Analysis of information sources in references of the Wikipedia article "SECD抽象机" in Chinese language version.
Algol 60 is not normally thought of as a functional language but its rules for procedures (the Algol equivalent of functions) and variable binding were closely related to those of λ-calculus.
The Revised Report on Algol 60 (Naur 1963) is a model of precise technical writing. It defines the effect of a procedure call by a copying rule with a requirement for systematic change of identifiers where needed to avoid variable capture — exactly like β-reduction.
Although formal parameters could be declared value the default parameter passing mode was call by name, which required the actual parameter to be copied unevaluated into the procedure body at every occurrence of the formal parameter. This amounts to normal order reduction (but not graph reduction, there is no sharing). The use of call by name allowed an ingenious programming technique: Jensen’s Device.
Algol 60 allowed textually nested procedures and passing procedures as parameters (but not returning procedures as results). The requirement in the copying rule for systematic change of identifiers has the effect of enforcing static (that is lexicographic) binding of free variables.
In their book “Algol 60 Implementation”, Randell and Russell (1964, Sect. 2.2) handle this by two sets of links between stack frames. The dynamic chain links each stack frame, representing a procedure call, to the frame that called it. The static chain links each stack frame to that of the textually containing procedure, which might be much further away. Free variables are accessed via the static chain.
This mechanism works well for Algol 60 but in a language in which functions can be returned as results, a free variable might be held onto after the function call in which it was created has returned, and will no longer be present on the stack.
Landin (1964) solved this in his SECD machine. A function is represented by a closure, consisting of code for the function plus the environment for its free variables. The environment is a linked list of name-value pairs. Closures live in the heap.
Algol 60 is not normally thought of as a functional language but its rules for procedures (the Algol equivalent of functions) and variable binding were closely related to those of λ-calculus.
The Revised Report on Algol 60 (Naur 1963) is a model of precise technical writing. It defines the effect of a procedure call by a copying rule with a requirement for systematic change of identifiers where needed to avoid variable capture — exactly like β-reduction.
Although formal parameters could be declared value the default parameter passing mode was call by name, which required the actual parameter to be copied unevaluated into the procedure body at every occurrence of the formal parameter. This amounts to normal order reduction (but not graph reduction, there is no sharing). The use of call by name allowed an ingenious programming technique: Jensen’s Device.
Algol 60 allowed textually nested procedures and passing procedures as parameters (but not returning procedures as results). The requirement in the copying rule for systematic change of identifiers has the effect of enforcing static (that is lexicographic) binding of free variables.
In their book “Algol 60 Implementation”, Randell and Russell (1964, Sect. 2.2) handle this by two sets of links between stack frames. The dynamic chain links each stack frame, representing a procedure call, to the frame that called it. The static chain links each stack frame to that of the textually containing procedure, which might be much further away. Free variables are accessed via the static chain.
This mechanism works well for Algol 60 but in a language in which functions can be returned as results, a free variable might be held onto after the function call in which it was created has returned, and will no longer be present on the stack.
Landin (1964) solved this in his SECD machine. A function is represented by a closure, consisting of code for the function plus the environment for its free variables. The environment is a linked list of name-value pairs. Closures live in the heap.
