In tro duction to F unctional Programming: Lecture 8 1 In tro duction to F unctional Programming John Harrison Univ ersit y of Cam bridge Lecture 8 Imp erativ e features of ML T opics co v ered: � Output � Sequencing commands � Exceptions � References � Arra ys � Imp erativ e t yp es John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 2 ML's imp erativ e features ML is not a pure functional programming language. No w at last w e will discuss its imp erativ e features. Whether a certain feature is really imp er ative is partly a matter of taste. W e group together here some features that ma y not b e found in pure languages. The main reasons for ha ving these features is that they can mak e programs (a) more e�cien t and/or (b) easier to write. In an y case, it's hard to imagine doing certain things lik e I/O in ML in a purely functional w a y . John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 3 Output Input-output and other kinds of in teraction with the en vironmen t seem essen tially imp erativ e. F rom a certain p oin t of view, w e can imagine the input and output as p oten tially in�nite streams and handle them in a purely functional st yle | this is done in some lazy languages lik e Miranda or Hask ell. It's not v ery con v enien t in ML. ML has v arious sp ecial functions whose ev aluation causes a side-e�ect of in teracting with the en vironmen t. W e will sho w one, the print function whic h prin ts a string: - print; > val it = fn : string -> unit - print "hello"; hello> val it = () : unit - print "goodbye\n"; goodbye > val it = () : unit John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 4 Sequencing No w that w e ha v e some expressions whose ev aluation causes a side-e�ect, w e care ev en more ab out ev aluation order. Since w e kno w the rules, w e can predict for example: - let val x = print "first " in print "second\n" end; first second Ho w ev er, ML also pro vides a sequencing op eration ; as do most imp erativ e languages lik e Mo dula-3. In e1 ; e2 , expression e1 is ev aluated and the result discarded, then e2 is ev aluated and is the v alue of the whole expression: - (print "first "; print "second\n"); first second > val it = () : unit - (1;2); > val it = 2 : int John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 5 Exceptions (1) ML's errors, e.g. matc hing failures and division b y zero, are all signalled b y propagating exc eptions : - 1 div 0; ! Uncaught exception: ! Div In all these cases the compiler complains ab out an `uncaugh t exception'. As the error message suggests, it is p ossible to `catc h' them. There is a t yp e exn of exc eptions , whic h is e�ectiv ely a recursiv e t yp e. Unlik e with ordinary t yp es, one can add new constructors for the t yp e exn at an y p oin t in the program via an exception declaration, e.g. - exception Died; > exn Died = Died : exn - exception Failed of string; > exn Failed = fn : string -> exn John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 6 Exceptions (2) One can explicitly generate an exception using the raise construct. raise <exception> F or example, w e migh t in v en t our o wn exception to co v er the case of taking the head of an empt y list: > exn Head_of_empty = Head_of_empty : exn - fun hd [] = raise Head_of_empty | hd (h::t) = h; > val hd = fn : 'a list -> 'a - hd(tl [1]); ! Uncaught exception: ! Head_of_empty John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 7 Exceptions (3) One can c atch exceptions using <expr> handle <patterns> , where the patterns to matc h exceptions are just as for ordinary recursiv e t yp es. - fun headstring sl = hd sl handle Head_of_empty => "" | Failed s => "Failure because "^s; > val headstring = fn : string list -> string - headstring ["hi","there"]; > val it = "hi" : string - headstring []; > val it = "" : string On one view, exceptions are not really imp erativ e features. W e can imagine a hidden t yp e of exceptions tac k ed on to the return t yp e of eac h function. An yw a y , they are often quite useful! John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 8 Exceptions (4) Exceptions can normally b e treated just as other ML v alues. F or example, supp ose w e w an t to de�ne a function to \trace" other functions: - fun trace name f x = (print ("entering "^name^"\n"); let val y = f x handle ex => (print (name^" gave an exception\n"); raise ex) in (print (name^" finished\n"); y) end); > val trace = fn : string -> ('a -> 'b) -> 'a -> 'b - fun hd' l = trace "hd" hd l; > val hd' = fn : 'a list -> 'a - hd' [1,2]; entering hd hd finished > val it = 1 : int John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 9 References (1) ML do es ha v e real assignable v ariables, and expressions can, as a side-e�ect, mo dify the v alues of these v ariables. They are explicitly accessed via r efer enc es (p oin ters in C parlance) and the p oin ters themselv es b eha v e more lik e ordinary ML v alues. One sets up a new assignable memory cell with the initial con ten ts x b y writing ref x . This expression returns the corresp onding reference, i.e. a p oin ter to it. One manipulates the con ten ts via the p oin ter. This is quite similar to C: here one often sim ulates `v ariable parameters' and passes bac k comp osite ob jects from functions via explicit use of p oin ters. T o get at the con ten ts of a ref , use the dereferencing (indirection) op erator ! . T o mo dify its v alue, use := . John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 10 References (2) Here is an example of ho w w e can create and pla y with a reference cell: - val x = ref 1; > val x = ref 1 : int ref - !x; > val it = 1 : int - x := 2; > val it = () : unit - !x; > val it = 2 : int - x := !x + !x; > val it = () : unit - x; > val it = ref 4 : int ref - !x; > val it = 4 : int In most resp ects ref b eha v es lik e a t yp e constructor, so one can pattern-matc h against it. John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 11 References (3) References are useful in ML for t w o di�eren t reasons. First, as y ou migh t exp ect, they allo w us to mo dify the state of the program as w e go along, in a more con v en tional st yle. If w e didn't use references, functions w ould often ha v e to ha v e additional argumen ts. Secondly , they can b e used to construct data structures that are shar e d or cyclic . Whic hev er w a y y ou use them, it is sometimes useful to ha v e reference v ariables inside a function for con v enience or e�ciency , but use them in suc h a w a y that the function as a whole is still a true function, i.e. returns the same v alue on m ultiple calls. John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
In tro duction to F unctional Programming: Lecture 8 12 References (4) F or example, it's ofen con v enien t to memoize or c ache the result of a previous function call, so that if w e get the same argumen t again, w e can return the stored v alue instead of recalculating it. W e start b y de�ning a function to �nd an item in a list of pairs: - exception Not_found; > exn Not_found = Not_found : exn - fun assoc a [] = raise Not_found | assoc a ((x,y)::rest) = if x = a then y else assoc a rest; > val assoc = fn : ''a -> (''a * 'b) list -> 'b John Harrison Univ ersit y of Cam bridge, 30 Jan uary 1998
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