implement language system

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1 Outlie Priciples of programmig laguages Lecture 3 Part I. Laguage systems Part II. Fuctioal programmig. First look at ML. Natalia Silvis-Cividjia silvis@few.vu.l vrije Uiversiteit amsterdam Imagie: A ew laguage have bee desiged ad Part I your task is to implemet this laguage. Laguage systems You have to build a laguage system Laguage systems The classical sequece The classical sequece Variatios o the classical sequece Bidig Debuggers Rutime support 1

2 1. Creatig 1. Creatig it i; void mai() { The programmer uses a editor to create a text file cotaiig the program A high-level laguage: machie idepedet 2. Compilig 2. Compilig Compiler traslates to assembly laguage Machie-specific Each lie represets either a piece of data, or a sigle machie-level istructio Programs used to be writte directly i assembly laguage, before Fortra (1957) Now used directly oly rarely it i; void mai() { C 3. Assemblig compiler assembler i: data word 0 mai: move 1 to i t1: compare i with 100 jump to t2 if greater push i call fred add 1 to i go to t1 t2: retur 2

3 3. Assemblig Assembly laguage is still ot directly executable Still text format, readable by people Still has ames, ot memory addresses Assembler coverts each assemblylaguage istructio ito the machie s biary format: its machie laguage Resultig object file ot readable by people i: data word 0 mai: move 1 to i t1: compare i with 100 jump to t2 if greater push i call fred add 1 to i go to t1 t2: retur assembler mai: i: 0 m o v e i c m i m j m p i f g p u s h i c fred a d d t i r e t u r 4. Likig 4. Likig Object file still ot directly executable Missig some parts Still has some ames Mostly machie laguage, but ot etirely Liker collects ad combies all the differet parts I our example, fred was compiled separately, ad may eve have bee writte i a differet high-level laguage Result is the executable file i: 0 mai: m o v e i c m i m mai: m o v e i c m i m j m p i f g p u s h i c fred a d d t i r e t u r liker i: 0 j m p i f g p u s h i c fred a d d t i r e t u r fred: j m p i f g r e t u r 5. Loadig 3

4 5. Loadig Executable file still ot directly executable Still has some ames Mostly machie laguage, but ot etirely Fial step: whe the program is ru, the loader loads it ito memory ad replaces ames with addresses mai: i: 0 m o v e i c m i m j m p i f g p u s h i c fred a d d t i r e t u r fred: j m p i f g r e t u r loader 0: 20: (mai) 60: (fred) 80: (i) m o v e 80 c m 80 m j m p i f g p u s h 80 c 60 a d d t 80 r e t u r j m p i f g r e t u r 0 6. Ruig 6. Ruig After loadig, the program is etirely machie laguage Processor begis executig its istructios, ad the program rus Optimizatio Usually after compilatio step Code geerated by a compiler is usually optimized to make it faster, smaller, or both First used for Fortra i 1957 Other optimizatios may be doe by the assembler, liker, ad/or loader A misomer: the resultig code is better, but ot guarateed to be optimal Example: loop ivariat removal Origial code: it i = 0; while (i < 100) { a[i++] = x*x*x; Improved code, with loop ivariat moved: it i = 0; it temp = x*x*x; while (i < 100) { a[i++] = temp; 4

5 #iclude <stdio.h> double power (double d, usiged ) { double x = 1.0; usiged j; for (j = 1; j <= ; j++) x *= d; retur x; it mai (void) { double sum = 0.0; usiged i; for (i = 1; i <= ; i++) { sum += power (i, i % 5); pritf ("sum = %g\", sum); retur 0; Laguage systems The classical sequece Variatios o the classical sequece $ gcc -Wall -O0 test.c -lm s $ gcc -Wall -O1 test.c -lm s $ gcc -Wall -O2 test.c -lm s $ gcc -Wall -O3 test.c -lm s $ gcc -Wall -O3 -furoll-loops test.c -lm s Variatios o the classical sequece The gcc C++ example Hidig steps Iterpreters Virtual machies (JVM=Java virtual machie) Delayed likig (.dll i Widows) Dyamic compilatio (JIT=just i time compilatio) Hidig steps example: the GNU C++ laguage system Laguage systems The classical sequece Variatios o the classical sequece Bidig 5

6 What is bidig? it i; void mai() { Bidig Associatio betwee two thigs (like a ame ad a property) Example: What set of values is associated with it? What is the type of fred? What is the address of the object code for mai? What is the value of i? Bidig Times 1.Laguage Desig Time Bidig time = the time whe a bidig is created (or a implemetatio decisio is made, or a aswer to a questio is give) Very importat for a laguage sematics A bidig ca happe at differet times: Laguage desig time Laguage implemetatio time Compile time Lik time Load time Rutime Some properties are boud whe the laguage is defied: Ex: Meaigs of keywords: void, for it i; void mai() { 2. Laguage Implemetatio Time Some properties are boud whe the laguage system is writte: rage of values of type it i C (but i Java, these are part of the laguage defiitio) implemetatio limitatios: max idetifier legth, max umber of array dimesios, etc it i; void mai() { 3. Compile Time Some properties are boud whe the program is compiled or prepared for iterpretatio: Types of variables, i laguages like C ad ML that use static typig Declaratio that goes with a give use of a variable, i laguages that use static scopig (most laguages) it i; void mai() { 6

7 4. Lik Time 5. Load Time Some properties are boud whe separately-compiled program parts are combied ito oe executable file by the liker: Object code for exteral fuctio ames it i; void mai() { Some properties are boud whe the program is loaded ito the computer s memory, but before it rus: Memory locatios for code for fuctios Memory locatios for static variables it i; void mai() { 6. Ru Time Practice (Exercise 3, Ch.4) Some properties are boud oly whe the code i questio is executed: Values of variables Types of variables, i laguages like Lisp that use dyamic typig Bidig at ru time is called late or dyamic bidig Bidig before ru time is called early or static! #" $ % #& ' ' ' ' ' ' # ( )$ * & # ( +, - # & * #./(, 0 & # 1 (2 $ #* ( % & ( % # )$ 3, & $ 3 # # $ & Practice (Exercise 3, Ch.4) Trade-off What is the bidig time for each of the followig i a Java program? The locatio i memory of a local variable i a method? The locatio i memory of a o-static field i a class? The meaig of the keyword while? The size i memory of a variable of type it? The type of a local variable i a fuctio? The value assiged to a variable? Rutime Rutime Laguage defiitio Laguage defiitio Compile time Ru time Early Bidig vs. Late Bidig Early: faster ad more secure at rutime (less to do, less that ca go wrog) (compilers) Late: flexible at rutime (as with types, dyamic loadig, etc.) (iterpreters) 7

8 Laguage systems Rutime Support The classical sequece Variatios o the classical sequece Bidig Debuggers Rutime support Additioal code the liker icludes eve if the program does ot refer to it explicitly Startup processig: iitializig the machie state Exceptio hadlig: reactig to exceptios Memory maagemet: allocatig memory, reusig it whe the program is fiished with it Operatig system iterface: commuicatig betwee ruig program ad operatig system for I/O, etc. A importat hidde player i laguage systems Part II Fuctioal laguages: A first look to ML Fuctioal laguages emphasize the defiitio ad applicatio of mathematical fuctios, i cotrast to imperative programmig, which emphasizes the executio of sequetial commads well-defied sematics. Make much easier to formally prove the correctess of fuctioal programs based o lambda calculus Examples: Lisp, ML, Scheme, Haskell, Mirada Read more about : Discussios forum: comp.lag.fuctioal y = f(x) * f(x) Ca you say it is the same with: z = f(x) y = z*z ;? About ML ML=Meta Laguage used for a theorem prover Ediburgh, 1974, Robi Miler s group There are a umber of dialects We are usig Stadard ML (istalled o Liux machies) Good book: Ulma, Elemets of ML programmig Aother popular dialect is OCaml = Objective Caml (used by Ileidig i Theoretische Iformatica) 8

9 Gettig started To ru SML i iteractive mode, type to the Uix prompt $sml Type a expressio after - prompt; ML replies with value ad type Do t forget ; after the expressio! Variable it is a special variable that is boud to the value of the expressio you type Stadard ML of New Jersey - 1+2*3; val it = 7 : it Outlie Costats Operators Variables Tuples ad Lists Fuctios Costats ; val it = 1234 : it ; val it = : real - true; - "fred"; val it = "fred" : strig - "H"; val it = "H" : strig - #"H"; val it = #"H" : char ML is case-sesitive it : Iteger costats: real : Real costats bool: Boolea costats true ad false strig: Strig costats: text iside double quotes char: Character costats: put # before a 1-character strig Operators -~ * 4 div 5 mod 6; val it = ~1 : it - ~ * 4.0 / 5.0; val it = ~1.4 : real -"bibity" ^ "bobity" ^ "boo"; val it = "bibitybobityboo" : strig Stadard operators for itegers, usig ~ for uary egatio ad - for biary subtractio Same operators for reals, but use / for divisio Strig cocateatio: ^ operator Left associative, precedece is {+,- < {*,/,div,mod < {~. Orderig operators - 2 < 3; <= 1.0; - #"d" > #"c"; - "abce" >= "abd"; val it = false : bool Orderig comparisos: <, >, <=, >=, apply to strig, char, it ad real Order o strigs ad characters is lexicographic Equality compariso operators - 1 = 2; val it = false : bool - true <> false; = 1.3; Error: operator ad operad do't agree [equality type required] operator domai: ''Z * ''Z operad: real * real i expressio: 1.3 = 1.3 Most types are equality testable: these are equality types Type real is ot a equality type 9

10 Boolea operators Short circuitig operators - 1 < 2 orelse 3 > 4; - 1 < 2 adalso ot (3 < 4); val it = false : bool Boolea operators: adalso, orelse, ot. (Ad we ca also use = for equivalece ad <> for exclusive or.) Precedece so far: {orelse < {adalso < {=,<>,<,>,<=,>= < {+,-,^ < {*,/,div,mod < {~,ot - true orelse 1 div 0 = 0; adalso ad orelse are short-circuitig operators = if the first operad of orelse is true, the secod is ot evaluated; likewise if the first operad of adalso is false Coditioal expressios - if 1 < 2 the #"x" else #"y"; val it = #"x" : char - if 1 > 2 the 34 else 56; val it = 56 : it - (if 1 < 2 the 34 else 56) + 1; val it = 35 : it Coditioal expressio (ot statemet) usig if the else Similar to C's terary operator: (1<2)? 'x' : 'y' Value of the expressio is the value of the the part, if the test part is true, or the value of the else part otherwise There is o if the costruct Overloaded operators - 1 * 2; val it = 2 : it * 2.0; val it = 2.0 : real * 2; Error: operator ad operad do't agree [literal] operator domai: real * real operad: real * it i expressio: 1.0 * 2 The * operator, ad others like + ad <, are overloaded to have oe meaig o pairs of itegers, ad aother o pairs of reals Type coversios A little about fuctios ML does ot perform implicit type coversio ML has builti coversio fuctios: real (it to real), floor (real to it), ceil (real to it), roud (real to it), truc (real to it), ord (char to it), chr (it to char), str (char to strig) - real(123); val it = : real - floor(3.6); val it = 3 : it - floor 3.6; val it = 3 : it - str #"a"; val it = "a" : strig - square 2+1; val it = 5 : it - square (2+1); val it = 9 : it Fuctio applicatio has higher precedece tha ay operator Fuctio applicatio is leftassociative 10

11 Defiig variables - val x = 1+2*3; val x = 7 : it - x; val it = 7 : it - val y = if x = 7 the 1.0 else 2.0; val y = 1.0 : real val creates a bidig val defies a ew variable ad bids it to a value. ML has No assigmet val defiitio i ML is ot the same as assigmet i C or Java. val defies a ew variable ad bids it to a value BUT it does ot chage the old defiitio. It adds a ew defiitio op top of the other. - val radius = 4.0; val radius = 4.0 : real - radius; val it = 4.0 : real - val radius = 5.0; val radius = 5.0 : real - radius; val it = 5.0 : real Fuctios have o side effects -val x = 0 ; val x=0; it -fu ic(x) = x+1 ; val ic = f : it -> it - ic x; val it = 1 : it - ic x; val it = 1 : it The value of x is ot chaged by the fuctio = referetial trasparecy Tuples Tuple = collectio of values of differet types (like barey,poit1) Tuples ca cotai other tuples To get i'th elemet of a tuple x, use #i x There is o such thig as a tuple of oe - val barey = (1+2, 3.0*4.0, "brow"); val barey = (3,12.0,"brow") : it * real * strig - val poit1 = ("red", (300,200)); val poit1 = ("red",(300,200)) : strig * (it * it) - #2 barey; val it = 12.0 : real - #1 (#2 poit1); val it = 300 : it Tuple Type Costructor ML gives the type of a tuple usig * as a type costructor For example, it * bool is the type of pairs (x,y) where x is a it ad y is a bool Note that paretheses have structural sigificace here: it * (it * bool) is ot the same as (it * it) * bool, ad either is the same as it * it * bool Lists List = collectio of values of the same type -[1,2,3]; val it = [1,2,3] : it list - [1.0,2.0]; val it = [1.0,2.0] : real list - [(1,2),(1,3)]; val it = [(1,2),(1,3)] : (it * it) list - [[1,2,3],[1,2]]; val it = [[1,2,3],[1,2]] : it list list - []; val it = [] : 'a list - il; val it = [] : 'a list Empty list is [] or il. The type of the empty list is 'a list 'a list meas a list of elemets, type ukow 11

12 The ull test - ull []; - ull [1,2,3]; val it = false : bool ull tests whether a give list is empty You could also use a equality test, as i x = [] However, ull x is preferred; we will see why i a momet List Type Costructor ML gives the type of lists usig list as a type costructor For example, it list is the type of lists of thigs, each of which is of type it A list is ot a tuple operator cocateates 2 lists of the same type - [1,2,3]@[4,5,6]; val it = [1,2,3,4,5,6] : it list List-builder (cos) operator is :: It takes a elemet of ay type, ad a list of elemets of that same type, ad produces a ew list by puttig the ew elemet o the frot of the old list - val x = #"c"::[]; val x = [#"c"] : char list - val y = #"b"::x; val y = [#"b",#"c"] : char list - val z = 1::2::3::[]; val z = [1,2,3] : it list The :: operator is right-associative The hd fuctio gets the head of a list: the first elemet The tl fuctio gets the tail of a list: the whole list after the first elemet The explode fuctio coverts a strig to a list of characters, ad the implode fuctio does the reverse - hd z; val it = 1 : it - tl z; val it = [2,3] : it list - tl(tl z); val it = [3] : it list - tl(tl(tl z)); val it = [] : it list - explode "hello"; val it = [#"h",#"e",#"l",#"l",#"o"] : char list - implode [#"h",#"i"]; val it = "hi" : strig Defiig fuctios - fu firstchar s = hd (explode s); val firstchar = f : strig -> char - firstchar "abc"; val it = #"a" : char fu defies a ew fuctio ad bids it to a variable Here f meas a fuctio, the thig itself, cosidered separately from ay ame we've give it. The value of firstchar is a fuctio whose type is strig -> char Fuctio Defiitio Sytax <fu-def> ::= fu <fuctio-ame> <parameter> = <expressio> ; <fuctio-ame> ca be ay legal ML ame The simplest <parameter> is just a sigle variable ame: the formal parameter of the fuctio The <expressio> is ay ML expressio; its value is the value the fuctio returs This is oly a subset of ML fuctio defiitio sytax; 12

13 Fuctio Type Costructor ML gives the type of fuctios usig -> as a type costructor For example, it -> real is the type of a fuctio that takes a it parameter (the domai type) ad produces a real result (the rage type) - fu quot(a,b) = a div b; val quot = f : it * it -> it - quot (6,2); val it = 3 : it - val pair = (6,2); val pair = (6,2) : it * it - quot pair; val it = 3 : it All ML fuctios take exactly oe parameter To pass more tha oe thig, you ca pass a tuple Example 1 - fu fact = = if = 0 the 1 = else * fact(-1); val fact = f : it -> it - fact 5; val it = 120 : it Recursive factorial fuctio Example 2 - fu listsum x = = if ull x the 0 = else hd x + listsum(tl x); val listsum = f : it list -> it - listsum [1,2,3,4,5]; val it = 15 : it Recursive fuctio to add up the elemets of a it list A commo patter: base case for ull x, recursive call o tl x Example 3 - fu legth x = = if ull x the 0 = else 1 + legth (tl x); val legth = f : 'a list -> it - legth [true,false,true]; val it = 3 : it - legth [4.0,3.0,2.0,1.0]; val it = 4 : it Recursive fuctio to compute the legth of a list Note type: this works o ay type of list. It is polymorphic. - fu badlegth x = = if x=[] the 0 = else 1 + badlegth (tl x); val badlegth = f : ''a list -> it - badlegth [true,false,true]; val it = 3 : it - badlegth [4.0,3.0,2.0,1.0]; Error: operator ad operad do't agree [equality type required] Same as previous example, but with x=[] istead of ull x Type variables that begi with two apostrophes, like ''a, are restricted to equality types. ML isists o that restrictio because we compared x for equality with the empty list. That s why you should use ull x istead of x=[]. It avoids uecessary type restrictios. 13

14 Example 4 - fu reverse L = = if ull L the il = else reverse(tl [hd L]; val reverse = f : 'a list -> 'a list - reverse [1,2,3]; val it = [3,2,1] : it list Recursive fuctio to reverse a list That patter agai Summary Costats ad primitive types: it, real, bool, char, strig Operators: ~, +, -, *, div, mod, /, ^, <, >, <=, >=, =, <>, ot, adalso, orelse Coditioal expressio Fuctio applicatio Predefied fuctios: real, floor, ceil, roud, truc, ord, chr, str, hd, tl, explode, implode, ad ull Summary, Cotiued Defiig ew variable bidigs usig val Tuple costructio usig (x,y,,z) ad selectio usig # List costructio usig [x,y,,z] Type costructors *, list, ad -> Fuctio declaratio usig fu It s easy; it s fu All real programmig i ML is coducted by defiitio of fuctios ad applicatio of these fuctios to argumets. Fuctioal laguages are also called applicative laguages Practice What is the value ad ML type for each of these expressios? 1 * * 4 "abc" ^ "def" if (1 < 2) the 3.0 else < 2 orelse (1 div 0) = 0 What is wrog with each of these expressios? 10 / 5 #"a" = #"b" or 1 = = 1.0 if (1<2) the 3 Practice Suppose we make these ML declaratios: val a = "123"; val b = "456"; val c = a ^ b ^ "789"; val a = 3 + 4; The what is the value ad type of each of these expressios? a b c 14

15 Practice What are the values of these expressios? #2(3,4,5) hd(1::2::il) hd(tl(#2([1,2],[3,4]))); What is wrog with the followig expressios? hd(tl(tl [1,2])) [1]::[2,3] 15