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guide: clarify eval function and ARGV.

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Joel Martin 2015-03-15 11:20:05 -05:00
parent 10034e8272
commit 6fef8e58dd
1 changed files with 29 additions and 12 deletions
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process/guide.md
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@ -858,12 +858,18 @@ diff -urp ../process/step5_tco.txt ../process/step6_file.txt
unmarshall (extract) the string parameter to get the raw file name
string and marshall (wrap) the result back to a mal string type.
* In your main program, add a new `eval` (symbol) entry to your REPL
environment. The value of the new entry is a regular function
closure with a single argument `ast`. The closure calls the real
`EVAL` function using the `ast` as the first argument and the REPL
environment (closed over from outside) as the second argument. The
result of the `EVAL` call is returned.
* In your main program, add a new symbol "eval" to your REPL
environment. The value of this new entry is a function that takes
a single argument `ast`. The closure calls the your `EVAL` function
using the `ast` as the first argument and the REPL environment
(closed over from outside) as the second argument. The result of
the `EVAL` call is returned. This simple but powerful addition
allows your program to treat mal data as a mal program. For example,
you can now to this:
```
(def! mal-prog (list + 1 2))
(eval mal-prog)
```
* Define a `load-file` function using mal itself. In your main
program call the `rep` function with this string:
@ -889,11 +895,22 @@ make test^quux^step6
```
Congratulations, you now have a full-fledged scripting language that
can run other mal programs. However, the set of functions that are
available (from `core.qx`) is fairly limited. The bulk of the
functions you will add are described in step 9, but you will begin to
flesh them out over the next few steps to support quoting (step 7) and
macros (step 8).
can run other mal programs. The `slurp` function loads a file as
a string, the `read-string` function calls the mal reader to turn that
stirng into data, and the `eval` function takes data and evaluates it
as a normal mal program. However, it is important to note that the
`eval` function is not just for running external programs. Because mal
programs are regular mal data structures, you can dynamically generate
or manipulate those data structures before calling `eval` on them.
This isomorphisism (same shape) between data and programs is known as
"homoiconicity". Lisp languages are homoiconic and this property
distinguishes them from most other programming languages.
You mal implementation is quite powerful already but the set of
functions that are available (from `core.qx`) is fairly limited. The
bulk of the functions you will add are described in step 9 and step A,
but you will begin to flesh them out over the next few steps to
support quoting (step 7) and macros (step 8).
#### Deferrable:
@ -906,7 +923,7 @@ macros (step 8).
* Add the rest of the command line arguments to your REPL environment
so that programs that are run with `load-file` have access to their
calling environmnet. Add a new "*ARGV*" (symbol) entry to your REPL
calling environmnet. Add a new "\*ARGV\*" (symbol) entry to your REPL
environment. The value of this entry should be the rest of the
command line arguments as a mal list value.