= ShowBasis

MLton has a flag, `-show-basis <file>`, that causes MLton to pretty
print to _file_ the basis defined by the input program.  For example,
if `foo.sml` contains
[source,sml]
----
fun f x = x + 1
----
then `mlton -show-basis foo.basis foo.sml` will create `foo.basis`
with the following contents.
----
val f: int -> int
----

If you only want to see the basis and do not wish to compile the
program, you can call MLton with `-stop tc`.

== Displaying signatures

When displaying signatures, MLton prefixes types defined in the
signature them with `_sig.` to distinguish them from types defined in the
environment.  For example,
[source,sml]
----
signature SIG =
   sig
      type t
      val x: t * int -> unit
   end
----
is displayed as
----
signature SIG =
   sig
      type t
      val x: _sig.t * int -> unit
   end
----

Notice that `int` occurs without the `_sig.` prefix.

MLton also uses a canonical name for each type in the signature, and
that name is used everywhere for that type, no matter what the input
signature looked like.  For example:
[source,sml]
----
signature SIG =
   sig
      type t
      type u = t
      val x: t
      val y: u
   end
----
is displayed as
----
signature SIG =
   sig
      type t
      type u = _sig.t
      val x: _sig.t
      val y: _sig.t
   end
----

Canonical names are always relative to the "top" of the signature,
even when used in nested substructures.  For example:
[source,sml]
----
signature S =
   sig
      type t
      val w: t
      structure U:
         sig
            type u
            val x: t
            val y: u
         end
      val z: U.u
   end
----
is displayed as
----
signature S =
   sig
      type t
      val w: _sig.t
      val z: _sig.U.u
      structure U:
         sig
            type u
            val x: _sig.t
            val y: _sig.U.u
         end
   end
----

== Displaying structures

When displaying structures, MLton uses signature constraints wherever
possible, combined with `where type` clauses to specify the meanings
of the types defined within the signature.  For example:
[source,sml]
----
signature SIG =
   sig
      type t
      val x: t
   end
structure S: SIG =
   struct
      type t = int
      val x = 13
   end
structure S2:> SIG = S
----
is displayed as
----
signature SIG =
   sig
      type t
      val x: _sig.t
   end
structure S: SIG
             where type t = int
structure S2: SIG
              where type t = S2.t
----
