We Lost Something: 1970s REPLs Were Better Than Modern Development Environments

Summary

In the 1970s, Lisp and APL REPLs represented something we’ve lost: true interactive, exploratory programming environments where developer feedback was instantaneous, context was persistent, and the entire system was designed around the developer’s cognitive workflow. Today’s programming environments—scattered across language-specific interpreters, IDEs, and build pipelines—fragment this experience. We optimized for production efficiency at the cost of development experience. It’s time to ask: what would it look like to reclaim what early REPLs did right, while leveraging modern technology to make it universally available?REPLsREPLs prioritize developer productivity and feedback speed (fast iteration, quick testing, interactive debugging) over production-level performance (low memory usage, fast execution, minimal startup time).In a REPL, you want:Instant feedback: Type an expression, see the result immediatelyQuick iteration: Modify a function, re-test it, all in the same sessionMinimal context-switching: Stay in the language environment rather than repeatedly invoking the compiler/interpreterThis is fundamentally different from production code, where you want:The Trade-OffA REPL might maintain large in-memory symbol tables, keep debug information readily available, and avoid aggressive optimizations that would slow down recompilation—all to serve the interactive developer. A production system would strip these away[1][3][2].Is the Command Line a REPL?The UNIX command line does resemble a REPL in structure, but it lacks critical features to be a true REPL.How the UNIX Shell Resembles a REPLThe UNIX command line does follow a read-eval-print loop:Read: It reads commands you typeEval: It executes those commands (usually by spawning programs)Print: It displays outputLoop: It waits for the next commandWhat’s Missing: Some Critical REPPL FeaturesTo make the UNIX shell a proper REPL (in the sense of Lisp or APL), you would need, at least:Persistent Language StateA REPL ma...