mirror of https://github.com/nealey/irc-bot
137 lines
4.1 KiB
Plaintext
137 lines
4.1 KiB
Plaintext
1. General
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Ocs is an implementation of Scheme, as defined by R5RS. It is
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written entirely in OCaml and can be trivially embedded in any
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OCaml program.
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Known deviations from R5RS:
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- transcript-on and transcript-off are currently not implemented
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- scheme-report-environment and null-environment ignore their
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argument
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Anything else that does not work as specified in R5RS is a bug.
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2. Installation
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Requirements:
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- GNU make or pmake (BSD make)
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- OCaml 3.x (versions 3.06 and newer tested)
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Type make or gmake in the src directory. This should produce the
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following:
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- A bytecode library (ocs.cma)
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- A native library (ocs.cmxa, ocs.a)
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- A stand-alone, native interpreter (ocscm)
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2.1 The 'ocscm' command
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If invoked without arguments, the interpreter will run in interactive
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mode.
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If invoked with arguments, the interpreter will read and evaluate
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the files listed as arguments and exit. The evaluation results are
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not printed.
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3. Implementation Details
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Implementing Scheme in OCaml is so straightforward that it hardly
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needs any documentation. The following mappings between languages
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are done:
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- Scheme is dynamically typed. Scheme values are represented by
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the OCaml type Ocs_types.sval.
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- In Scheme, top-level bindings are global and all variables are
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mutable. Variables references are bound through environments
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(Ocs_types.env) to global slots (Ocs_types.gvar) or frame indices
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(the actual frames are visible at evaluation-time through
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Ocs_types.thread).
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- Scheme has capturable, first-class continuations. Most of the
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evaluator is written in continuation-passing style in order to allow
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this.
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Where discussing types, the rest of this section assumes that the
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types defined in the module Ocs_types are visible.
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3.1 Evaluation
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Scheme values (S-expressions) are of the type sval.
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Before evaluation Scheme values are compiled to internal representations
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of the type code. This is done by the function
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Ocs_compile.compile : env -> sval -> code
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The env type is used during compilation for variable bindings. A
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new env is created for each new scope and frame. The base
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environment with the basic language bindings can be created using
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Ocs_top.make_env : unit -> env
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Evaluation is done by
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Ocs_eval.eval : thread -> (sval -> unit) -> code -> unit
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where the second argument is a continuation to pass the result to.
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The thread type is used during evaluation for storing the current
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frame and display for local variables, the input/output ports and
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the current dynamic extent. It does not represent a thread in the
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concurrent sense, but rather the evaluation state, and is copied and
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changed rather than modified in place. The initial thread to be
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passed to the evaluator can be created using
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Ocs_top.make_thread : unit -> thread.
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3.2 Continuations and I/O
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Any continuations captured are associated with the thread at the
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time of capture, so if a continuation is used to escape a
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with-input-from-file or with-output-to-file, the input/output port
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is restored to those of the time of capture.
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If a continuation is used to return to a with-input-from-file or
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with-output-to-file, the port is once again set to the one
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opened by the with-...-file call. However, if the thunk has
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already exited once, the port will be closed and no longer be
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valid for I/O calls.
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3.3 Numbers
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The full R5RS numeric tower is implemented, with the following
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internal representations:
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Exact numbers are
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- 31- or 63-bit integers (OCaml int)
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- Big_int objects from the Num library when unboxed integers are
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too small
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- Ratio objects from the Num library for rationals
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Inexact numbers are
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- 64-bit IEEE floats for reals (OCaml float)
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- Pairs of 64-bit IEEE floats for complex numbers (OCaml Complex.t)
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Since inexact numbers are represented internally as binary floating
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point, conversions to exact numbers are most precise for fractions of
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powers of two
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(inexact->exact 2.125) ==> 17/8
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compared to
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(inexact->exact 0.3) ==> 5404319552844595/18014398509481984
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And in fact many rationals will not satisfy
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(= (inexact->exact (exact->inexact r)) r)
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However
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(rationalize (inexact->exact 0.3) (expt 2 -54)) ==> 3/10
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