The let-syntax is used to neatly encapsulate intermediate variables, so they are only visible in the parts of the program that need to see them. It has two versions:

• let-expression: let DECLARATIONS in EXPRESSIONS endlet
• let-declaration: let DECLARATIONS endlet

Let Expressions

The let-expression is the commonest form of let-syntax. It is used to define some intermediate variables that are then used inside an expression. For example:

let d := sqrt( x**2, y**2 )
in
    x/d + y/d
end

Let Declarations

Modifiers

In a let-expression, the declarations can be annotated by the modifiers private, public or common with the following rules:

• Any variable declared as private is entirely local to that scope, as you might expect.
• Variables declared as public or common are visible outside that scope, as if they were declared in the parent scope.
• And variables declared as common cannot directly reference private variables. This is a modifier that is unique to Nutmug that is mainly useful when defining classes.

Note that, as an alternative to putting private, public or common in front of the name, the declaration can be marked using the annotations @private/@public/@common.

block/endblock

The block/endblock form is most useful for defining some functions whilst hiding some auxiliary variables. For example:

    block
        private x := ref(initial)
        public countup := fun(): x!, x! <-- $$ + 1 end
    endblock

Within block scopes, you may use let as a synonym for private. So the above code can be written more tersely like this:

    block
        let x := ref(initial)
        countup := fun(): x!, x! <-- $$ + 1 end
    end

Files as block scopes

There are a couple of other block scopes in Nutmeg. Each compiled file forms a block scope with public default. This means it is possible to have private top-level definitions that are hidden from the rest of the program. e.g.

def factorial( n ):
    fact( n, 1 )
enddef

@private
def fact( n, sofar ) =>>
    if n <= 1:
        sofar
    else:
        fact( n-1, b=n*sofar )
    endif
enddef

Note that the default visibility for files is public.

Classes as Block Scope

Class definitions also introduce block scopes with a default of public.

Nesting Block Scopes