Param

Parameter presence and type verification

Import

_ <- fat.extra.Param

Error type is automatically imported with this import

Types

This library introduces the Param type and the Using/UsingStrict/Implicitly utilities for implicit parameter declaration.

Constructors

Both Param and Using/UsingStrict constructors take two arguments:

• _exp: Text: the parameter name to check in context.
• _typ: Type: the expected type of the evaluated value.

Additionally Param accepts an optional argument:

• strict: Boolean: disable flexible match (default is false).

Using has this flag set as false, and UsingStrict has it set as true

Param

The Param type provides mechanisms for checking the presence and type of parameters in the execution context.

Prototype members

the get method throws KeyError if the parameter is not defined, and TypeError if the type does not match

Example

_ <- fat.extra.Param
_ <- fat.type.Text  # the desired type must be loaded

currentUser = Param('userId', Text)

...

# Assuming userId is defined in the context and is a text,
# safely retrieve it's value from the current namespace
userId = currentUser.get

Using

Apply Using/UsingStrict to suppress implicit parameter hints on method declarations for entries expected to be in scope.

alternatively, to suppress warnings about implicit parameters, name the implicit entry starting with an underscore (_)

Example

_ <- fat.extra.Param
_ <- fat.type.Text

printUserIdFromContext = <> {
  Using('userId', Text)
  console.log(userId)
}

if the implicit parameter is missing or mismatched, an error will be raised at runtime when the method is called

Implicitly

A zero‑overhead, parse‑time hint that declares a name as implicitly used so the parser doesn't warn about an undeclared or unused entry. When parsed, the call is stripped out entirely. It doesn't evaluate or assign anything at runtime.

Example

_ <- fat.extra.Param

printUserIdFromContext = <> {
  Implicitly('userId')
  console.log(userId)
}

the only exception where Implicitly is not stripped out is when code when a method is declared in the REPL prompt; yet, provided you have Param import, it should still be a no-op

See also

• Extra package