Morel

Standard ML interpreter, with relational extensions, implemented in Java

(Morel was known as smlj until version 0.1, and hosted at GitHub/julianhyde until version 0.2.)

Requirements

Java version 11 or higher.

Get Morel

From Maven

Get Morel from Maven Central:

<dependency>
  <groupId>net.hydromatic</groupId>
  <artifactId>morel</artifactId>
  <version>0.4.0</version>
</dependency>

Download and build

$ git clone git://github.com/hydromatic/morel.git
$ cd morel
$ ./mvnw install

On Windows, the last line is

> mvnw install

If you are using Java 8, you should add parameters -Dcheckstyle.version=9.3 -Dhsqldb.version=2.5.1.

Run the shell

$ ./morel
morel version 0.4.0 (java version "21", JLine terminal, xterm-256color)
- "Hello, world!";
val it = "Hello, world!" : string
- exit
$

Within the shell, the use function reads and evaluates source from a file:

- use "script.sml";

Documentation

• Morel language reference
• "How to" guide
• Change log
• Reading test reference logs can be instructive; try, for example, relational.sml.out

Status

Implemented:

• Literals
• Variables
• Comments ((* block *) and (*) line)
• let (expression that lets you define local variables and functions)
• val (including val rec)
• fun (declare function)
• Operators: = <> < > <= >= ~ abs + - * / div mod ^ :: o @ andalso orelse
• Type derivation
• fn, function values, and function application
• if
• case
• Primitive, list, tuple and record types
• Type variables (polymorphism) (except in let and datatype)
• Algebraic types (datatype)
• Tuples and unit, record, list, and vector values
• Patterns (destructuring) in val, case, fun and from, matching constants, wildcards, tuples, records, and lists
• Basis library types and functions:
  • Top: eqtype int, eqtype word, type real, datatype bool = false | true, app, ceil, concat, explode, floor, foldl, foldr, getOpt, hd, ignore, implode, isSome, length, map, not, null, real, round, size, str, substring, tl, trunc, use, valOf, vector
  • General: eqtype unit, exception Size, exception Subscript, datatype order (LESS, EQUAL, GREATER), op o, ignore
  • List: exception Empty, datatype 'a list = nil | :: of ('a * 'a list), null, length, @, hd, tl, last, getItem, nth, take, drop, rev, concat, revAppend, app, map, mapPartial, find, filter, partition, foldl, foldr, exists, all, tabulate, collate
  • Math: type real, acos, asin, atan, atan2, cos, cosh, e, exp, ln, log10, pi, pow, sin, sinh, sqrt, tan, tanh
  • Option: exception Option, datatype 'a option = NONE | SOME of 'a, getOpt, isSome, valOf, filter, join, app, valOf, map, mapPartial, compose, composePartial
  • Real: op *, op +, op -, op /, op <, op <=, op >, op >=, op ~, abs, ceil, checkFloat, compare, copySign, floor, fromInt, real, fromManExp, fromString, isFinite, isNan, isNormal, max, maxFinite, min, minNormalPos, minPos, negInf, posInf, precision, radix, realCeil, realFloor, realMod, realRound, realTrunc, rem, round, sameSign, sign, signBit, split, trunc, toManExp, toString, unordered
  • String: eqtype char, eqtype string, maxSize, size, sub, extract, substring, ^, concat, concatWith, str, implode, explode, map, translate, isPrefix, isSubstring, isSuffix
  • Vector: eqtype 'a vector, maxLen, fromList, tabulate, length, sub, update, concat, appi, app, mapi, map, foldli, foldri, foldl, foldr, findi, find, exists, all, collate
• Non-basis built-ins:
  • Interact: use
  • Relational: count, op elem, op notelem, exists, notExists, only, max, min, sum
  • System: env, plan, set, show, unset

Not implemented:

• type, eqtype, exception
• structure, struct, signature, sig, open
• local
• raise, handle
• while
• References, and operators ! and :=
• Operators: before
• User-defined operators (infix, infixr)
• Type annotations in expressions and patterns

Bugs:

• Prevent user from overriding built-in constants and functions: true, false, nil, ref, it, ::; they should not be reserved
• Access parameters and variables by offset into a fixed-size array; currently we address them by name, in a map that is copied far too often
• Runtime should raise Div when divide by zero
• Validator should give good user error when it cannot type an expression
• See also GitHub issues

Extensions

Morel has a few extensions to Standard ML: postfix labels, implicit labels in record expressions, and relational extensions. Postfix labels and implicit labels are intended to make relational expressions more concise and more similar to SQL but they can be used anywhere in Morel, not just in relational expressions.

Postfix labels

Morel allows '.' for field references. Thus e.deptno is equivalent to #deptno e.

(Postfix labels are implemented as syntactic sugar; both expressions become an application of label #deptno to expression e.)

Because '.' is left-associative, it is a more convenient syntax for chained references. In the standard syntax, e.address.zipcode would be written #zipcode (#address e).

Implicit labels in record expressions

In standard ML, a record expression is of the form {label1 = exp1, label2 = exp2, ...}; in Morel, you can omit label = if the expression is an identifier, label application, or field reference.

Thus

{#deptno e, e.name, d}

is short-hand for

{deptno = #deptno e, name = e.name, d = d}

In the relational extensions, group and compute expressions also use implicit labels. For instance,

from e in emps
  group e.deptno compute sum of e.salary, count

is shorthand for

from e in emps
  group deptno = e.deptno compute sum = sum of e.salary, count = count

and both expressions have type {count:int,deptno:int,sum:int} list.

Relational extensions

The from expression (and associated in, join, where, group, compute, order and yield keywords) is a language extension to support relational algebra. It iterates over a list and generates another list.

In a sense, from is syntactic sugar. For example, given emps and depts, relations defined as lists of records as follows

val emps =
  [{id = 100, name = "Fred", deptno = 10},
   {id = 101, name = "Velma", deptno = 20},
   {id = 102, name = "Shaggy", deptno = 30},
   {id = 103, name = "Scooby", deptno = 30}];
val depts =
  [{deptno = 10, name = "Sales"},
   {deptno = 20, name = "Marketing"},
   {deptno = 30, name = "Engineering"},
   {deptno = 40, name = "Support"}];

the expression

from e in emps where e.deptno = 30 yield e.id

is equivalent to standard ML

map (fn e => (#id e)) (filter (fn e => (#deptno e) = 30) emps)

with the where and yield clauses emulating the filter and map higher-order functions without the need for lambdas (fn).

Relational expressions are an experiment bringing the features of query languages such as SQL into a functional language. We believe that a little syntactic sugar, backed by a relational query planner, makes ML into a powerful and convenient tool for querying large data sets. Conversely, we want to see how SQL would look if it supported lambdas, function-values, polymorphism, pattern-matching, and removed the syntactic distinction between tables and collection-valued columns.

You can iterate over more than one collection, and therefore generate a join or a cartesian product:

from e in emps,
    d in depts
  where e.deptno = d.deptno
  yield {e.id, e.deptno, ename = e.name, dname = d.name};

As in any ML expression, you can define functions within a from expression, and those functions can operate on lists. Thus we can implement equivalents of SQL's IN and EXISTS operators:

let
  fun in_ e [] = false
    | in_ e (h :: t) = e = h orelse (in_ e t)
in
  from e in emps
    where in_ e.deptno (from d in depts
        where d.name = "Engineering"
        yield d.deptno)
    yield e.name
end;

let
  fun exists [] = false
    | exists (hd :: tl) = true
in
  from e in emps
    where exists (from d in depts
        where d.deptno = e.deptno
        andalso d.name = "Engineering")
    yield e.name
end;

In the second query, note that the sub-query inside the exists is correlated (references the e variable from the enclosing query) and skips the yield clause (because it doesn't matter which columns the sub-query returns, just whether it returns any rows).

There are now built-in operators elem and exists, so you can write

from e in emps
  where e.deptno elem (from d in depts
      where d.name = "Engineering"
      yield d.deptno)
  yield e.name;

from e in emps
  where exists (from d in depts
      where d.deptno = e.deptno
      andalso d.name = "Engineering");

More information

• License: Apache License, Version 2.0
• Author: Julian Hyde (@julianhyde)
• Blog: http://blog.hydromatic.net
• Project page: http://www.hydromatic.net/morel
• API: http://www.hydromatic.net/morel/apidocs
• Source code: https://github.com/hydromatic/morel
• Developers list: dev at calcite.apache.org (archive, subscribe)
• Issues: https://github.com/hydromatic/morel/issues
• Release notes and history