exprl_cpp

A simple expression language written in C++.

Syntax

Literals

• true or false for booleans
• 42, 31.4 or -1 for numbers
• 'hello world' for text
• variable-name for literal variables

Functions

• function(argA) for unary functions
• function(argA, argB) for binary functions
• function(argA, argB, ..., argN) for n-ary functions

Functions and literals can be combined to express complex expressions :

• concat('abc', 'def', 'ghi')
• cond(and(true, true), 'abcdef', '')
• len(substr('test', 0, sub(len('test'), 1)))

Available functions

• not(argA: boolean): boolean : logical NOT
  • not(true) ==> false
  • not(false) ==> true
• and(argA: boolean, argB: boolean, ...argN: boolean): boolean : logical AND
  • and(true, true) ==> true
  • and(true, true, false) ==> false
• or(argA: boolean, argB: boolean, ...argN: boolean): boolean : logical OR
  • or(false, false) ==> false
  • or(true, true, false) ==> true
• add(argA: number, argB: number, ...argN: number): number : addition
  • add(1, 2) ==> 3
  • add(1, -2, 3) ==> 2
• sub(argA: number, argB: number): number : substraction
  • sub(1, 2) ==> -1
  • sub(3, 1) ==> 2
• mul(argA: number, argB: number, ...argN: number): number : multiplication
  • mul(1, 2) ==> 2
  • mul(1, -2, 3) ==> -6
• div(argA: number, argB: number): number : division
  • div(9, 3) ==> 3
  • div(9, 2) ==> 4.5
• mod(argA: number, argB: number): number : modulus division
  • mod(9, 3) ==> 0
  • mod(9, 2) ==> 1
• starts(str: text, prefix: text): boolean : textual starts with check
  • starts('abcdef', 'a') ==> true
  • starts('abcdef', 'ab') ==> true
  • starts('abcdef', 'f') ==> false
• ends(str: text, suffix: text): boolean : textual ends with check
  • ends('abcdef', 'a') ==> false
  • ends('abcdef', 'ab') ==> false
  • ends('abcdef', 'f') ==> true
• in(str: text, content: text): boolean : textual contains check
  • in('abcdef', 'a') ==> true
  • in('abcdef', 'ab') ==> true
  • in('abcdef', 'cd') ==> true
  • in('abcdef', 'z') ==> false
• substr(str: text, begin: number, end: number): text : textual sub-string extraction based on indexes
  • substr('abcdef', 1, 5) ==> 'bcde'
  • substr('abcdef', 0, -1) ==> abcdef
  • substr('abcdef', 1, -2) ==> bcde
• substrl(str: text, begin: number, end: number): text : textual sub-string extraction based on index and length
  • substrl('abcdef', 1, 5) ==> 'bcdef'
  • substrl('abcdef', -3, 2) ==> 'ef'
• concat(str: text, append: text): text : textual concatenation
  • concat('abc', 'def') ==> 'abcdef'
• len(str: text): number : textual length
  • len('abcdef') ==> 6
  • len('abc') ==> 3
• eq(argA: any, argB: any): boolean : equality check
  • eq('abcdef', 'abcdef') ==> true
  • eq(1, 1) ==> true
  • eq(1, 2) ==> false
• neq(argA: any, argB: any): boolean : inequality check
  • neq('abcdef', 'abcdef') ==> false
  • neq(1, 1) ==> false
  • neq(1, 2) ==> true
• lt(argA: number, argB: number): boolean : less than check
  • lt(1, 2) ==> true
  • lt(2, 2) ==> false
  • lt(3, 2) ==> false
• lte(argA: number, argB: number): boolean : less than or equal to check
  • lte(1, 2) ==> true
  • lte(2, 2) ==> true
  • lte(3, 2) ==> false
• gt(argA: number, argB: number): boolean : greater than check
  • gt(1, 2) ==> false
  • gt(2, 2) ==> false
  • gt(3, 2) ==> true
• gte(argA: number, argB: number): boolean : greater than or equal to check
  • gte(1, 2) ==> false
  • gte(2, 2) ==> true
  • gte(3, 2) ==> true
• min(argA: number, argB: number, ...argN: number): minimum of given numbers
  • min(0, 1) ==> 0
  • min(0, -1, -5) ==> -5
• max(argA: number, argB: number, ...argN: number): maximum of given numbers
  • max(0, 1) ==> 1
  • max(0, -1, 5)==> 5
• var(variable: text): any: access the content of a variable
  • var('variable-boolean') ==> true with variable-boolean set to true
  • var('variable-text') ==> 'abcdef' with variable-text set to 'abcdef'
• cond(condition: boolean, then: any, else: any): any : conditional selection between two expressions
  • cond(true, 1, 2) ==> 1
  • cond(false, 1, 2) ==> 2
  • cond(false, 'a', 'b') ==> 'b'
• debug(debugged: any, label: text): any: reports the result of the execution as a labeled debug layer and transparently forwards the result to the caller
  • debug(true, 'dbg1') ==> true + reports dbg1 : true to the context
  • debug(cond(true, 1, 2), 'dbg-cond') ==> 1 + reports dbg-cond : 1 to the context
  • debug(cond(debug(and(true, true), 'dbg-and'), 'abcdef', ''), 'dbg-cond') ==> 'abcdef' + reports dbg-and : true and dbg-cond : abcdef to the context

Accessing variables

Variables can either be accessed using var('variable-name') function or by using literal variable like variable-name. For example and(var('variable-name'), true) and and(variable-name, true) are equivalent.

However, accessing a dynamically named variable (i.e. a variable whose name is built from sub-expressions) can only be accessed using var() function. For example var(concat('constant-prefix.', variable-part)).

Usage

ExpressionEvaluator evaluator;
evaluator.setVariable("state", true);

// Parse and evaluate expression
auto result = evaluator.evaluateAsBoolean("var('state')");

// Parse expression
auto parsedExpression = Parser::parse("var('state')");

// Evaluate already parsed expression
result = evaluator.evaluateAsBoolean(parsedExpression);

The evaluator guarantees that variables will keep consistent during the evaluation of an expression.