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Evaluation Order

Disciple uses a strict/call-by-value evaluation order, where compound expressions are evaluated from left to right in data-dependency order.

This function:

f a b = f1 (f2 a) (f3 b)

is desugared to:

f a b
 = do   x1 = f2 a
        x2 = f3 b
        f1 x1 x2

where `x1` and `x2` are fresh variables.

do Expressions

`do` expressions contain a sequence of bindings and statements, and must end with a statement. These elements are evaluated in turn, from top to bottom. Bindings may not be mutually recursive. The final statement is taken as the value of the entire expression.

_Differences to Haskell_: The `do` expression does not perform monadic desugaring, though there is a plan (ticket) to overload the syntax in the future.

Introduction of Laziness

Although strict evaluation is the default, any function application may be suspended with the `@` operator, so long as the application does not cause visible, impure SideEffects. The attempted suspension of an application with visible side effects will result in a compile-time type error.

The expression `f @ x` (read as: f suspend x) creates a thunk containing the function `f` and its argument `x`. When the value of this thunk is demanded at runtime, the function will be applied to its argument, yielding the result.

If a value is not suspended it is called _direct_. Both _direct_ and _lazy_ objects have the same type, are first class and interchangable. The forcing of thunks is transparent and does not require extra code in the source program.

The suspension operator `@` accepts a variable number of arguments. Uses of `@` map onto a set of primitive `suspend` functions, which can also be used if desired.

To suspend the applications of `f1` and `f3` in the above example we can write:

{{{ f a b = f1 @ (f2 a) (f3 @ b) }}}

which is equivalent to:

{{{ f a b

= do  x1 = f2 a
      x2 = suspend1 f3 b
      suspend2 f1 x1 x2