Re:Functional Python Programming
by kazinator on Monday June 12, 2023 @06:15PM
(#36302039)
Many mainstream programming languages implement procedures and functions with the same syntax, and call procedures functions. Procedures can take arguments and return a value, like functions, but also have side effects. Nobody asked for this terminology mixup; it just happened in the reference manuals of programming languages. Lisp calls procedures functions; so does C.
Programming with procedures isn't functional programming; it is procedural programming. This is true even when the procedures are first-class procedures, and are used in declarative patterns like mapping, reducing and so on. There is no word which refers to programming with first-class procedures in a declarative style. It is different from "Fortran-like procedural", and is rooted in OOP. First class procedures are objects; and in fact in many languages you can take a solution based on first class procedures and replace those procedures with objects. Mutated lexical variables become member variables (or slots) and so on.
The rhetoric in this area conflates two meanings of "pure" and "purely", causing equivocation.
- "pure X" as in "consisting of nothing but X" and "purely X" meaning "solely X"
- "pure" as in having no side effects: a pure expression can be replaced by its value without a change in meaning.
A "not purely functional language" uses "purely" in the former sense. The language emphasizes functional features, but has procedural features also. It doesn't refer to a language which emphasizes declarative programming patterns around first-class procedures, though such a language can fit the description.
A "not purely functional program" likewise: it emphasizes functional programming but has procedural bits in there. It doesn't refer to the program being composed entirely of the declarative use of higher order procedures, like mapping over collections while doing I/O or mutation and such.
If we take the value of a variable and then filter it through functional stages, only to then assign the transformed value into the same variable by assignment, that is an example of "not purely functional". The data transformation is functional; the assignment isn't.
Programming with procedures isn't functional programming; it is procedural programming. This is true even when the procedures are first-class procedures, and are used in declarative patterns like mapping, reducing and so on. There is no word which refers to programming with first-class procedures in a declarative style. It is different from "Fortran-like procedural", and is rooted in OOP. First class procedures are objects; and in fact in many languages you can take a solution based on first class procedures and replace those procedures with objects. Mutated lexical variables become member variables (or slots) and so on.
The rhetoric in this area conflates two meanings of "pure" and "purely", causing equivocation.
- "pure X" as in "consisting of nothing but X" and "purely X" meaning "solely X"
- "pure" as in having no side effects: a pure expression can be replaced by its value without a change in meaning.
A "not purely functional language" uses "purely" in the former sense. The language emphasizes functional features, but has procedural features also. It doesn't refer to a language which emphasizes declarative programming patterns around first-class procedures, though such a language can fit the description.
A "not purely functional program" likewise: it emphasizes functional programming but has procedural bits in there. It doesn't refer to the program being composed entirely of the declarative use of higher order procedures, like mapping over collections while doing I/O or mutation and such.
If we take the value of a variable and then filter it through functional stages, only to then assign the transformed value into the same variable by assignment, that is an example of "not purely functional". The data transformation is functional; the assignment isn't.