

    func spec block
]