Add support for WebAssembly target (#4)

[This pull request against
MLton](MLton/mlton#550) enables compiling to
WebAssembly, so that twelf can be run from, e.g. inside a web browser.

A demo can be seen at:
https://jcreedcmu.github.io/twelf-wasm/

Test plan:
- ensure that the make target `twelf-server-mlton` still produces a
twelf-server binary
- ensure that the make target `wasi` produces
`bin/twelf.wasm`, when an appropriate version of `mlton` is installed.
Detailed instructions for building follow. Comments on the above pull
request have more details pertaining to build and install of GMP.

Checked out
https://github.com/agoode/mlton/tree/wasm2
at commit
MLton/mlton@d2b9e5d
put into
`$BUILD/dev-mlton`

and then did the following:

```
MLTON=$BUILD/mlton-wasm2-INSTALL

cd $BUILD/dev-mlton
make clean
make CC=$WASISDK/bin/clang \
     AR=$WASISDK/bin/ar \
     RANLIB=$WASISDK/bin/ranlib \
     TARGET_OS=wasi \
     TARGET_ARCH=wasm32 \
     TARGET=wasm32-unknown-wasi \
     WITH_GMP_DIR=$BUILD/gmp-wasi-INSTALL \
     PREFIX=$MLTON \
     dirs runtime install-runtime

cd $BUILD/dev-mlton
make clean
make all
make PREFIX=$MLTON install

cd $BUILD/twelf
mlton=$BUILD/mlton-wasm2-INSTALL/bin/mlton make wasi
```

Co-authored-by: Jason Reed <[email protected]>

Makefile

@@ -11,7 +11,7 @@
 polyml = poly
 smlnj = sml
 oldnj = sml
-mlton = mlton -default-ann 'nonexhaustiveMatch ignore'
+mlton ?= mlton -default-ann 'nonexhaustiveMatch ignore'
 make = make
 
 twelfdir = `pwd`
@@ -55,6 +55,14 @@ twelf-server-mlton:
 	fi;								\
 	$(mlton) -output bin/$(twelfserver) build/$${cmfileid}
 
+.PHONY: twelf-lib-mlton-wasi
+twelf-lib-mlton-wasi:
+	$(mlton) -target wasm32-unknown-wasi \
+		-format libexecutable \
+		-output bin/twelf.wasm \
+		-default-ann 'allowFFI true' \
+		build/twelf-lib-mlton-wasi.mlb
+
 .PHONY: twelf-server-smlnj
 twelf-server-smlnj:
 	$(smlnj) < build/twelf-server-smlnj.sml ;
@@ -82,6 +90,8 @@ smlnj : twelf-server-announce buildid twelf-server-smlnj twelf-emacs
 
 mlton : twelf-server-announce buildid twelf-server-mlton twelf-emacs 
 
+wasi : twelf-server-announce buildid twelf-lib-mlton-wasi
+
 .PHONY: twelf-regression check
 twelf-regression: buildid
 	$(mlton) -output bin/twelf-regression TEST/mlton-regression.mlb

build/twelf-core-mlton.mlb

@@ -343,6 +343,3 @@ $(SML_LIB)/basis/sml-nj.mlb
 ../src/frontend/unknownexn.fun
 ../src/frontend/unknownexn-smlnj.sml
 ../src/frontend/frontend.sml
-../src/server/sigint.sig
-../src/server/sigint-mlton.sml
-../src/server/server.sml

build/twelf-lib-mlton-wasi.mlb

@@ -0,0 +1,3 @@
+
+twelf-core-mlton.mlb
+twelf-wasi.sml

build/twelf-server-mlton.mlb

@@ -1,4 +1,6 @@
 
 twelf-core-mlton.mlb
+../src/server/sigint.sig
+../src/server/sigint-mlton.sml
+../src/server/server.sml
 twelf-server-mlton.sml
-

build/twelf-wasi.sml

@@ -0,0 +1,37 @@
+(*
+ *
+ * This file makes twelf into a library that can be called directly from WASI Preview 1,
+ * which specifies an API by which WebAssembly programs can interact with the outside world.
+ * It is somewhat like POSIX.
+ *
+ * https://github.com/WebAssembly/WASI/blob/main/legacy/preview1/docs.md
+ *
+ * It provides two entry points:
+ *
+ * allocate : int -> CharArray.array
+ *   expects to be called with the length of the twelf source string
+ *   we want to evaluate, so that the sml side can allocate a buffer.
+ *   The caller is then expected to write the twelf source into the
+ *   buffer returned.
+ *
+ * execute : unit -> unit
+ *   has the side effect of loading the string found in the allocated
+ *   buffer, which will result in various print statements. The caller
+ *   is expected to implement the WASI fd_write endpoint so that they
+ *   can see the output so printed.
+ *)
+
+val bref: CharArray.array option ref = ref NONE
+
+val e = _export "allocate": (int -> CharArray.array) -> unit;
+val _ = e (fn size =>
+				  let
+					 val b = CharArray.tabulate (size, fn _ => Char.chr 0)
+				  in
+					 bref := SOME b; b
+				  end)
+
+val e = _export "execute": (unit -> unit) -> unit;
+val _ = e (fn () => (case !bref of
+								 NONE => print "No input buffer allocated"
+							 | SOME b => (Twelf.loadString (CharArray.vector b); ())))

src/server/sigint-noop.sml

@@ -0,0 +1,10 @@
+structure SigINT :> SIGINT =
+struct
+
+  fun interruptLoop (loop:unit -> unit) =
+     let
+     in
+	loop ()
+     end
+
+end;