MQL beginner: MQL recipes

Recipes

Shopping cart total: variables and arithmetic
Grade a class: for loop, list and case
Savings plan: while, repeat, break and continue
Check a sign-up form with boolean operators
Your first try and exception: a bank withdrawal
Split a bill: functions inside functions
Loop over the lines of a JSON order
Your first script with typed parameters
Check typed quantities: try inside a loop
Your first web service, called with execute

executed on MentDB

Shopping cart total: variables and arithmetic

Your first MQL script: a cart holds 3 items at 19.90 with a 10% discount and 4.99 of shipping. The script stores the values in variables, computes the total with the four operators, shows why money needs the exact bd operators, decides with if whether shipping is free, and uses the bi operators for integers of any size.

Pipeline

01 · storeValues in variablesvariable→02 · computeTotal with + - * /*→03 · exactExact decimals for moneybd*→04 · decideFree shipping or notif

Script

admin

#1. Store the values in variables;
-> "[price]" 19.90;
-> "[qty]" 3;
-> "[shipping]" 4.99;
#2. The four operators take two numbers each;
-> "[subtotal]" (* [price] [qty]);
-> "[discount]" (/ [subtotal] 10);
-> "[total]" (+ (- [subtotal] [discount]) [shipping]);
#3. Same thing with exact decimal operators;
-> "[bd_subtotal]" (bd* [price] [qty]);
-> "[bd_total]" (bd+ (bd- [bd_subtotal] (bd/ [bd_subtotal] 10)) [shipping]);
#4. Free shipping from 50;
if (>= [bd_subtotal] 50) {-> "[note]" "free shipping: remove the 4.99";} {-> "[note]" "add items to reach 50";};
#5. Integers of any size with bi operators;
-> "[big]" (bi* 987654321987 123456789123);
-> "[back]" (bi/ (bi- (bi+ [big] 7) 7) 123456789123);
concat "Subtotal with *: " [subtotal] [_n_] "Subtotal with bd*: " [bd_subtotal] [_n_] "Total with + - /: " [total] [_n_] "Total with bd+ bd- bd/: " [bd_total] [_n_] "Note: " [note] [_n_] "bi*: " [big] ", then bi+ bi- bi/ give back " [back];

Result

mentdb

Subtotal with *: 59.699999999999996
Subtotal with bd*: 59.70
Total with + - /: 58.72
Total with bd+ bd- bd/: 58.72
Note: free shipping: remove the 4.99
bi*: 121932631355968601347401, then bi+ bi- bi/ give back 987654321987

Go further

Very large integers: bi* and bi+ never overflow, try bi* 123456789123 987654321987
Round a double for display with math round

executed on MentDB

Grade a class: for loop, list and case

A teacher has five scores out of 20 in a comma-separated list. The script loops over the list with for, reads each score with atom get, gives a mention with case and computes the class average.

Pipeline

01 · listNumber of scoresatom size→02 · loopOne turn per scorefor→03 · readScore number iatom get→04 · mentionMention from the scorecase

Script

admin

#1. A list of scores separated by commas;
-> "[scores]" "15,8,12,19,10";
-> "[n]" (atom size [scores] ",");
-> "[sum]" 0;
-> "[report]" "";
#2. Loop from 1 to n: init, condition, increment, action;
for (-> "[i]" 1) (<= [i] [n]) (++ "[i]") {
	-> "[s]" (atom get [scores] [i] ",");
	-> "[sum]" (+ [sum] [s]);
	#3. case returns the action of the first true condition;
	-> "[mention]" (case (>= [s] 16) {"Very good"} (>= [s] 12) {"Good"} (>= [s] 10) {"Pass"} {"Fail"});
	-> "[report]" (concat [report] "Student " [i] ": " [s] "/20 " [mention] [_n_]);
};
concat [report] "Class average: " (/ [sum] [n]) "/20";

Result

mentdb

Student 1: 15/20 Good
Student 2: 8/20 Fail
Student 3: 12/20 Good
Student 4: 19/20 Very good
Student 5: 10/20 Pass
Class average: 12.8/20

Go further

Keep only the failed students with continue when the score is >= 10
Return the report as JSON with json load and json iarray

executed on MentDB

Savings plan: while, repeat, break and continue

You save 120 a month on an account paying 1% a month, but skip August, and you want 1200. The script loops with while until the goal, skips a month with continue, stops at 24 months with break, then shows that repeat runs its block at least once even when the condition is false.

Pipeline

01 · loopUntil the goal is reachedwhile→02 · skipNo saving in Augustcontinue→03 · guardStop after 24 monthsbreak→04 · comparerepeat runs at least oncerepeat

Script

admin

#1. Start empty, goal 1200;
-> "[balance]" 0;
-> "[month]" 0;
-> "[skipped]" 0;
while (< [balance] 1200) {
	++ "[month]";
	#2. Safety guard: never loop forever;
	if (> [month] 24) {break;};
	-> "[balance]" (math round (* [balance] 1.01) 2);
	#3. Month 8 is August: no deposit;
	if (== [month] 8) {++ "[skipped]"; continue;};
	-> "[balance]" (+ [balance] 120);
};
#4. while checks first, repeat checks after the block;
-> "[w]" 0;
while (< [w] 0) {++ "[w]";};
-> "[r]" 0;
repeat (< [r] 0) {++ "[r]";};
concat "Goal reached in month " [month] " with " [balance] ", " [skipped] " month skipped." [_n_] "while with a false condition ran " [w] " time, repeat ran " [r] " time.";

Result

mentdb

Goal reached in month 11 with 1264.38, 1 month skipped.
while with a false condition ran 0 time, repeat ran 1 time.

Go further

Count down with -- and a for loop
Log each month with log trace and read it back with log show 12

executed on MentDB

Check a sign-up form with boolean operators

A sign-up form sends an e-mail, a nickname, an age, a country and an optional coupon. The script checks each field with the null and empty tests, compares strings and numbers, and combines the checks with and, or, not and xor.

Pipeline

01 · emptyNull and empty testsis null or empty→02 · compareStrings and numbersequal→03 · combineand / or / not / xorand→04 · reportOne line per ruleconcat

Script

admin

#1. The values sent by the form;
-> "[email]" "ana@example.com";
-> "[nickname]" "";
-> "[age]" "17";
-> "[country]" "FR";
-> "[coupon]" null;
#2. Null and empty tests return 1 or 0;
-> "[has_email]" (is not empty [email]);
-> "[no_nick]" (is empty [nickname]);
-> "[no_coupon]" (is null [coupon]);
-> "[coupon_ok]" (is not null [coupon]);
-> "[nick_missing]" (is null or empty [nickname]);
#3. Compare strings with equal, numbers with == != < >;
-> "[adult]" (>= [age] 18);
-> "[france]" (equal [country] "FR");
-> "[not_us]" (not equal [country] "US");
-> "[age_17]" (and (== [age] 17.0) (!= [age] 18));
#4. Combine the rules;
-> "[can_buy]" (and [has_email] (or [adult] (not [france])));
-> "[one_of]" (xor [no_coupon] [coupon_ok]);
concat "email given: " [has_email] ", nickname null or empty: " [nick_missing] ", nickname empty: " [no_nick] [_n_] "adult: " [adult] ", age is 17: " [age_17] ", from France: " [france] ", not US: " [not_us] [_n_] "coupon: none " [no_coupon] ", given " [coupon_ok] ", exactly one is true: " [one_of] [_n_] "can buy alcohol online: " [can_buy] ", minor in France: " (and (< [age] 18) [france]);

Result

mentdb

email given: 1, nickname null or empty: 1, nickname empty: 1
adult: 0, age is 17: 1, from France: 1, not US: 1
coupon: none 1, given 0, exactly one is true: 1
can buy alcohol online: 0, minor in France: 1

Go further

Check the e-mail format with string matches and a regular expression
Stop at the first error with exception and catch it with try

executed on MentDB

Your first try and exception: a bank withdrawal

A customer with 120 on the account asks for 200. The script raises an exception with a code when the funds are too low, catches it with try, reads the message from the error variable and shows that the balance did not change.

Pipeline

01 · raiseError with a codeexception→02 · catchCatch the errortry→03 · readMessage of the errorerror→04 · resultStatus and balanceconcat

Script

admin

#1. Account and request;
-> "[balance]" 120;
-> "[amount]" 200;
try {
	#2. Raise an error with a code and a message;
	if (> [amount] [balance]) {
		exception (1001) (concat "Insufficient funds: balance " [balance] ", asked " [amount]);
	};
	-> "[balance]" (- [balance] [amount]);
	-> "[status]" "Withdrawal accepted";
} {
	#3. The error text ends with code: message;
	-> "[status]" (concat "Refused, error " (string regexp_extract [err] "([0-9]+): [^\n]*$" 1) " - " (string regexp_extract [err] "[0-9]+: ([^\n]*)$" 1));
} "[err]";
concat [status] [_n_] "Balance still " [balance];

Result

mentdb

Refused, error 1001 - Insufficient funds: balance 120, asked 200
Balance still 120

Go further

Change the amount to 50: no error, the balance becomes 70
Without try, the same exception stops the script and the editor shows the error

executed on MentDB

Split a bill: functions inside functions

Four friends share a restaurant bill of 187.45. The script first works step by step with one function per line, then nests the calls in one line, and makes the last friend pay the rounding gap so the total stays exact.

Pipeline

01 · variablesOne result per variablevariable→02 · splitInteger and decimal partsmath trunc→03 · centsThe cents leftmath frac_part→04 · nestRound inside a divisionmath round→05 · exactExact decimal rest and its signmath sign

Script

admin

#1. A restaurant bill shared by 4 friends;
-> "[bill]" 187.45;
-> "[guests]" 4;
#2. Step by step: one function per line, each result in a variable;
-> "[share]" (/ [bill] [guests]);
-> "[euros]" (math trunc [share]);
-> "[cents]" (math frac_part [share]);
#3. The same work in one line: the innermost parenthesis runs first;
-> "[each]" (math round (/ [bill] [guests]) 2);
#4. The last friend pays what is left, so the total is exact;
-> "[last]" (bd- [bill] (bd* [each] (- [guests] 1)));
-> "[gap]" (bd- [last] [each]);
#5. A function result can be the parameter of another function;
concat "Share: " [share] " = " [euros] " euros + " [cents] [_n_] (- [guests] 1) " friends pay " [each] ", the last one pays " [last] [_n_] "Gap sign: " (math sign [gap]) ", gap in cents: " (string lpad (math round (* [gap] 100) 0) "0" 2) [_n_] (string upper (concat "total checked: " (bd+ (bd* [each] (- [guests] 1)) [last])));

Result

mentdb

Share: 46.8625 = 46 euros + 0.8625
3 friends pay 46.86, the last one pays 46.87
Gap sign: 1, gap in cents: 01
TOTAL CHECKED: 187.45

Go further

Read a nested call from the inside out: (math round (/ [bill] [guests]) 2) divides first, then rounds.
Replace the last concat by string lpad and string rpad to print aligned columns of a receipt.

executed on MentDB

Loop over the lines of a JSON order

An order arrives as JSON with an array of lines. The script loads it in memory, loops over the array with a for and json select, prints a small receipt, visits the fields of the first line with json parse_obj, writes the total back into the order and frees the memory.

Pipeline

01 · loadLoad the order in memoryjson load→02 · countNumber of linesjson count→03 · loopfor from 0 to n-1 with pathsjson select→04 · visitKey and value of each fieldjson parse_obj→05 · writeAdd the total, then the average pricemath avg

Script

admin

#1. Load an order in memory: a JSON object with an array of lines;
json load "rx_p4l_order" "{\"id\": \"A-102\", \"customer\": \"Ana\", \"lines\": [{\"item\": \"Pen\", \"qty\": 3, \"price\": 1.5}, {\"item\": \"Notebook\", \"qty\": 2, \"price\": 4.9}, {\"item\": \"Bag\", \"qty\": 1, \"price\": 24}]}";
-> "[n]" (json count "rx_p4l_order" "/lines");
#2. A for loop on the array: positions start at 0;
-> "[total]" 0;
-> "[prices]" "";
-> "[receipt]" "";
for (-> "[i]" 0) (< [i] [n]) (++ "[i]") {
	-> "[path]" (concat "/lines[" [i] "]");
	-> "[item]" (json select "rx_p4l_order" (concat [path] "/item"));
	-> "[amount]" (* (json select "rx_p4l_order" (concat [path] "/qty")) (json select "rx_p4l_order" (concat [path] "/price")));
	-> "[total]" (+ [total] [amount]);
	-> "[prices]" (concat [prices] (if (> [i] 0) {", "} {""}) (json select "rx_p4l_order" (concat [path] "/price")));
	-> "[receipt]" (concat [receipt] (string rpad [item] "." 12) " " [amount] [_n_]);
};
#3. json parse_obj visits each field of an object: key and value;
-> "[first]" "";
json parse_obj "rx_p4l_order" "/lines[0]" "[k]" "[v]" {
	-> "[first]" (concat [first] [k] "=" [v] " ");
};
#4. Write the total into the order, read it back, free the memory;
json iobject "rx_p4l_order" "/" "total" [total] NUM;
-> "[saved]" (json select "rx_p4l_order" "/total");
-> "[fields]" (json fields "rx_p4l_order" "/");
json unload "rx_p4l_order";
concat [receipt] "Total: " [saved] " for " [n] " lines" [_n_] "Average unit price: " (math round (math avg (concat "[" [prices] "]")) 2) [_n_] "First line: " [first] [_n_] "Order fields: " (string replace (string replace [fields] [_n_] "") " " "");

Result

mentdb

Pen......... 4.5
Notebook.... 9.8
Bag......... 24
Total: 38.3 for 3 lines
Average unit price: 10.13
First line: item=Pen price=1.5 qty=3
Order fields: ["total","id","lines","customer"]

Go further

json parse_array "rx_key" "/lines" "[line]" {...} gives each element of an array in [line], with no index to manage.
Always end with json unload on your key: the document stays in the server memory otherwise.

executed on MentDB

Your first script with typed parameters

Create a small greeting script that takes a first name and the hour of the day, each checked by a type rule. Execute it twice, ask the server for its parameters, then delete it.

Pipeline

01 · createScript, parameters and codescript create→02 · executeRun it with valuesexecute→03 · branchPick the greetingcase→04 · describeRead its parameters as JSONscript show params→05 · deleteRemove the tutorial scriptscript delete

Script

admin

#1. Create a script: method post, name, activity log off, 1 attempt in the stack;
script create post "tuto.p4l.hello.greet" false 1
	(param
		(var "[name]" {type is_varchar [name] 30} "First name, 30 chars max" is_null:false is_empty:false "ana")
		(var "[hour]" {type is_integer [hour] 2} "Hour of the day, 0 to 23" is_null:false is_empty:false "9")
	;)
	"Say hello at the right time of the day"
{
	-> "[word]" (case (< [hour] 12) {"Good morning"} (< [hour] 18) {"Good afternoon"} {"Good evening"});
	concat [word] ", " (string capitalize_words [name]) "!";
} "A greeting sentence";
#2. Run it: the method is now the end of its name;
-> "[r1]" (execute "tuto.p4l.hello.greet.post" "[name]" "ana" "[hour]" 9);
#The parameters are given in the order of the definition;
-> "[r2]" (execute "tuto.p4l.hello.greet.post" "[name]" "léo martin" "[hour]" 20);
#3. The server knows the script and its parameters;
-> "[exists]" (script exist "tuto.p4l.hello.greet.post");
json load "rx_p4l_params" (script show params "tuto.p4l.hello.greet.post");
-> "[p1]" (concat (json select "rx_p4l_params" "/[0]/name") ": " (json select "rx_p4l_params" "/[0]/desc") ", checked by " (json select "rx_p4l_params" "/[0]/type"));
-> "[p2]" (concat (json select "rx_p4l_params" "/[1]/name") ": " (json select "rx_p4l_params" "/[1]/desc") ", checked by " (json select "rx_p4l_params" "/[1]/type"));
json unload "rx_p4l_params";
#4. A tutorial script is deleted at the end;
script delete "tuto.p4l.hello.greet.post";
concat [r1] [_n_] [r2] [_n_] "exists: " [exists] ", parameters: " [p1] " | " [p2] [_n_] "after delete, exists: " (script exist "tuto.p4l.hello.greet.post");

Result

mentdb

Good morning, Ana!
Good evening, Léo Martin!
exists: 1, parameters: [name]: First name, 30 chars max, checked by type is_varchar [name] 30; | [hour]: Hour of the day, 0 to 23, checked by type is_integer [hour] 2;
after delete, exists: 0

Go further

The values must be given in the order of the definition: [name] then [hour], or the server refuses the call.
Try execute "tuto.p4l.hello.greet.post" "[name]" "ana" "[hour]" "nine" before the delete: the type rule type is_integer rejects it.

executed on MentDB

Check typed quantities: try inside a loop

A stock form sends five quantities, some of them wrong. Each value is checked inside its own try, so one bad value raises a coded exception without stopping the loop; a server error (a text given to math round) is caught the same way.

Pipeline

01 · listValues separated by |atom get→02 · checkNumber, sign and whole numbertype is_number→03 · raiseYour own error codesexception→04 · catchCode and message of the errortry→05 · parseRead the error textstring regexp_extract

Script

admin

#1. Quantities typed in a stock form, one per product;
-> "[inputs]" "12|abc|-4|7.5|30";
-> "[n]" (atom size [inputs] "|");
-> "[stock]" 0;
-> "[ok]" 0;
-> "[report]" "";
for (-> "[i]" 1) (<= [i] [n]) (++ "[i]") {
	-> "[q]" (atom get [inputs] [i] "|");
	#2. One try per value: an error stops this value only, the loop goes on;
	try {
		if (not (type is_number [q])) {exception (2001) (concat "'" [q] "' is not a number");};
		if (< (math sign [q]) 1) {exception (2002) (concat [q] " must be positive");};
		if (not (type is_integer [q] 4)) {exception (2003) (concat [q] " is not a whole number");};
		-> "[stock]" (+ [stock] [q]);
		++ "[ok]";
		-> "[report]" (concat [report] "line " [i] ": +" [q] [_n_]);
	} {
		#3. The catch block reads the code and the message at the end of the error;
		-> "[code]" (string regexp_extract [err] "([0-9]+): [^\n]*$" 1);
		-> "[msg]" (string regexp_extract [err] "[0-9]+: ([^\n]*)$" 1);
		-> "[report]" (concat [report] "line " [i] ": error " [code] ", " [msg] [_n_]);
	} "[err]";
};
#4. An error raised by the server itself is caught the same way;
try {
	-> "[x]" (math round "twelve" 2);
} {
	-> "[server]" (string trim (string regexp_extract [err] "\[\[\[ (.*) \]\]\]" 1));
} "[err]";
concat [report] [ok] "/" [n] " lines accepted, stock +" [stock] [_n_] "Server error caught: " [server];

Result

mentdb

line 1: +12
line 2: error 2001, 'abc' is not a number
line 3: error 2002, -4 must be positive
line 4: error 2003, 7.5 is not a whole number
line 5: +30
2/5 lines accepted, stock +42
Server error caught: Sorry, an error appears (For input string: "twelve").

Go further

Put the try around the whole loop instead: the first bad value then stops everything, compare both outputs.
Use the code in the catch with switch to answer 400 for an input error and 500 for a server error.

executed on MentDB

Your first web service, called with execute

Write a GET web service that returns the weather of a city as JSON, call it from MQL with execute exactly as the web server would, read one field of the answer, and print how a browser or another program calls it over HTTP.

Pipeline

01 · createA get script with an enum parameterscript create→02 · buildBuild the JSON answerjson iobject→03 · callCall the serviceexecute→04 · readRead the answerjson select→05 · httpThe HTTP call for other programsscript generate url

Script

admin

#1. A web service is a script with a method: get to read data;
script create get "tuto.p4l.weather.city" false 1
	(param
		(var "[city]" {type is_enum [city] "paris,lyon,saint-denis"} "City code" is_null:false is_empty:false "paris")
	;)
	"Weather of a city, as a JSON object"
{
	-> "[temp]" (switch ([city]) ("paris") {12.5} ("lyon") {10} {27.8});
	json load "rx_p4l_ws" "{}";
	json iobject "rx_p4l_ws" "/" "city" (string capitalize_words [city]) STR;
	json iobject "rx_p4l_ws" "/" "celsius" [temp] NUM;
	json iobject "rx_p4l_ws" "/" "fahrenheit" (math round (+ (* [temp] 1.8) 32) 1) NUM;
	json iobject "rx_p4l_ws" "/" "date" (date sysdate) STR;
	-> "[answer]" (json doc "rx_p4l_ws");
	json unload "rx_p4l_ws";
	[answer];
} "JSON with city, celsius, fahrenheit and date";
#2. Call it from MQL with execute, like the web server does;
-> "[json]" (execute "tuto.p4l.weather.city.get" "[city]" "saint-denis");
#3. The answer is a JSON string: load it to read one field;
json load "rx_p4l_answer" [json];
-> "[f]" (json select "rx_p4l_answer" "/fahrenheit");
-> "[c]" (json select "rx_p4l_answer" "/city");
json unload "rx_p4l_answer";
#4. How a browser or another program calls the same service over HTTP;
-> "[http]" (script generate url "tuto.p4l.weather.city.get");
script delete "tuto.p4l.weather.city.get";
concat "Answer: " (string replace (string replace [json] [_n_] "") "  " "") [_n_] [c] " is at " [f] " F" [_n_] "HTTP call:" [_n_] [http];

Result

mentdb

Answer: {"date": "2026-11-01","celsius": 27.8,"city": "Saint-Denis","fahrenheit": 82.0}
Saint-Denis is at 82.0 F
HTTP call:
Method
  get
Header
  x-token = (empty or not define the first time)
  x-user = admin (required the first time)
  x-password = ***** (required the first time)
URL
  https://localhost:9999/api/tuto/p4l/weather/city?x-token=&x-user=&city=

Go further

The URL path is the script name: tuto.p4l.weather.city.get is served at /api/tuto/p4l/weather/city with the GET method.
Call it with a city outside the enum, "nice", and catch the refusal with try.