read

(read &optional stream)

Reads and parses a single S-expression. With no argument it reads from standard input; given a stream opened by open or with-open-file it reads from that stream. Blank and comment-only lines are skipped, the datum must fit on a single line, and EOF returns nil. The compiled backends emit a runtime reader with frontend parity: lists (dotted pairs included), ', #', strings, symbols, numbers (ratios and #x/#o/#b radix integers included), #\ character literals, #(...)/#nA(...) arrays, #* bit vectors, #f(/#d( packed float arrays, #S(...) structure literals and #|...|# block comments. #., #+/#- and #n=/#n# need an evaluator or the feature set at read time, so the compiled reader signals a catchable error on them (the interpreter still resolves them; binding *read-eval* to nil there makes #. signal instead, per the standard). The WASM reader's numbers are narrower (64-bit integers, decimal floats without exponents, static error messages) -- see Compiled read/load Limitations.

(print (read))

Reading the line (+ 1 2) from standard input parses it into the list (+ 1 2), which print then echoes back. At end of input read returns nil. Symbols read at run time follow the reader's upcasing -- your symbols and the standard names alike upcase (there is no fold) -- identically on every backend.