Functional Programming Using F# by Hansen Michael R. & Rischel Hans

Author:Hansen, Michael R. & Rischel, Hans [Hansen, Michael R.]
Language: eng
Format: epub
Publisher: Cambridge University Press
Published: 2013-04-29T22:00:00+00:00

8.4 Sequential composition

The semicolon symbol “;” denotes the sequential composition operator (while the double semicolon “;;” is a terminator symbol). This operator combines two expressions exp1 and exp2 to form a new expression:

exp1; exp2

The expression exp1 ; exp2 is evaluated as follows:

Evaluate exp1 and discard the result.

Evaluate exp2 and supply the result as the result of evaluating exp1; exp2.

Hence, if exp2 has type τ then exp1 ; exp2 has type τ as well.

The F# compiler issues a warning if exp1 is of type different from unit as the result of the evaluation might be of some use. This warning is avoided by using the ignore function:

ignore(exp1) ; exp2 or exp1|> ignore ; exp2

where ignore a = () for any value a.

We may combine two assignments using sequential composition:

let mutable x = 5;; let mutable y = 7;; x <- y + 1 ; y <- x + 2;; (x,y);; val it : int * int = (8 ,10)

Note that the second assignment uses the new value stored in the location denoted by x.

The operator “;” may be omitted if the expressions are written on separate lines, that is,

exp1

exp2

means (exp1 ) ; exp2.

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