= Expr
Expressions calculator
:authors: Celestino Amoroso
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toc::[]

== Expr
_Expr_ is a GO package capable of analysing, interpreting and calculating expressions.

=== Usage


[source,go]
----
import (
	"fmt"
	"strings"

	"git.portale-stac.it/go-pkg/expr"
)

func main() {
	ctx := expr.NewSimpleVarStore()
	ctx.SetValue("var", int64(4))

	source := `(3-1)*(10/5) == var`

    r := strings.NewReader(source)
    scanner := expr.NewScanner(r, DefaultTranslations())
	parser := expr.NewParser(ctx)

	if ast, err := parser.parse(scanner); err == nil {
		if result, err := ast.eval(ctx); err == nil {
            fmt.Printf("%q -> %v [%T]\n", source, result, result)
        } else {
            fmt.Println("Error calculating the expression:", err)
        }
	} else {
        fmt.Println("Error parsing the expression:", err)
    }
}
----
The above program is equivalent to the following one.

[source,go]
----
import (
	"fmt"
	"git.portale-stac.it/go-pkg/expr"
)

func main() {
	ctx := expr.NewSimpleVarStore()
	ctx.SetValue("var", int64(4))

	source := `(3-1)*(10/5) == var`

    if result, err := expr.evalString(ctx, source); err == nil {
        fmt.Printf("%q -> %v [%T]\n", source, result, result)
    } else {
        fmt.Println("Error calculating the expression:", err)
    }
}
----

=== Data types
_Expr_ supports numerical, string, relational, and boolean expressions.

==== Numbers
Numbers can be integers (GO int64) and float (GO float64). In mixed operations involving integers and floats, integers are automatically promoted to floats.

.Arithmetic operators
[cols="^1,^2,6,4"]
|===
| Symbol    | Operation     | Description               | Examples

| `+` / `-` | _change sign_ | Change the sign of values | `-1` _[-1]_, `-(+2)` _[-2]_

| `+` | _sum_ | Add two values | `-1 + 2` _[1]_, `4 + 0.5` _[4.5]_

| `-` | _subtracion_ | Subtract the right value from the left one | `3 - 1` _[2]_, `4 - 0.5` _[3.5]_

| `*` | _product_ | Multiply two values | `-1 * 2` _[-2]_, `4 * 0.5` _[2.0]_

| `/` | _Division_ | Divide the left value by the right one | `-1 / 2` _[0]_, `1.0 / 2` _[0.5]_

| `./` | _Float division_ | Force float division | `-1 ./ 2` _[-0.5]_

| `%` | _Modulo_ | Remainder of the integer division | `5 % 2` _[1]_

|=== 

==== String
Strings are character sequences enclosed between two double quote `"`.

==== Boolean
Boolean data type has two values only: _true_ and _false_. Relational and Boolean expressions produce a Boolean values.


.Relational operators
[cols="^1,^2,6,4"]
|===
| Symbol    | Operation     | Description               | Examples

| `<` | _Less_ | True if the left value is less than the right one | `5 < 2` _[false]_, `"a" < "b"` [true]
| `\<=` | _Less or Greater_ | True if the left value is less or greater than the right one | `5 <= 2` _[false]_, `"b" <= "b"` [true]
|===


.Boolean operators
[cols="^1,^2,6,4"]
|===
| Symbol    | Operation     | Description               | Examples

| `NOT` | _Not_ | True if the right value is false | `NOT true` _[false]_, `NOT (2 < 1)` _[true]_

| `AND` / `&&` | _And_ | True if both left and right values are true  | `false && true` _[false]_, `"a" < "b" AND NOT (2 < 1)` _[true]_

| `OR` / `\|\|` | _Or_ | True if at least one of the left and right values integers true| `false or true` _[true]_, `"a" == "b" OR (2 == 1)` _[false]_
|===