Alice ML extends the SML module system in various ways, by providing:
A syntax summary is given below.
Unlike SML, Alice ML has no stratification between core and module language. Core declarations (dec), structure declarations (strdec), and toplevel declarations (topdec) are collapsed into one class. As a consequence, structures can be declared local to an expression (via let) and functors as well as signatures can be nested into structures. For example:
fun sortWithoutDups compare l = let structure Set = MkRedBlackSet (type t = string; val compare = compare) in Set.toList (foldr (flip Set.insert) Set.empty l) end
Alice ML generalises SML's open declaration and allows not only structure identifiers, but arbitrary structure expressions:
open MkRedBlackSet(Int) open IntInf : INTEGER open unpack p : (val it : int -> int)
Note that, for backwards compatibility with SML, a sequence of identifiers is not parsed as a functor application, but as a multiple open declaration:
open MkRedBlackSet Int
is an attempt to open two structures MkRedBlackSet and Int, and will thus produce an error. Parentheses are needed to disambiguate, as in either of the following:
open MkRedBlackSet(Int) open (MkRedBlackSet Int)
A direct consequence of allowing local functor declarations is the presence of higher-order functors, even if a bit cumbersome:
functor F (X : S1) = struct functor G (Y : S2) = struct (* ... *) end end structure M = let structure Z = F (X) in G (Y) end
Functors are truly higher-order, so that the above example may be simplified to curried form:
functor F (X : S1) (Y : S2) = struct (* ... *) end
Application can be written as in the core language, without parentheses (see below):
structure M = F X Y
The class of structure expressions (strexp) has been extended to include functor expressions. Similar to fun declarations, functor declarations are mere derived forms. The declaration for F above is just sugar for:
structure F = fct X : S1 => fct Y : S2 => struct (* ... *) end
The keyword fct starts a functor expression very much like fn begins a function expression in the core language. Functor expressions can be arbitrarily mixed with other structure expressions. In contrast to SML, there is no distinction between structure and functor identifiers (see incompatibilities).
Note: The keyword structure and the syntactic classes strid, strexp, etc. are kept for compatibility reasons, despite module now being more appropriate.
The syntax of signatures has been extended to contain functor types. For example, functor F can be described by the following signature:
structure F : fct X : S1 -> fct Y : S2 -> sig (* ... *) end
If the parameters X and Y do not appear in S2 or the rest of the signature, it may be abbreviated as follows:
structure F : S1 -> S2 -> sig (* ... *) end
As another derived form, the following SML/NJ compatible syntax is provided for functor descriptions in signatures:
functor F (X : S1) (Y : S2) : sig (* ... *) end
Like structures and functors, signatures can also be declared anywhere. In particular, this allows signatures inside structures, and consequently, nested signatures:
signature S = sig signature T = sig (* ... *) end end structure X :> S = struct signature T = sig (* ... *) end end
Nested signatures must always be matched exactly. More precisely, a signature S1 with nested T = T1 matches another signature S2 with nested T = T2 only if T1 matches T2, and also T2 matches T1.
Analogously to types, nested signatures may be specified abstractly:
signature S = sig signature T structure X : T end
An abstract signature can be matched by any signature. Abstract signatures are particularly useful as functor parameters, because they allow declaring "polymorphic" functors:
functor F (signature S structure X : S) = (* ... *)
The Alice library contains several examples of such polymorphic functors, for example the functors provided by the component manager.
Note: Abstract signatures render the type system of Alice ML undecidable. We do not consider this a problem in practice, since already the simplest program to make the type checker loop is highly artificial:
signature I = sig signature A functor F(X : sig signature A = A functor F(X : A) : sig end end) : sig end end signature J = sig signature A = I functor F(X : I) : sig end end (* Try to check J ≤ I *) functor Loop(X : J) = X : I
Currently, the Alice compiler has no upper limit on the number of substitutions it does during signature matching, so this example will actually make it loop.
Modules can be wrapped up and passed as first-class values as so-called packages. The signature of packed modules can be dynamically checked against given signatures. See the section on packages for a description of the package mechanism.
Like core expressions, modules can be evaluated concurrently or lazily. Like in the core language, this can be accomplished by just preceding a module expression with the respective keyword:
lazy strexp spawn strexp
Evaluating these expressions returns a lazy or concurrent module future, respectively.
Note however that the most frequent cause of lazy module evaluation is the component system. Every structure that is imported from another component is evaluated lazily.
Derived forms analogous to the core language are provided for defining functors that should evaluate lazily or in a separate threads:
functor lazy F (X : S) = strexp functor spawn G (X : S) = strexp
An application of F is evaluated lazily, while G will spawn a new thread for evaluation. See below for a precise definition of this derived form.
Long identifiers have lazy semantics: accessing a structure X via the dot notation X.l does not request X. The structure is only requested if X.l itself is requested.
Signatures may contain fixity specifications:
signature S = sig type t infix 2 ++ val x : t val op++ : t * t -> t end
To match a signature with infix specifications, a structure must provide the same infix status directives. The infix environment is part of a structure's principal signature.
Opening a structure with infix specifications pulls in the according infix status into the local environment:
structure M :> S = struct (* ... *) end open M val z = x ++ x
Some modules of the Alice library, e.g., in the constraint modeling structure, define infix operators that can be used conveniently this way.
Note: This feature produces a syntactic incompatibility with SML showing up in some rare cases.
Parentheses may be used freely in module and signature expressions:
structure X :> (S' where type t = int) = (F (A))
Parentheses may be dropped from functor arguments:
structure Y = F A B
The derived form for functor arguments, allowing a list dec of declarations being given instead of a structure expression, has been generalized: in a structure expression, parentheses may either enclose another strexp, or a dec. For example,
structure Z = (type t = int val x = 9)
Analogously, in a signature expression, parentheses may enclose either another sigexp, or a spec:
signature S = (type t val x : int)
Functor parameters may also be specified as a spec, generalising the SML functor binding derived forms:
structure F = fct (type t val x : int) => struct val x = x end
Structure bindings may contain a wildcard instead of a structure identifier:
structure _ = Pickle.SaveVal (type t = int val x = 43)
In this example, the functor application is performed solely for its side effect, and does not return any interesting result.
Similarly, wildcards are allowed for functor parameters:
functor F (_ : S) = struct (* don't actually need argument *) end
They are also allowed in signatures:
signature FF = fct (_ : A) -> B
However, the same signature may be written more compactly as:
signature FF = A -> B
As in the core language, datatype specifications may be made recursive with type declarations using the withtype keyword:
signature S = sig datatype 'a tree = TREE of 'a forest withtype 'a forest = 'a tree list end
The keyword op is allowed in value, constructor and exception specifications:
signature S = sig datatype 'a list = nil | op:: of 'a * 'a list val op+ : int * int -> int exception op!!! end
The syntax for modules very much resembles the syntax of core language expressions. Derived forms have been marked with (*).
|pack atstrexp : atsigexp||transparent packing|
|pack atstrexp :> atsigexp||opaque packing (*)|
|atstrexp||::=||struct dec end||structure|
|let dec in strexp end||local declarations|
|( strexp )||parentheses|
|( dec )||structure (*)|
|appstrexp atstrexp||functor application|
|strexp : sigexp||transparent constraint|
|strexp :> sigexp||opaque constraint|
|unpack infexp : sigexp||unpacking|
|fct strpat => strexp||functor|
|atstrpat||::=||( strid : sigexp )||parameter (*)|
|( _ : sigexp )||anonymous parameter (*)|
|( spec )||signature as parameter (*)|
|strid : sigexp||parameter|
|_ : sigexp||anonymous parameter (*)|
|atsigexp||::=||sig spec end||ground signature|
|( sigexp )||parentheses|
|( spec )||signature (*)|
|sigexp where rea||specialization|
|fct strpat -> sigexp||functor|
|atsigexp -> sigexp||non-dependent functor (*)|
|rea||::=||type tyvarseq longtycon = ty|
|functor fundesc||functor specification (*)|
|signature sigdesc||signature specification|
|fundesc||::=||strid atstrpat1 ... atstrpatn : sigexp <and fundesc>||functor description (n≥1) (*)|
|sigdesc||::=||sigid <= sigexp> <and sigdesc>||signature description|
|open strexp||generalised open1 (*)|
|_ <: sigexp> = strexp <and strbind>||anonymous structure (*)|
|funbind||::=||<lazy | spawn> strid atstrpat1 ... atstrpatn < : | :> sigexp> = strexp <and funbind>||functor binding (n≥1) (*)|
1) The structure expression strexp does not have the form longstrid1 ... longstridn.
|( dec )||struct dec end|
|_ : sigexp||strid : sigexp 1|
|( strid : sigexp )||strid : sigexp|
|( _ : sigexp )||strid : sigexp 1|
|( spec )||strid : ( spec ) 12|
|open strexp||local structure strid = strexp in open strid end 1|
|functor funbind||structure funbind|
|<lazy|spawn> strid atstrpat1 ... atstrpatn < : | :> sigexp> = strexp <and funbind>||strid = fct atstrpat1 => ... fct atstrpatn => <lazy|spawn> strexp < : | :> sigexp> <and funbind>|
|_ <: sigexp> = strexp <and strbind>||strid <: sigexp> = strexp <and strbind> 1|
|( spec )||sig spec end|
|atsigexp -> sigexp||fct strid : atsigexp -> sigexp 1|
|functor fundesc||structure fundesc|
|strid atstrpat1 ... atstrpatn: sigexp <and fundesc>||strid :fct atstrpat1 -> ... fct atstrpatn -> sigexp <and fundesc>|
1) The identifier strid is new.
2) If the strpat occurs in a functor expression fct strpat => strexp, then strexp is rewritten to strexp' by replacing any occurrence of an identifier x bound in spec to strid.x. Likewise, if it occurs in a functor signature fct strpat -> sigexp, then sigexp is rewritten to sigexp' by similar substitution.