// Evaluate the ast in the supplied environment. This is a bit of a monster but
// has been left as per the guide to simplify Tail Call Optimisation (TCO)
static MalVal EVAL(MalVal InitialAst, Env IntialEnv)
{
// The ast and env to be EVAL'd. Initially set to those passed to EVAL but
// may be updated if there is a TCO loop.
MalVal ast = InitialAst;
Env env = IntialEnv;
// Some eval cases will return out of the function, others keep looping (as per TCO) with
// an updated ast / Env. The alternative is to call EVAL recursively, which has simpler
// logic but which can blow up the stack.
while (true)
{
if (ast is MalList astList)
{
// Console.WriteLine("EVAL: ast is " + astList.ToString(true));
if (astList.Count() <= 0)
{
// Empty list, return unchanged.
return(ast);
}
else
{
// I used to check astList[0] was a symbol but this dissallows forms whose first el is an anon (fn..) special form.
string symbolZero = astList[0] is MalSym ? ((MalSym)astList[0]).getName() : "__<*fn*>__";
switch (symbolZero)
{
case "def":
// Evaluate all elements of list using eval_ast, retun the final eval'd element.
// Should be something like (def a b). Set() symbol a to be the result of evaluating b.
if (astList.Count() != 3 || !(astList[1] is MalSym symbol))
{
throw new MalEvalError("'def' should be followed by a symbol and a value");
}
// Must add the result of the EVAL to the environment so can't TCO loop here.
MalVal result = EVAL(astList[2], env);
env.Set(symbol, result);
return(result);
case "let":
// Let syntax is (let <bindings-list> <result>), e.g. (let (p ( + 2 3) q (+ 2 p)) (+ p q)).
// Bindings can refer to earlier bindings in the new let environment or to symbols
// in the outer environment. Let defs hide hide same-named symbols in the outer scope.
if (astList.Count() != 3)
{
throw new MalEvalError("'let' should have two arguments (bindings-list and result), but instead had " + (astList.Count() - 1));
}
// Extract the first parameter - the bindings list. E.g. (p (+ 2 3) q (+ 2 p))
if (!(astList[1] is MalList bindingsList))
{
throw new MalEvalError("'let' should be followed by a non-empty bindings list and a result form");
}
if (bindingsList.Count() <= 1 || bindingsList.Count() % 2 != 0)
{
throw new MalEvalError("'let' bindings list should have an even number of entries");
}
// Create a new Env - the scope of the Let form. It's discarded when done.
Env TempLetEnv = new Env(env);
// Process each pair of entries in the bindings list.
for (int i = 0; i < bindingsList.Count(); i += 2)
{
// The first element should be a 'key' symbol. E.g. 'p'.
if (!(bindingsList[i] is MalSym bindingKey))
{
throw new MalEvalError("'let' expected symbol but got: '" + bindingsList[i].ToString(true) + "'");
}
// The second element (e.g. (+ 2 3)) is the value of the key symbol in Let's environment
MalVal val = EVAL(bindingsList[i + 1], TempLetEnv);
// Store the new value in the environment.
TempLetEnv.Set(bindingKey, val);
}
// TCO loop instead of EVAL(astList[2], TempLetEnv)
ast = astList[2];
env = TempLetEnv;
continue;
case "do":
// Something like (do <form> <form>)
MalList formsToDo = astList.Rest();
int formsToDoCount = formsToDo.Count();
if (formsToDoCount == 0)
{
// Empty (do )
return(malNil);
}
else
{
// EVAL all forms in the (do ) body.
// up to but not including the last one.
if (formsToDoCount > 1)
{
eval_ast(formsToDo.GetRange(0, formsToDoCount - 2), env);
}
// EVAL the last form in the (do ) body via TCO loop, keeping the same env.
ast = formsToDo[formsToDoCount - 1];
// TCO loop.
continue;
}
case "if":
// If has the syntax (if <cond> <true-branch> <optional-false-branch>)
if (astList.Count() < 3 || astList.Count() > 4)
{
throw new MalEvalError("'if' should have a condition, true branch and optional false branch");
}
// Evaluate the Cond part of the if.
MalVal cond = EVAL(astList[1], env);
if (cond is MalNil || cond is MalFalse)
{
// Cond is nil or false.
if (astList.Count() == 4)
{
// Eval the 'false' branch via TCO loop.
ast = astList[3];
continue;
}
else
{
// No 'false' branch.
return(malNil);
}
}
else
{
// Eval the 'true' branch via TCO loop.
ast = astList[2];
continue;
}
case "fn":
// e.g. (def a (fn (i) (* i 2))) or (def b (fn () (prn 1) (prn 2)))
if (astList.Count() < 3)
{
throw new MalEvalError("fn must have an arg list and at least one body form");
}
if (!(astList[1] is MalList FnArgList))
{
// The 2nd element must be an arg list (possibly empty).
throw new MalEvalError("Expected arg list but got: '" + astList[1].ToString(true) + "'");
}
MalVal FnBodyExprs;
// Unlike the Mal reference, this handles fns that have multiple expr forms.
if (astList.Count() == 3)
{
FnBodyExprs = astList[2];
}
else
{
// Either I've misunderstood something or the Reference doesn't handle some functions correctly.
FnBodyExprs = astList.GetRange(2, astList.Count() - 1);
}
Env cur_env = env;
// This is different from C#-Mal but their version doesn't work.
return(new MalFunc(FnBodyExprs, env, FnArgList, args => EVAL(FnBodyExprs, new Env(cur_env, FnArgList, args))));
default:
// Ast isn't a special form, so try to evaluate it as a function.
MalList evaledList = (MalList)eval_ast(ast, env);
MalVal listFirst = evaledList.First();
if (listFirst is MalFunc func)
{
if (func.IsCore)
{
// Can't TCO a core function so directly apply func.
return(func.Apply(evaledList.Rest()));
}
else
{
// Non-ore function.
// The ast is the one stored by the func.
ast = func.ast;
// Create a new env using func's env and params as the outer and binds arguments
// and the rest of the current ast as the exprs argument.
env = new Env(func.env, func.fparams, evaledList.Rest());
// and now 'apply' via the TCO loop.
continue;
}
}
else
{
return(listFirst);
}
}
}
}
else
{
// If ast is not a list (e.g. a vector), return the result of calling eval_ast on it.
return(eval_ast(ast, env));
}
}
}
// EVALuate the ast in the supplied env. EVAL directly handles special forms
// and functions (core or otherwise) and passes symbols and sequences to its helper
// function eval_ast (with which it is mutually recursive).
// Conceptually, EVAL invokes itself recursively to evaluate some forms, sometimes in their
// own new environment (e.g. (let)). In this implementation, Tail Call Optimisation (TCO)
// is used to avoid recursion if possible. Specifically, the TCO version EVAL has ast and env
// variables. It sets these to those passed in by the caller and then starts a while(true)
// loop. If the last act of a pass through EVAL would have been to invoke EVAL recursively and
// return that value, the TCO version sets the ast and env to whatever would have been
// passed recursively, and goes back to the beginning of its loop, thus replacing
// recursion with (stack efficient) iteration. Recursion is still used in cases where
// the recursive call isn't the last thing EVAL does before returning the value, e.g. in
// (def ) clause where the returned value must be stored in the env.
// The downside of TCO is the increased complexity of the EVAL function itself, which
// is now harder to refactor into helper-functions for the various special forms.
static MalVal EVAL(MalVal InitialAst, Env IntialEnv)
{
// The ast and env to be EVAL'd (and which might be reset in a TCO loop).
MalVal ast = InitialAst;
Env env = IntialEnv;
// The TCO loop.
while (true)
{
if (ast is MalList astList)
{
// Console.WriteLine("EVAL: ast is " + astList.ToString(true));
if (astList.Count() <= 0)
{
// Empty list, return unchanged.
return(ast);
}
else
{
// See if we have a special form or something else.
string astListHead = astList[0] is MalSym ? ((MalSym)astList[0]).getName() : "__<*fn*>__";
switch (astListHead)
{
// Look for cases where the head of the ast list is a symbol (e.g. let, do, if) associated
// with a special form. If it is, we can discard th head symbol, and the remaining elements of the
// astList are the forms in the body of the special form. What happens to these depends on the form
// that has been encountered,
case "def":
// Should be something like (def a b). Set() symbol a to be the result of evaluating b
// and store the result in the env.
// Evaluate all elements of list using eval_ast, retun the final eval'd element.
if (astList.Count() != 3 || !(astList[1] is MalSym symbol))
{
throw new MalEvalError("'def' should be followed by a symbol and a value");
}
// Can't TCO loop because we must store the result of the EVAL before returning it.
MalVal result = EVAL(astList[2], env);
env.Set(symbol, result);
return(result);
case "let":
// Let syntax is (let <bindings-list> <result>), e.g. (let (p ( + 2 3) q (+ 2 p)) (+ p q)).
// Bindings can refer to earlier bindings in the new let environment or to symbols
// in the outer environment. Let defs can hide same-named symbols in the outer scope.
if (astList.Count() != 3)
{
throw new MalEvalError("'let' should have two arguments (bindings-list and result), but instead had " + (astList.Count() - 1));
}
// Extract the first parameter - the bindings list. E.g. (p (+ 2 3) q (+ 2 p))
if (!(astList[1] is MalList bindingsList))
{
throw new MalEvalError("'let' should be followed by a non-empty bindings list and a result form");
}
if (bindingsList.Count() <= 1 || bindingsList.Count() % 2 != 0)
{
throw new MalEvalError("'let' bindings list should have an even number of entries");
}
// Create a new Env - the scope of the Let form. It's discarded when done.
Env TempLetEnv = new Env(env);
// Process each pair of entries in the bindings list.
for (int i = 0; i < bindingsList.Count(); i += 2)
{
// The first element should be a 'key' symbol. E.g. 'p'.
if (!(bindingsList[i] is MalSym bindingKey))
{
throw new MalEvalError("'let' expected symbol but got: '" + bindingsList[i].ToString(true) + "'");
}
// The second element (e.g. (+ 2 3)) is the value of the key symbol in Let's environment
MalVal val = EVAL(bindingsList[i + 1], TempLetEnv);
// Store the new value in the environment.
TempLetEnv.Set(bindingKey, val);
}
// TCO loop instead of EVAL(astList[2], TempLetEnv)
ast = astList[2];
env = TempLetEnv;
continue;
case "do":
// Something like (do <form> <form>). EVAL all forms in the (do ) body in current environment.
// Some vars that make meaning clearer.
MalList formsInDoBody = astList.Rest();
int formsInDoBodyCount = formsInDoBody.Count();
if (formsInDoBodyCount == 0)
{
// Empty (do )
return(malNil);
}
else
{
// Use eval_ast to evaluate the DoBody forms up to but NOT including the last one.
if (formsInDoBodyCount > 1)
{
// If there is just one DoBody form, this won't get called.
eval_ast(formsInDoBody.GetRange(0, formsInDoBodyCount - 2), env);
}
// Prep for the TCO by using the last DoBody form as the new ast.
ast = formsInDoBody[formsInDoBodyCount - 1];
// TCO loop to process the final DoBody form.
continue;
}
case "if":
// If has the syntax (if <cond> <true-branch> <optional-false-branch>)
if (astList.Count() < 3 || astList.Count() > 4)
{
throw new MalEvalError("'if' should have a condition, true branch and optional false branch");
}
// Evaluate the Cond part of the if.
MalVal cond = EVAL(astList[1], env);
if (cond is MalNil || cond is MalFalse)
{
// Cond is nil or false.
if (astList.Count() == 4)
{
// Eval the 'false' branch via TCO loop.
ast = astList[3];
continue;
}
else
{
// No 'false' branch.
return(malNil);
}
}
else
{
// Eval the 'true' branch via TCO loop.
ast = astList[2];
continue;
}
case "fn":
// e.g. (def a (fn (i) (* i 2))) or (def b (fn () (prn 1) (prn 2)))
if (astList.Count() < 3)
{
throw new MalEvalError("fn must have an arg list and at least one body form");
}
if (!(astList[1] is MalList FnParamsList))
{
// The 2nd element must be an arg list (possibly empty).
throw new MalEvalError("Expected arg list but got: '" + astList[1].ToString(true) + "'");
}
MalVal FnBodyForms;
// Make a list of the forms in the funcion body (following the fn name and paramd list).
if (astList.Count() == 3)
{
FnBodyForms = astList[2];
}
else
{
// Either I've misunderstood something or the Reference doesn't handle some functions correctly.
FnBodyForms = astList.GetRange(2, astList.Count() - 1);
}
Env cur_env = env;
// This is different from C#-Mal but their version doesn't compile. Perhaps it's a C# version thing?
return(new MalFunc(FnBodyForms, env, FnParamsList, args => EVAL(FnBodyForms, new Env(cur_env, FnParamsList, args))));
case "quote":
if (astList.Count() == 2)
{
// Return the quoted thing.
return(astList[1]);
}
throw new MalEvalError("quote expected 1 arg but had " + (astList.Count() - 1).ToString());
case "quasiquote":
if (astList.Count() == 2)
{
// Return the result of processing the quasiquote.
ast = ProcessQuasiquote(astList[1]);
continue;
}
throw new MalEvalError("quasiquotequote expected 1 arg but had " + (astList.Count() - 1).ToString());
default:
// Ast isn't a special form (it mey be a symbol or a function).
// eval_ast the ast
MalList evaledList = (MalList)eval_ast(ast, env);
// Look at the head of the result.
MalVal listFirst = evaledList.First();
if (listFirst is MalFunc func)
{
// It's a mal func.
if (func.IsCore)
{
// Core funcs can be immediately applied and the value returned.
return(func.Apply(evaledList.Rest()));
}
else
{
// Non-core function, i.e. one created by (fn...).
// Use the Fn's ast and env info to set up a new EVAL (via TCO).
// The ast to EVAL is the func's stored ast.
ast = func.ast;
// The new EVAL Env is made as follows...
// The outer env is the func's env
// The 'binds' are the func's params
// The exprs are are the rest of the current ast.
env = new Env(func.env, func.fparams, evaledList.Rest());
// Apply = EVAL the function indirectly via the TCO loop.
continue;
}
}
else
{
// This is typically the value of a symbol looked up by eval_ast.
return(listFirst);
}
}
}
}
else
{
// If ast is not a list (e.g. a vector), return the result of calling eval_ast on it.
return(eval_ast(ast, env));
}
}
}
