set (variable-type? variable-name indexer?) | (file file-name) to value
  OR
  set stream-output-function-call open-modifiers to string-expression 

You use set to assign a value to a variable. set replaces the value of a variable with the new value:

  set my-stream to "Mary had a little lamb."
  set my-counter to 78
  set my-switch to true
  set referant "my-referant" to "Its fleece was white as snow."
  set my-float to fp_v "10.56"

You can also use set to read and write files:

  set file "mary.txt" to my-buffer
  set my-buffer to file "mary.txt"
  set file "mary.txt" to file "lamb.txt"

When used with streams, set is equivalent to opening the stream, writing to it, then closing it. For this reason, any modifiers that can be used with open can also be used with set:

  set my-stream with referents-allowed defaulting {""}
      to "Mary had a little lamb."
  set file "mary.txt" with referents-displayed
      to my-stream

You can also use set to provide input to an external output function :

  set my-external-function to "Mary had a little lamb"