Editing SECD machine (section)

==Landin's contribution==
[[David Turner (computer scientist)|D. A. Turner]] (2012)<ref name=tfp12>{{cite web |url=https://www.cs.kent.ac.uk/people/staff/dat/tfp12/tfp12.pdf |title=D. A. Turner "Some History of Functional Programming Languages" in an invited lecture '''TFP12''', St Andrews University, 12 June 2012. See the section on Algol 60}}</ref> points out that the [[ALGOL 60]] programming language could not return functions from other functions (rendering functions no longer first-class). A function nested inside another function could refer to a variable living on the outer function's stack. If the nested function were returned from the outer function, then it would be referring to a variable in a stack frame that is no longer present. Turner notes that Landin's SECD machine solves this problem (thus allowing functions to return functions), as a function value is now represented with a [[closure (computer programming)|closure]] on the heap that can store the environment of variables it should use irrespective of what happens on the stack.<ref name=tfp12 />