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‎docs/configuration.md‎

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# Configuration
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Expr can be configured with options. For example, you can pass the environment with variables and functions.
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## Return type
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## AllowUndefinedVariables()
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Usually, the return type of expression is anything. But we can instruct type checker to verify the return type of the
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expression.
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For example, in filter expressions, we expect the return type to be a boolean.
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This option allows undefined variables in the expression. By default, Expr will return an error
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if the expression contains undefined variables.
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```go
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program, err := expr.Compile(code, expr.AsBool())
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if err != nil {
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panic(err)
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}
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```go
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program, err := expr.Compile(`foo + bar`, expr.AllowUndefinedVariables())
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```
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output, err := expr.Run(program, env)
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if err != nil {
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panic(err)
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}
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## AsBool()
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ok := output.(bool) // It is safe to assert the output to bool, if the expression is type checked as bool.
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```
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This option forces the expression to return a boolean value. If the expression returns a non-boolean value,
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Expr will return an error.
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If `code` variable for example returns a string, the compiler will return an error.
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```go
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program, err := expr.Compile(`Title contains "Hello"`, expr.AsBool())
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```
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Expr has a few options to specify the return type:
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## AsFloat64()
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- [expr.AsBool()](https://pkg.go.dev/github.com/expr-lang/expr#AsBool) - expects the return type to be a bool.
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- [expr.AsInt()](https://pkg.go.dev/github.com/expr-lang/expr#AsInt) - expects the return type to be an int (float64,
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uint, int32, and other will be cast to int).
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- [expr.AsInt64()](https://pkg.go.dev/github.com/expr-lang/expr#AsInt64) - expects the return type to be an int64 (
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float64, uint, int32, and other will be cast to int64).
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- [expr.AsFloat64()](https://pkg.go.dev/github.com/expr-lang/expr#AsFloat64) - expects the return type to be a float64 (
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float32 will be cast to float64).
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- [expr.AsAny()](https://pkg.go.dev/github.com/expr-lang/expr#AsAny) - expects the return type to be anything.
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- [expr.AsKind(reflect.Kind)](https://pkg.go.dev/github.com/expr-lang/expr#AsKind) - expects the return type to be a
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specific kind.
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This option forces the expression to return a float64 value. If the expression returns a non-float64 value,
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Expr will return an error.
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:::tip Warn on any
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By default, type checker will accept any type, even if the return type is specified. Consider following examples:
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```go
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program, err := expr.Compile(`42`, expr.AsFloat64())
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```expr
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let arr = [1, 2, 3]; arr[0]
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```
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:::note
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If the expression returns integer value, Expr will convert it to float64.
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:::
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## AsInt()
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The return type of the expression is `any`. Arrays created in Expr are of type `[]any`. The type checker will not return
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an error if the return type is specified as `expr.AsInt()`. The output of the expression is `1`, which is an int, but the
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type checker will not return an error.
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This option forces the expression to return an int value. If the expression returns a non-int value,
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Expr will return an error.
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But we can instruct the type checker to warn us if the return type is `any`. Use [`expr.WarnOnAny()`](https://pkg.go.dev/github.com/expr-lang/expr#WarnOnAny) to enable this behavior.
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```go
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program, err := expr.Compile(`42`, expr.AsInt())
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program, err := expr.Compile(code, expr.AsInt(), expr.WarnOnAny())
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```
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:::note
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If the expression returns a float value, Expr truncates it to int.
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:::
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## AsInt64()
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The type checker will return an error if the return type is `any`. We need to modify the expression to return a specific
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type.
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Same as `AsInt()` but returns an int64 value.
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```go
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program, err := expr.Compile(`42`, expr.AsInt64())
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```expr
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let arr = [1, 2, 3]; int(arr[0])
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```
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:::
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## AsKind()
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This option forces the expression to return a value of the specified kind.
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If the expression returns a value of a different kind, Expr will return an error.
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## WithContext
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```go
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program, err := expr.Compile(`42`, expr.AsKind(reflect.String))
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```
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Although the compiled program is guaranteed to be terminated, some user defined functions may not be. For example, if a
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user defined function calls a remote service, we may want to pass a context to the function.
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## ConstExpr()
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This is possible via the [`WithContext`](https://pkg.go.dev/github.com/expr-lang/expr#WithContext) option.
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This option tells Expr to treat specified functions as constant expressions.
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If all arguments of the function are constants, Expr will replace the function call with the result
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during the compile step.
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This option will modify function calls to include the context as the first argument (only if the function signature
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accepts a context).
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```go
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program, err := expr.Compile(`fib(42)`, expr.ConstExpr("fib"))
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```expr
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customFunc(42)
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// will be transformed to
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customFunc(ctx, 42)
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```
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[ConstExpr Example](https://pkg.go.dev/github.com/expr-lang/expr?tab=doc#ConstExpr)
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## Env()
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This option passes the environment with variables and functions to the expression.
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Function `expr.WithContext()` takes the name of context variable. The context variable must be defined in the environment.
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```go
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program, err := expr.Compile(`foo + bar`, expr.Env(Env{}))
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env := map[string]any{
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"ctx": context.Background(),
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}
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program, err := expr.Compile(code, expr.Env(env), expr.WithContext("ctx"))
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```
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## Function()
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## ConstExpr
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This option adds a function to the expression.
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For some user defined functions, we may want to evaluate the expression at compile time. This is possible via the
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[`ConstExpr`](https://pkg.go.dev/github.com/expr-lang/expr#ConstExpr) option.
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```go
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atoi := expr.Function(
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"atoi",
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func(params ...any) (any, error) {
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return strconv.Atoi(params[0].(string))
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},
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)
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program, err := expr.Compile(`atoi("42")`, atoi)
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func fib(n int) int {
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if n <= 1 {
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return n
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}
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return fib(n-1) + fib(n-2)
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}
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env := map[string]any{
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"fib": fib,
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}
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program, err := expr.Compile(`fib(10)`, expr.Env(env), expr.ConstExpr("fib"))
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```
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Expr sees the `atoi` function as a function with a variadic number of arguments of type `any` and returns a value of type `any`. But, we can specify the types of arguments and the return value by adding the correct function
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signature or multiple signatures.
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If all arguments of the function are constants, the function will be evaluated at compile time. The result of the function
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will be used as a constant in the expression.
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```go
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atoi := expr.Function(
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"atoi",
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func(params ...any) (any, error) {
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return strconv.Atoi(params[0].(string))
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},
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new(func(string) int),
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)
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```expr
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fib(10) // will be transformed to 55 during the compilation
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fib(12+12) // will be transformed to 267914296 during the compilation
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fib(x) // will **not** be transformed and will be evaluated at runtime
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```
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Or we can simply reuse the `strconv.Atoi` function.
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```go
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atoi := expr.Function(
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"atoi",
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func(params ...any) (any, error) {
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return strconv.Atoi(params[0].(string))
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},
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strconv.Atoi,
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)
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```
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## Options
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Here is another example with a few function signatures:
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Compiler options can be defined as an array:
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```go
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toInt := expr.Function(
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"toInt",
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func(params ...any) (any, error) {
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switch params[0].(type) {
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case float64:
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return int(params[0].(float64)), nil
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case string:
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return strconv.Atoi(params[0].(string))
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}
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return nil, fmt.Errorf("invalid type")
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},
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new(func(float64) int),
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new(func(string) int),
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)
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options := []expr.Option{
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expr.Env(Env{})
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expr.AsInt(),
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expr.WarnOnAny(),
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expr.WithContext("ctx"),
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expr.ConstExpr("fib"),
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}
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program, err := expr.Compile(code, options...)
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```
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## Operator()
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This options defines an [operator overloading](operator-overloading).
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## Optimize()
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This option enables [optimizations](internals.md). By default, Expr will optimize the expression.
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## Patch()
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This option allows you to [patch the expression](visitor-and-patch) before compilation.
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Full list of available options can be found in the [pkg.go.dev](https://pkg.go.dev/github.com/expr-lang/expr#Option) documentation.

‎docs/environment.md‎

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# Environment
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The environment is a map or a struct that contains the variables and functions that the expression can access.
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## Struct as Environment
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Let's consider the following example:
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```go
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type Env struct {
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UpdatedAt time.Time
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Posts []Post
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Map map[string]string `expr:"tags"`
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}
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```
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The `Env` struct contains 3 variables that the expression can access: `UpdatedAt`, `Posts`, and `tags`.
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:::info
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The `expr` tag is used to rename the `Map` field to `tags` variable in the expression.
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:::
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The `Env` struct can also contain methods. The methods defined on the struct become functions that the expression can
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call.
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```go
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func (Env) Format(t time.Time) string {
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return t.Format(time.RFC822)
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}
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```
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:::tip
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Methods defined on embedded structs are also accessible.
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```go
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type Env struct {
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Helpers
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}
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type Helpers struct{}
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func (Helpers) Format(t time.Time) string {
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return t.Format(time.RFC822)
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}
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```
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:::
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We can use an empty struct `Env{}` to with [expr.Env](https://pkg.go.dev/github.com/expr-lang/expr#Env) to create an environment. Expr will use reflection to find
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the fields and methods of the struct.
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```go
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program, err := expr.Compile(code, expr.Env(Env{}))
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```
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Compiler will type check the expression against the environment. After the compilation, we can run the program with the environment.
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You should use the same type of environment that you passed to the `expr.Env` function.
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```go
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output, err := expr.Run(program, Env{
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UpdatedAt: time.Now(),
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Posts: []Post{{Title: "Hello, World!"}},
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Map: map[string]string{"tag1": "value1"},
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})
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```
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## Map as Environment
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You can also use a map as an environment.
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```go
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env := map[string]any{
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"UpdatedAt": time.Time{},
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"Posts": []Post{},
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"tags": map[string]string{},
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"sprintf": fmt.Sprintf,
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}
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program, err := expr.Compile(code, expr.Env(env))
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```
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A map defines variables and functions that the expression can access. The key is the variable name, and the type
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is the value's type.
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```go
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env := map[string]any{
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"object": map[string]any{
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"field": 42,
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},
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}
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```
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Expr will infer the type of the `object` variable as `map[string]any`.
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By default, Expr will return an error if unknown variables are used in the expression.
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You can disable this behavior by passing [`AllowUndefinedVariables`](https://pkg.go.dev/github.com/expr-lang/expr#AllowUndefinedVariables) option to the compiler.

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