studious-potato

A build tool written in Guile Scheme.

Description

Studious Potato is a scheme library that aims to simplify the task of maintaining, updating, and regenerating programs. It is inspired by the make utility in POSIX. With this library, you can write a build script in Guile Scheme.

Like POSIX make, these scripts update files that are derived from other files. The makefile script typically will describe how a file is built from shell commands, and it will describe the relationships between components to be built, so that they are built in order. In a typical script, the script will check the prerequisites to a target, and if the prerequisites are newer than the target, it will rebuild the target.

There are two types of rules that a makefile script can contain.

1. Target rules, which describe how a specific named file is to be built from prerequisites using a set of shell commands.
2. Suffix rules, which generically describe how to convert files with one filename suffix into files with another filename suffix.

The makefile script will make use of a custom variable type which can be set either in the script, by environment variables, or in command line arguments. Let's call them makevars, to reduce confusion with standard scheme variables.

Setting up the Scheme Script

To write a build script with this library, one needs to add the following boilerplate code at the top of an executable scheme script. Throughout this documentation we will presume that this scheme script is named makefile.scm; however, you may choose any name.

#!/usr/bin/env sh
exec guile -s "$0" "$@"
!#

(use-modules (studious-potato))
(setup (command-line))

This boilerplate loads the library functions and it parses the command-line arguments. The command-line arguments are the following,

makefile.scm [-efiknprs] [var=value...] [target_name...]
         -e  makevars from the environment override makefiles
             in the script.
         -f  rebuild even if the prerequisite timestamps are
             earlier than the target timestamps
         -i  ignore error codes return from system commands
         -k  continue to update other targets that don't depend
             on a target that has reported a non-ignored error
         -n  write commands to be executed, but, do not execute
             them
         -p  write debug output
         -r  do not use any built-in rules
         -s  use a terse output
         
         [var=value...]
           set the value of makevars
         [target_name...]
           Set one or more targets to be executed.  If no target
           is specified, the first target found will be executed.

Environment Variables

Certain environment variables affect the execution of the makefile script.

LANG - affects the current locale

MAKEFLAGS - This will be parsed similar to command-line arguments. If it contains the single, space-separated letters 'e', 'f', 'i', 'k', 'n', 'p', 'r', or 's', those options will be enabled as if set on the command line. If it contains strings of the form VAR=VALUE, it will set those makevars.

SHELL - The shell environment variable is always ignored.

All other environment variables, including those with null values, shall initialize makevars.

Signals

SIGHUP, SIGTERM, SIGINT, and SIGQUIT shall interrupt any processing.

Rules

The purpose of a makefile script is to run rules, which describe how programs act on prerequisites to create targets. There are two types of rules: target rules and suffix rules.

Target Rules

Target rules are defined and manipulated with the following commands.

target-rule name [prerequisites] [commands...]
: name [prerequisites] [commands ...]

target-rule (aka :) adds a target rule to the target rule list. There are 3 components

• NAME is a string that names the target. If this rule is being used to create a file, NAME is the name of the file to be output.

  NAME can also be a predicate procedure that maps string->boolean. But if NAME is a procedure, this rule cannot be used at the top-level target of a build.

• PREREQUISITES, if provided, is a list of strings or procedures of zero arguments that evaluate to strings. Each entry is the name of a target that needs to be exist before this target is attempted. It may be an empty list, indicating that there are no prerequisites.

• COMMANDS, if provided, are recipes that will be executed that are intended to cause the target to be created. The recipe can be either a string or a procedure.

If the COMMAND recipe is a string, it will be passed to the system procedure for execution by the shell. If any call to system returns a non-zero return value, processing will end. (This behavior is modified by the '-i' and '-k' command-line arguments.)

If the COMMAND recipe is a procedure, it will be executed. If it returns #f or a non-zero integer, failure is assumed. If the COMMAND recipe returns a string, the resulting string is passed to system and is process as above.

If the COMMAND recipe is a pair, and the CAR of the pair is one of 'ignore-error, 'silent, or 'always-execute, it will have the extra effect of ignoring errors, not printing the command line, or always executing even when the -n option is enabled. The CDR must be a string or procedure as above.

There are a set of helper functions and variables that can be used to construct recipes.

compose element ...
~ element ...
ignore-error-compose element ...
~- element ...
silent-compose element ...
~@ element ...
always-execute-compose element ...
~+ element ...

compose (aka ~) takes as arguments one or more elements, each of which is a string or a procedure of zero arguments that returns a string. It executes any procedure arguments and concatenates the resulting strings, appending spaces in between them.

ignore-error-compose (aka ~-) is like compose but returns a pair with the first argument of 'ignore-error. When passed as a recipe, it causes the recipe not to end execution, even if an error is signaled.

silent-compose (aka ~@) is like compose, but, it does not print the resulting string to the output port.

always-execute-compose (aka ~+) is like compose, but, it forces the line to always be executed, even if the -n option was chosen.

target-name
$@

target-name (aka $@) is a global variable. If called from within the context of a recipe, it contains as a string the name of the target. target-name is not thread safe.

newer-prerequisites
$?

newer-prerequisites (aka $?) returns the list of prerequisites that are newer than the target.

primary-prerequisite
$<

primary-prerequisite (aka $<) returns the first prerequisite.

target-basename
$*

target-basename (aka $*) returns the target with the suffix elided.

prerequisites
$^

prerequisites (aka $^) return all the prerequisites.

%target-rule-list`

%target-rule-list is list of targets rules encountered in the build script in the order in which they were listed, converted into an internal format.

Here are some example target rules that with recipes meant to be executed by system.

(: "foo.o" '("foo.c" "foo.h")
   (~ "cc -o" $@ $<))
   
(: "clean" '()
   "rm *.o"
   "rm *~")

Target rules may take advantage of makevars.

(: "foo.o" '("foo.c" "foo.h")
   (~ ($ CC) ($ CFLAGS)  "-o" $@ $<))

Target rules may also have recipes that execute scheme code

(: "clean" '()
    (lambda ()
      (delete-file "foo.o")))

Suffix Rules

Unlike target rules which are for one specific target and may have multiple prerequisites, suffix rules describe how to create a target from a single prerequisite with the assumption that they have the same basename and differ only in the filename suffixes. The are applied to implicit prerequisites to other rules, or to explicit prerequisites to other rules that have no target rules defined.

For example, one could have a suffix rule to convert a *.c file into a *.o file. The syntax for suffix rules are similar to target rules above.

suffix-rule source-suffix target-suffix [commands...]
-> source-suffix target-suffix [commands ...]

suffix-rule (aka ->) adds a suffix rule to the suffix rule list. There are 3 components

• SOURCE-SUFFIX is a string that names the filename suffix of the file used to create the target. Commonly, this string begins with a period.

  SOURCE-SUFFIX can also be a conversion procedure that takes in a target name string and converts it into a source name string.

• TARGET-SUFFIX, is a string that is the filename suffix of the file to be created. The TARGET-SUFFIX could be an empty string, indicating that the target is just the basename with no suffix.

  TARGET-SUFFIX can also be a predicate procedure that takes in a potential target name string and returns #t or #f if the target name string should be processed with this suffix rule.

• COMMANDS, if provided, are recipes that will be executed that are intended to cause the target to be created. The recipe can be either a string or a procedure.

If the COMMAND recipe is a string, it will be passed to the system procedure for execution by the shell. If any call to system returns a non-zero return value, ending processing.

If the COMMAND recipe is a procedure, it will be executed. If it returns #f or a non-zero integer, failure is assumed. If the COMMAND recipe returns a string, the resulting string is passed to system and is process as above.

%suffix-rule-list

%suffix-rule-list is list of suffix rules encountered in the build script in the order in which they were listed, converted into an internal format.

Example suffix rules are

(-> ".c" ".o"
    (~ ($ CC) ($ CFLAGS) "-c" "-o" $@ $<))
    
(-> ".sh" ""
    (~ "cp" $< $@)
    (~ "chmod a+x" $@))

makevars

Makefile scripts may take advantage of a special variable type called a makevar. In scheme terms, makevars are entries in a %makevars hash table that have special accessor syntax.

• The makevar names -- the keys -- are strings.
• The makevar values are either strings or procedures that take no arguments that return strings.

There are five ways a makevar can be initialized.

1. Set directly in the script using the ?= or := syntax.
2. Set in command-line arguments
3. Extracted from the MAKEFLAGS environment variable
4. Generated from the environment variables
5. Or be one of the predefined variables built into this library

There is a priority to makevars. The variables from category five above are set first, then four, then three, etc. Each lower category may overwrite variables set in the higher category.

This priority is modified by the -e command-line argument. If -e is set, category 1 variables do not override variables from categories 2, 3, and 4. They do override variables set in category 5.

The library provides the following procedures for makevars

lazy-assign key [val]

lazy-assign sets a entry in the makevars hash table. KEY must be a string or a thunk that evaluates to a string. Likewise VAL must be a string or a thunk that evaluates to a string.

If KEY is a thunk, it is immediately evaluated to a string to use as the key in the hash table entry.

If VAL is a thunk, it is stored as a promise to be evaluated later. The promise will be evaluated the first time this key is referenced.

If VAL is not given, the empty string will be used.

?= key [val]

This is a syntax version of lazy-assign where KEY should be a string without quotes, e.g.

    (?= foo "bar")  ==>  (lazy-assign "foo" "bar")

assign key [val]

assign is the same as lazy-assign above, except that if VAL is a thunk it is immediately evaluated to a string and that string is used as the value in the hash table entry.

:= key [val]

This is a syntax version of assign where KEY should be a string without quotes, e.g.

    (:= foo "bar")  ==>  (assign "foo" "bar")

reference key [transformer]

reference looks up KEY in the %makevar hash table. If it is found, VALUE is returned as a string. If it is not found, #f is returned.

If the value was stored using lazy-assign and is a promise, this procedure is forced to return a string. Also, the value in the hash table is updated to this string.

The optional transfomer should be a function the takes a string and returns a string. It will be applied to every space-separated word in the value.

$ key

This is a syntax version of reference, where KEY should be a string without quotes, e.g.

    ($ key) ==> (reference "key")
    
reference-func key

reference-func returns a procedure of zero arguments that will, when called, look up a key as described in reference above.

$$ key

This is a syntax version of reference-func, where KEY should be a string without quotes, e.g.

    ($$ key) ==> (reference-func "key")

%makevars

This is the hash table. You are not meant to access it directly, but, with the functions above. If you do use it directly, the VALUE is a cons where the CAR is string or promise and the CDR is private data.

%environment-overrides?

This boolean variable, when #t prevents lazy-assign and assign from overwriting a makevar set by the environment.

The build algorithm

The initial target is given on the command line. If no target was given on the command line, the first entry in the target list is used.

For each top-level target, create a n-ary tree of prerequisites. If a target doesn't have an explicit rule, but has a suffix that appears in one or more suffix rules, it searches for possible prerequisites that would fulfill a suffix rule. Continue until the tree is populated.

Then for each node, try to compute timestamps for each target, if they exist.

Mark as 'skip' each node that is a real file that is older than the parent file.

In a depth-first search, build each node unless the node target is older than the parent.

If a build recipe fails... If '-i', mark current node as 'skip', then keep going. If '-k', mark all siblings as 'skip', and mark the direct ancestors 'skip', keep going. Else, quit.

If we're not quit, once reaching the end, start with the next top-level target (which only happens is multiple targets are given in the command line).

Built-in rules and makevars

Unless the -r option was given, there are some builtin suffix rules and makevars that are present by default. These are

MAKE=make
AR=ar
ARFLAGS=-rv
YACC=yacc
YFLAGS=
LEX=lex
LFLAGS=
LDFLAGS=
CC=gcc
CFLAGS=-g -O2
FC=gfortran
FFLAGS=-g -O2

(-> ".c" ".o"
    (~ ($ CC) ($ CFLAGS) "-c" $<)))

(-> ".f90" ".o"
    (~ ($ FC) ($ FFLAGS) "-c" $<))
    
(-> ".y" ".o"
    (~ ($ YACC) ($ YFLAGS) $<)
    (~ ($ CC) ($ CFLAGS) "-c y.tab.c")
    "rm -f y.tab.c"
    (~ "mv y.tab.o" $@))

(-> ".l" ".o"
    (~ ($ LEX) ($ LFLAGS) $<)
    (~ ($ CC) ($ CFLAGS) "-c lex.yy.c")
    "rm -f lex.yy.c"
    (~ "mv lex.yy.o" $@))

(-> ".y" ".c"
    (~ ($ YACC) ($ YFLAGS) $<)
    (~ "mv y.tab.c" $@))

(-> ".l" ".c"
    (~ ($ LEX) ($ LFLAGS) $<)
    (~ "mv lex.yy.c" $@))

Debug commands

These commands modify how rules are interpreted or executed

FIXME