CPL

CPL

CPL is the most influential programming language that almost nobody ever ran. Designed jointly by Cambridge's Mathematical Laboratory and the University of London Computer Unit — its name migrating from "Cambridge Programming Language" to "Combined" as the collaboration formalized (and jokingly re-expanded as "Cambridge Plus London" and "Christopher's Programming Language," after Christopher Strachey) — it was published in August 1963 in a Computer Journal paper whose five authors read like the era's dean's list: Strachey, Barron, Buxton, Hartley, and Nixon.^[1]

Where ALGOL 60 had been small and elegant, CPL was deliberately big: a language meant to cover scientific calculation, industrial process control, business data processing — even, the paper admitted, command-line games — with both low-level access and high abstraction in one system. It was the most ambitious language design of its decade. It was also, on the evidence, too ambitious to build: implementations on Cambridge's Titan and London's Atlas proceeded slowly, the first CPL compiler probably wasn't written until around 1970, and the language faded during the seventies without ever gaining a user community to speak of.^[2]

The failure that parented C

CPL's real history happened to its child. In 1967 Martin Richards wrote BCPL — "Basic CPL" (originally "Bootstrap CPL") — a stripped-down, portable systems language distilled from CPL's ideas while CPL itself still barely existed in running form. BCPL was implemented before the first full CPL compiler; it became the bootstrap language of choice on half the machines in the world; and it led, through Ken Thompson's B on the PDP-7, to C. The lineage is exact: CPL → BCPL → B → C — meaning the largest failed language of the 1960s is the direct ancestor of the most successful one ever written.^[3]

Where it sits

CPL is the lesson ALGOL 68 would re-learn expensively: a language can be intellectually magnificent and still lose to something smaller that actually compiles. It exists now mostly as ancestry — but what ancestry. Autocode's British school produced it; B's lineage descends from it; and every C program ever written is, three removes down, a CPL program that finally shipped.

Check your understanding

• What did CPL originally stand for — and what did the "Combined" version combine?
• Why is CPL called a failure despite its pedigree?
• What language did Martin Richards distill from it, and when?
• Trace the full lineage from CPL to C.