Which one among the following is NOT a non-procedural computer language?

Correct — B, Python. A non-procedural, or declarative, language lets you state the result you want and leaves the order of execution to the language's own engine. A procedural language makes you spell out the steps. LISP and ML are functional languages in which a program is an expression to be evaluated, and Prolog is a logic language in which you assert facts and rules and let an inference engine search for an answer. All three are non-procedural. Python is imperative at its core: you write assignments, loops and calls, and they run in the order written. Python is therefore the single name on the list that is not non-procedural, which is exactly what the double negative in the stem asks for.

• (a)LISP — LISP, written by John McCarthy in 1958, is the original functional language. A LISP program is a nest of expressions to be evaluated rather than an ordered list of commands, which puts it in the non-procedural family.
• (c)Prolog — Prolog is the textbook logic language. The programmer supplies facts and rules; the built-in resolution engine decides how to search for a proof. Nothing in the source code fixes the order of execution.
• (d)ML — ML, the Meta Language designed at Edinburgh in the 1970s and the ancestor of OCaml and F#, is a statically typed functional language. Its style is declarative, like LISP's.

Languages are grouped by how much of the 'how' the programmer has to supply. In a procedural or imperative language the source is an ordered list of instructions that change the machine's state — C, FORTRAN, COBOL, Pascal and Python all work this way. In a non-procedural or declarative language the source describes the goal and a built-in engine works out the route: SQL asks for rows without saying how to fetch them, Prolog asks for a proof, LISP and ML evaluate expressions.

Two things are being tested at once. The first is the double negative — 'NOT a non-procedural language' means 'which one IS procedural', and reversing it once too often flips the answer. The second is whether you classify a language by its family rather than by how familiar it feels. Python does support functional constructs such as map and lambda, but its normal idiom is a loop with an assignment inside, and no syllabus places it beside LISP, Prolog and ML.

• Procedural or imperative languages state the sequence of steps: C, FORTRAN, COBOL, Pascal, Python.
• Non-procedural or declarative languages state the result wanted: SQL, Prolog, LISP, ML, Haskell.
• LISP dates from 1958 and ML from the 1970s; both are functional. Prolog dates from 1972 and is a logic language.
• Python is multi-paradigm but imperative in its core model, which is why it breaks the pattern here.
• SQL is the declarative language most candidates already use — you name the rows you want, never the search strategy.

• Reading the double negative once and answering 'which is non-procedural'.
• Assuming the newest language on a list must be the odd one out for some other reason.
• Treating Python's lambda and map as proof that it belongs with LISP.

A classification item dressed in a double negative — the examiner wants the paradigm of four named languages, and hides it behind 'NOT a non-procedural'.

• practice — not a real PYQ

  Which one of the following is a declarative language used to query relational databases?

  • (a)C
  • (b)SQL
  • (c)FORTRAN
  • (d)Assembly

  Answer(b) SQL — you state which rows you want and the database engine chooses how to find them.

• practice — not a real PYQ

  Prolog is best described as which type of language?

  • (a)Logic
  • (b)Assembly
  • (c)Machine
  • (d)Markup

  Answer(a) Logic — it works from facts and rules through an inference engine.