nomsu/core/control_flow.nom

#!/usr/bin/env nomsu -V6.13.12.8
#
    This file contains compile-time actions that define basic control flow structures
    like "if" statements and loops.
    
use "core/metaprogramming.nom"
use "core/operators.nom"

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# No-Op
test:
    do nothing
(do nothing) compiles to ""

# Conditionals
test:
    if (no):
        barf "conditional fail"

(if $condition $if_body) compiles to ("
    if \($condition as lua expr) then
        \($if_body as lua)
    end
")

test:
    unless (yes):
        barf "conditional fail"

(unless $condition $unless_body) parses as (if (not $condition) $unless_body)
[
    if $condition $if_body else $else_body, unless $condition $else_body else $if_body
] all compile to ("
    if \($condition as lua expr) then
        \($if_body as lua)
    else
        \($else_body as lua)
    end
")

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# Conditional expression (ternary operator)
#   Note: this uses a function instead of "(condition and if_expr or else_expr)"
    because that breaks if $if_expr is falsey, e.g. "x < 5 and false or 99"
test:
    assume ((1 if (yes) else 2) == 1)
    assume ((1 if (no) else 2) == 2)

[
    $when_true_expr if $condition else $when_false_expr
    $when_true_expr if $condition otherwise $when_false_expr
    $when_false_expr unless $condition else $when_true_expr
    $when_false_expr unless $condition then $when_true_expr
] all compile to:
    #   If $when_true_expr is guaranteed to be truthy, we can use Lua's idiomatic
        equivalent of a conditional expression: (cond and if_true or if_false)
    if {.Text, .List, .Dict, .Number}.($when_true_expr.type):
        return
            Lua ("
                (\($condition as lua expr) and \($when_true_expr as lua expr) or \
                ..\($when_false_expr as lua expr))
            ")
    ..else:
        #   Otherwise, need to do an anonymous inline function (yuck, too bad lua 
            doesn't have a proper ternary operator!)
            To see why this is necessary consider: (random()<.5 and false or 99)
        return
            Lua ("
                ((function()
                    if \($condition as lua expr) then
                        return \($when_true_expr as lua expr)
                    else
                        return \($when_false_expr as lua expr)
                    end
                end)())
            ")

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# GOTOs
test:
    $i = 0
    --- $loop ---
    $i += 1
    unless ($i == 10):
        go to $loop
    assume ($i == 10)
    --- (Loop) ---
    $i -= 1
    unless ($i == 0):
        go to (Loop)
    assume ($i == 0)

(--- $label ---) compiles to ("
    ::label_\(
        ($label.stub, as lua id) if ($label.type == "Action") else
            $label as lua identifier
    )::
")

(go to $label) compiles to ("
    goto label_\(
        ($label.stub, as lua id) if ($label.type == "Action") else
            $label as lua identifier
    )
")

# Basic loop control
(stop $var) compiles to:
    if $var:
        return (Lua "goto stop_\($var as lua identifier)")
    ..else:
        return (Lua "break")

(do next $var) compiles to:
    if $var:
        return (Lua "goto continue_\($var as lua identifier)")
    ..else:
        return (Lua "goto continue")

(---stop $var ---) compiles to "::stop_\($var as lua identifier)::"
(---next $var ---) compiles to "::continue_\($var as lua identifier)::"

# While loops
test:
    $x = 0
    repeat while ($x < 10): $x += 1
    assume ($x == 10)
    repeat while ($x < 20): stop
    assume ($x == 10)
    repeat while ($x < 20):
        $x += 1
        if (yes):
            do next
        barf "Failed to 'do next'"
    assume ($x == 20)

(repeat while $condition $body) compiles to:
    $lua =
        Lua ("
            while \($condition as lua expr) do
                \($body as lua)
        ")
    
    if ($body has subtree \(do next)):
        $lua, add "\n    ::continue::"
    
    $lua, add "\nend --while-loop"
    return $lua

(repeat $body) parses as (repeat while (yes) $body)
(repeat until $condition $body) parses as (repeat while (not $condition) $body)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

test:
    $nums = []
    for $x in 1 to 5:
        $nums, add $x
    assume ($nums == [1, 2, 3, 4, 5])
    $nums = []
    for $x in 1 to 5 via 2:
        $nums, add $x
    assume ($nums == [1, 3, 5])
    $nums = []
    for $outer in 1 to 100:
        for $inner in $outer to ($outer + 2):
            if ($inner == 2):
                $nums, add -2
                do next $inner
            $nums, add $inner
            if ($inner == 5):
                stop $outer
    assume ($nums == [1, -2, 3, -2, 3, 4, 3, 4, 5])

# Numeric range for loops
[
    for $var in $start to $stop by $step $body
    for $var in $start to $stop via $step $body
] all compile to:
    # This uses Lua's approach of only allowing loop-scoped variables in a loop
    $lua =
        Lua ("
            for \($var as lua identifier)=\($start as lua expr),\($stop as lua expr),\
            ..\($step as lua expr) do
        ")
    $lua, add "\n    " ($body as lua)
    if ($body has subtree \(do next)):
        $lua, add "\n    ::continue::"
    
    if ($body has subtree \(do next $var)):
        $lua, add "\n    " (\(---next $var ---) as lua)
    
    $lua, add "\nend -- numeric for " ($var as lua identifier) " loop"
    if ($body has subtree \(stop $var)):
        $lua =
            Lua ("
                do -- scope for (stop \($var as lua identifier))
                    \$lua
                    \(\(---stop $var ---) as lua)
                end -- scope for (stop \($var as lua identifier))
            ")
    return $lua

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(for $var in $start to $stop $body) parses as
    for $var in $start to $stop via 1 $body

test:
    $x = 0
    repeat 5 times:
        $x += 1
    assume $x == 5

(repeat $n times $body) parses as (for (=lua "_XXX_") in 1 to $n $body)
test:
    $a = [10, 20, 30, 40, 50]
    $b = []
    for $x in $a:
        $b, add $x
    assume ($a == $b)
    $b = []
    for $x in $a:
        if ($x == 10):
            do next $x
        
        if ($x == 50):
            stop $x
        
        $b, add $x
    assume ($b == [20, 30, 40])

# For-each loop (lua's "ipairs()")
(for $var in $iterable at $i $body) compiles to:
    # This uses Lua's approach of only allowing loop-scoped variables in a loop
    $lua =
        Lua ("
            for \($i as lua identifier),\($var as lua identifier) in ipairs(\($iterable as lua expr)) do
                \;
        ")
    $lua, add ($body as lua)
    if ($body has subtree \(do next)):
        $lua, add "\n    ::continue::"
    
    if ($body has subtree \(do next $var)):
        $lua, add "\n    " (\(---next $var ---) as lua)
    
    $lua, add "\nend --for \($var as lua identifier) loop"
    if ($body has subtree \(stop $var)):
        $inner_lua = $lua
        $lua = (Lua "do -- scope for stopping for-loop\n    ")
        $lua, add $inner_lua "\n    "
        $lua, add (\(---stop $var ---) as lua)
        $lua, add "\nend -- end of scope for stopping for-loop"
    return $lua

(for $var in $iterable $body) parses as
    for $var in $iterable at (=lua "__") $body

test:
    $d = {.a = 10, .b = 20, .c = 30, .d = 40, .e = 50}
    $result = []
    for $k = $v in $d:
        if ($k == "a"):
            do next $k
        
        if ($v == 20):
            do next $v
        
        $result, add "\$k = \$v"
    assume (($result sorted) == ["c = 30", "d = 40", "e = 50"])

# Dict iteration (lua's "pairs()")
[for $key = $value in $iterable $body, for $key $value in $iterable $body]
..all compile to:
    $lua =
        Lua ("
            for \($key as lua identifier),\($value as lua identifier) in pairs(\
            ..\($iterable as lua expr)) do
        ")
    $lua, add "\n    " ($body as lua)
    if ($body has subtree \(do next)):
        $lua, add "\n    ::continue::"
    
    if ($body has subtree \(do next $key)):
        $lua, add "\n    " (\(---next $key ---) as lua)
    
    if ($body has subtree \(do next $value)):
        $lua, add "\n    " (\(---next $value ---) as lua)
    
    $lua, add "\nend --foreach-loop"
    $stop_labels = (Lua "")
    if ($body has subtree \(stop $key)):
        $stop_labels, add "\n" (\(---stop $key ---) as lua)
    
    if ($body has subtree \(stop $value)):
        $stop_labels, add "\n" (\(---stop $value ---) as lua)
    
    if ((size of "\$stop_labels") > 0):
        $inner_lua = $lua
        $lua = (Lua "do -- scope for stopping for $ = $ loop\n    ")
        $lua, add $inner_lua $stop_labels "\nend"
    
    return $lua

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

test:
    when:
        (1 == 2) (100 < 0):
            barf "bad conditional"
        (1 == 0) (1 == 1) $not_a_variable.x: do nothing
        (1 == 1):
            barf "bad conditional"
        
        (1 == 2):
            barf "bad conditional"
        
        else:
            barf "bad conditional"

# Multi-branch conditional (if..elseif..else)
(when $body) compiles to:
    $code = (Lua "")
    $clause = "if"
    $else_allowed = (yes)
    unless ($body.type is "Block"):
        compile error at $body "'if' expected a Block, but got a \($body.type)."
            "Perhaps you forgot to put a ':' after 'if'?"
    
    for $line in $body:
        unless
            (($line.type is "Action") and ((size of $line) >= 2)) and
                $line.(size of $line) is "Block" syntax tree
        ..:
            compile error at $line "Invalid line for the body of an 'if' block." ("
                Each line should contain one or more conditional expressions followed by a block, or "else"\
                .. followed by a block.
            ")
        $action = $line.(size of $line)
        if (($line.1 is "else") and ((size of $line) == 2)):
            unless $else_allowed:
                compile error at $line "You can't have two 'else' blocks."
                    "Merge all of the 'else' blocks together."
            
            unless ((size of "\$code") > 0):
                compile error at $line
                .."You can't have an 'else' block without a preceding condition" ("
                    If you want the code in this block to always execute, you don't need a conditional block \
                    ..around it. Otherwise, make sure the 'else' block comes last.
                ")
            
            $code, add "\nelse\n    " ($action as lua)
            $else_allowed = (no)
        ..else:
            $code, add $clause " "
            for $i in 1 to ((size of $line) - 1):
                if ($i > 1):
                    $code, add " or "
                $code, add ($line.$i as lua expr)
            $code, add " then\n    " ($action as lua)
            $clause = "\nelseif"
    
    if ((size of "\$code") == 0):
        compile error at $body "'if' block has an empty body."
            "This means nothing would happen, so the 'if' block should be deleted."
    
    $code, add "\nend --when"
    return $code

test:
    if 5 is:
        1 2 3:
            barf "bad switch statement"
        
        4 5:
            do nothing
        
        5 6:
            barf "bad switch statement"
        
        else:
            barf "bad switch statement"

# Switch statement
[if $branch_value is $body, when $branch_value is $body] all compile to:
    $code = (Lua "")
    $clause = "if"
    $else_allowed = (yes)
    define mangler
    unless ($body.type is "Block"):
        compile error at $body "'if' expected a Block, but got a \($body.type)"
            "Perhaps you forgot to put a ':' after the 'is'?"
    
    for $line in $body:
        unless
            (($line.type is "Action") and ((size of $line) >= 2)) and
                $line.(size of $line) is "Block" syntax tree
        ..:
            compile error at $line "Invalid line for 'if' block." ("
                Each line should contain expressions followed by a block, or "else" followed by a block
            ")
        $action = $line.(size of $line)
        if (($line.1 is "else") and ((size of $line) == 2)):
            unless $else_allowed:
                compile error at $line "You can't have two 'else' blocks."
                    "Merge all of the 'else' blocks together."
            
            unless ((size of "\$code") > 0):
                compile error at $line
                .."You can't have an 'else' block without a preceding condition" ("
                    If you want the code in this block to always execute, you don't need a conditional block \
                    ..around it. Otherwise, make sure the 'else' block comes last.
                ")
            
            $code, add "\nelse\n    " ($action as lua)
            $else_allowed = (no)
        ..else:
            $code, add $clause " "
            for $i in 1 to ((size of $line) - 1):
                if ($i > 1):
                    $code, add " or "
                $code, add "\(mangle "branch value") == " ($line.$i as lua expr)
            $code, add " then\n    " ($action as lua)
            $clause = "\nelseif"
    
    if ((size of "\$code") == 0):
        compile error at $body "'if' block has an empty body."
            "This means nothing would happen, so the 'if' block should be deleted."
    
    $code, add "\nend --when"
    return
        Lua ("
            do --if $ is...
                local \(mangle "branch value") = \($branch_value as lua expr)
                \$code
            end -- if $ is...
        ")

# Do/finally
(do $action) compiles to ("
    do
        \($action as lua)
    end -- do
")

test:
    assume ((result of: return 99) == 99)

# Inline thunk:
(result of $body) compiles to "\(\(-> $body) as lua)()"
test:
    $t = [1, [2, [[3], 4], 5, [[[6]]]]]
    $flat = []
    for $ in recursive $t:
        if ((lua type of $) is "table"):
            for $2 in $:
                recurse $ on $2
        ..else:
            $flat, add $
    assume (sorted $flat) == [1, 2, 3, 4, 5, 6]

# Recurion control flow
(for $var in recursive $structure $body) compiles to:
    with local compile actions:
        define mangler
        (recurse $v on $x) compiles to
            Lua "table.insert(\(mangle "stack \($v.1)"), \($x as lua expr))"
        
        $lua =
            Lua ("
                do
                    local \(mangle "stack \($var.1)") = List{\($structure as lua expr)}
                    while #\(mangle "stack \($var.1)") > 0 do
                        \($var as lua expr) = table.remove(\(mangle "stack \($var.1)"), 1)
                        \($body as lua)
            ")
        
        if ($body has subtree \(do next)):
            $lua, add "\n        ::continue::"
        
        if ($body has subtree \(do next $var)):
            $lua, add "\n        \(\(---next $var ---) as lua)"
        
        $lua, add "\n    end -- Recursive loop"
        if ($body has subtree \(stop $var)):
            $lua, add "\n        \(\(---stop $var ---) as lua)"
        $lua, add "\nend -- Recursive scope"
        return $lua