What all of these languages–FORTRAN, C, the UNIX shell sh and its text manipulation tools such as awk, vi and sed–had in common was that they were procedural: you gave a series of commands in a procedure and ordered the computer to carry them out step by step …

In a functional language, in contrast, every statement is a function that takes input and transforms it into a different output, which in some generalized sense is what a program in toto is supposed to do

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