Relation to Standard ML

Moscow ML implements the Core language of Standard ML (revised 1996), and most of the required parts of the Standard ML Basis Library. Notable changes from the language of the 1990 Definition include:

• The use of `value polymorphism' in the type checker. This means that val x = [] will elaborate, with x having type 'a list, because the expression is non-expansive, but val x = (fn y => y) [] will not, because the expression is expansive.
• New types char and word, new constants such as character #"a", decimal word 0w99, hexadecimal word 0wxF1, hexadecimal integer 0x99, and new string escapes "\a".
• Functions chr, ord, explode, and implode produce resp. consume characters, not strings.
• The arithmetic exceptions Abs, Diff, Div, Exp, Floor, Mod, Neg, Prod, Quot, and Sum have been replaced by the two exceptions Overflow and Div, meaning `result too big' and `divisor is zero'.
• The exceptions Ln and Sqrt have been replaced by a new exception Domain, meaning `argument outside domain of the function'.
• The identifier @ associates to the right.
• The resolution of mixed infix operators of the same precedence has been changed: mixed left-associative operators associate to the left, mixed right-associative operators associate to the right, and mixed left- and right-associative operators are illegal. This permits expressions such as [1] @ 2 :: [3] and 1 :: [2] @ [3].
• The math functions sin, cos, arctan, ln, exp, and sqrt are no longer available at the top-level; they are in structure Math.
• The input/output functions open, input, close, etc. are no longer available at the top-level; they are in structures TextIO and BinIO.
• There is a type 'a vector of immutable vectors of elements, vector expressions #[ , and vector patterns #[ .
• There is a type 'a array of mutable arrays.
• There is a large library of utility functions on lists, arrays, vectors, strings, substrings, paths, file systems, characters, integers, reals, binary trees, etc.
• Moscow ML supports quotations and antiquotations such as `record ^fields end`, and permits installable pretty-printers. These are non-standard facilities.

Moscow ML has a simple module system, which deviates from the Standard ML Modules language as follows:

• There are no functors (and hence no sharing constraints), and no nested structures.
• Signatures may contain type abbreviations. In a signature, no type, value, or exception constructor may be specified twice at top-level; a local specification can be used only to restrict the scope of open specifications and type abbreviations; and an open specification can appear only inside local.
• Signature constraints are opaque: only the information given in the signature U.sig can be used when compiling another unit which uses unit U.
• Each signature must reside in a separate file with suffix ` .sig', and each structure must reside in a separate file with suffix `.sml'.