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alphabet, typeset, declare, import, type, define, autodeclare, autoconvert, -, rewrite, modules, types, usage, info, clear - typed shell interface


load alphabet type qname...
declare name [ usage ]
undeclare name...
define name expr
import qname...
typeset qname
autoconvert srctype dsttype expr
autodeclare 0|1
- {expression}
${rewrite {expression} [ dsttype ] }
${types typeset }
${usage qname }


Alphabet is a loadable sh(1) module providing an interface to a simple, experimental, typed shell. It initially provides a small set of basic types, which can be added to by loading new typesets. Types are atomic; alphabet provides no support for higher-order types. The root typeset, named /, consists of the following kinds of value:

A simple string literal, represented by itself, or quoted according to the usual shell quoting rules.
A shell command or uninterpreted alphabet expression, represented by the syntax "{block}.
A file open for reading.
A file open for reading and writing.
The status of a completed command.

Each typeset implements a set of types, and has an associated set of modules. Initially, types may only be referred to by their qualified names, consisting of the name of the type prefixed with the name of its typeset; for instance /string for the string type, or /grid/data for a type named data in the typeset /grid. An unqualified name is the qualified name without the typeset prefix; for instance string or data.

To make a type available as its unqualified name, use the type command, which imports the type named by the qualified name qname from its parent typeset. This is a no-op if the type has previously been imported from the same typeset; an error is raised if the type has previously been imported from a different typeset.

Declare declares the module name with type usage. If name is a qualified name, the module must exist in the specified typeset and be compatible with the specified usage. If usage is not given, the module itself will be loaded and queried to find it out. If name is not qualified, the declaration is virtual: the module cannot actually be used, but is available for typechecking and expression rewriting. Declare is a no-op if the module has already been declared with a compatible usage, otherwise an error is raised. The syntax of usage is similar to the usage messages printed by normal shell commands, and defines the argument types expected by a module. For example:

declare /foo/bar '[-a] [-x string] fd string [string...] -> fd'

The above declares the module bar from typeset /foo, which takes two possible options (one of which requires a single associated argument of type string), two mandatory arguments, of type fd and string respectively, and any number of additional arguments of type string. The module returns a value of type fd.

When first loaded, alphabet is lax about declaration requirements: if a module is referred to by its qualified name, and is not currently declared, the module will automatically be declared and used as appropriate (as long as the module actually exists). Use autodeclare to change this behaviour. Autodeclare 0 turns off all automatic declaration: all modules used in an expression must have previously been declared; autodeclare 1 reverts to the original behaviour.

Once a module is declared, it may be referred to by its qualified name. Import makes the module available under its unqualified name. It is an error if a module of the same name has already been imported from a different typeset. For instance:

declare /read 'string -> fd'
import /read

This would declare a module named read from the root typeset (checking that it accepts a single string argument and returns a file), and make it available under the name read.

Undeclare removes the previously declared name. Note that an imported module has two names: its qualified name and its unqualified name.

Typeset loads the new typeset qname. Typesets are hierarchical in the same way that types and modules are: a typeset adds some types to its parent typeset, and has an associated set of modules that provide transformations between those types and between those of its parent.

Autoconvert specifies an automatic conversion between srctype and dsttype, i.e. whereever a module expects an argument of type dsttype, and the argument is actually of type srctype, the module block (see below for definition) expr (which must be compatible with type srctype->dsttype), will be invoked to convert between the two. Several conversions will be applied atop one another if necessary. It is an error if adding the auto-conversion creates an ambiguous conversion path between two types. As a convenience, expr may simply be the name of a module, in which case the expression will be rewritten as its identity module block: {(srctype); expr}.

The - command evaluates the alphabet expression, of the form:

{command arg...}

Usually, command is the name of a previously declared module, which must also have been imported if it is not a qualified name. The arguments must conform to those expected by the module. Each argument is either a literal string or shell-command, as described earlier, or a subexpression of the same form as above. All subexpressions are evaluated fully before command is invoked. The result of the outermost expression must be convertible to the type /status; the shell status of the - command will reflect this when the command has completed.

As a convenience, alphabet provides a pipe-like syntax. It will rewrite any expression of the form m1 m1args|m2 m2args as m2 {m1 m1args}m2args. This occurs before any auto-conversions have been applied.

Command may also be a module block, of the form:

{(argtype...); command arg...}

The argtype values given in the brackets specify the types of the arguments expected by the module block; these can be referred to in the arguments to command (and subexpressions thereof) with values of the form $1, $2, etc. For instance,

{(/string); echo $1} hello}

is exactly equivalent to:

{echo hello}

In a module block with no arguments, the argument declaration section may be omitted.

Define defines a new module in terms of previously declared modules. Name (which must be an unqualified name) gives the name of the module to define, and expr is a module block giving the expression to evaluate when name is used. The usage of the module is taken from the types declared in the module block; its return type is inferred from the return type of expr. All modules used in the evaluation of expr are evaluated when the definition takes place, so evaluation of name is unaffected if any modules it uses are later undeclared.

To show the complete form of an expression, after pipe transformations and auto-conversions have been applied, and module definitions expanded, use ${rewrite}, which returns the expression's canonical form without actually executing it. If dsttype is given, auto-conversions will be applied to try to convert the result of expression to dsttype. Rewrite raises an error if it finds any declarations incompatible with expression or if the final type cannot be converted successfully.

Alphabet also provides some shell operations that give information on its current state: ${modules} yields a list of all the currently declared module names (including entries for both qualified and unqualified names); ${types} yields a list of all currently available types (giving only the types in typeset if specified); and ${usage} provides usage information on module qname, which need not be declared. Additionally, info searches the module directories on all currently loaded typesets, and prints usage information for everything it can find there, along with information on all currently installed auto-conversions.

Finally, clear clears all existing declarations and definitions, and starts again with a clean slate.


load alphabet
type /string
type /fd
type /status
import /cat /filter
autoconvert string fd /read
autoconvert fd status {(fd); /print $1 1}
define wc {(fd); /filter $1 "{wc}}
- {cat /lib/polyhedra /dis/sh.dis | wc}
echo ${rewrite {cat /lib/polyhedra /dis/sh.dis | wc} status}




Alphabet expressions involving external typesets and file descriptors cannot have their I/O redirected at the shell level. Unfortunately it is not possible to provide a diagnostic when this occurs.


sh(1), alphabet(2), alphabet-main(1), alphabet-fs(1), alphabet-grid(1)

SH-ALPHABET(1 ) Rev:  Thu Feb 15 14:42:47 GMT 2007