nomsu/core/control_flow.nom

#
    This file contains compile-time actions that define basic control flow structures
    like "if" statements and loops.

use "core/metaprogramming.nom"
use "core/text.nom"
use "core/operators.nom"

# No-Op
immediately
    compile [do nothing] to: Lua ""

# Conditionals
immediately
    compile [if %condition %if_body] to
        Lua ".."
            if \(%condition as lua expr) then
                \(%if_body as lua statements)
            end
    parse [unless %condition %unless_body] as: if (not %condition) %unless_body

    compile [if %condition %if_body else %else_body, unless %condition %else_body else %if_body] to
        Lua ".."
            if \(%condition as lua expr) then
                \(%if_body as lua statements)
            else
                \(%else_body as lua statements)
            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"
immediately
    compile [..]
        %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
    ..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: %when_true_expr.type in {Text:yes, List:yes, Dict:yes, Number:yes}
            return
                Lua value ".."
                    (\(%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 value ".."
                    (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
immediately
    compile [=== %label ===, --- %label ---, *** %label ***] to
        Lua "::label_\(%label as lua identifier)::"
    compile [go to %label] to
        Lua "goto label_\(%label as lua identifier)"

# Basic loop control
immediately
    compile [do next] to: Lua "continue"
    compile [stop] to: Lua "break"

# Helper function
immediately
    compile [%tree has subtree %subtree where %condition] to
        Lua value ".."
            (function()
                for \(%subtree as lua expr) in coroutine.wrap(function() \(%tree as lua expr):map(coroutine.yield) end) do
                    if \(%condition as lua expr) then
                        return true
                    end
                end
                return false
            end)()

# While loops
immediately
    compile [do next repeat] to: Lua "goto continue_repeat"
    compile [stop repeating] to: Lua "goto stop_repeat"
    compile [repeat while %condition %body] to
        %lua <-
            Lua ".."
                while \(%condition as lua expr) do
                    \(%body as lua statements)
        if
            %body has subtree % where: (%.type = "Action") and (%.stub is "do next repeat")
        ..: to %lua write "\n    ::continue_repeat::"
        to %lua write "\nend --while-loop"
        if
            %body has subtree % where: (%.type = "Action") and (%.stub is "stop repeating")
        ..
            %lua <-
                Lua ".."
                    do -- scope of "stop repeating" label
                        \%lua
                        ::stop_repeat::
                    end -- end of "stop repeating" label scope
        return %lua
    parse [repeat %body] as: repeat while (yes) %body
    parse [repeat until %condition %body] as: repeat while (not %condition) %body

    compile [..]
        repeat %n times %body
    ..to
        %lua <-
            Lua ".."
                for i=1,\(%n as lua expr) do
                    \(%body as lua statements)
        if
            %body has subtree % where: (%.type = "Action") and (%.stub is "do next repeat")
        ..: to %lua write "\n    ::continue_repeat::"
        to %lua write "\nend --numeric for-loop"
        if
            %body has subtree % where: (%.type = "Action") and (%.stub is "stop repeating")
        ..
            %lua <-
                Lua ".."
                    do -- scope of "stop repeating" label
                        \%lua
                        ::stop_repeat::
                    end -- end of "stop repeating" label scope
        return %lua

# For loop control flow
immediately
    compile [stop %var] to
        Lua "goto stop_\(%var as lua identifier)"
    compile [do next %var] to
        Lua "goto continue_\(%var as lua identifier)"

# Numeric range for loops
immediately
    compile [..]
        for %var in %start to %stop by %step %body
        for %var in %start to %stop via %step %body
    ..to
        # This uses Lua's approach of only allowing loop-scoped variables in a loop
        assume (%var.type is "Var") or barf "Loop expected variable, not: \%var"
        %lua <-
            Lua ".."
                for \(%var as lua expr)=\(%start as lua expr),\(%stop as lua expr),\(%step as lua expr) do
                    \(%body as lua statements)
        if
            %body has subtree % where
                (%.type = "Action") and
                    (%.stub is "do next %") and 
                        %.3 = %var
        ..: to %lua write "\n    ::continue_\(%var as lua identifier)::"
        to %lua write "\nend --numeric for-loop"

        if
            %body has subtree % where
                (%.type = "Action") and
                    (%.stub is "stop %") and
                        %.2 = %var
        ..
            %lua <-
                Lua ".."
                    do -- scope for stopping for-loop
                        \%lua
                        ::stop_\(%var as lua identifier)::
                    end -- end of scope for stopping for-loop

        return %lua

immediately
    parse [for %var in %start to %stop %body] as: for %var in %start to %stop via 1 %body

# For-each loop (lua's "ipairs()")
immediately
    compile [for %var in %iterable %body] to
        # This uses Lua's approach of only allowing loop-scoped variables in a loop
        assume (%var.type is "Var") or barf "Loop expected variable, not: \%var"
        %lua <-
            Lua ".."
                for i,\(%var as lua identifier) in ipairs(\(%iterable as lua expr)) do
                    \(%body as lua statements)
        if
            %body has subtree % where
                (%.type = "Action") and
                    (%.stub is "do next %") and
                        %.3.value = %var.value
        ..: to %lua write (Lua "\n    ::continue_\(%var as lua identifier)::")
        to %lua write "\nend --foreach-loop"
        if
            %body has subtree % where
                (%.type = "Action") and
                    (%.stub is "stop %") and
                        %.2.value = %var.value
        ..
            %lua <-
                Lua ".."
                    do -- scope for stopping for-loop
                        \%lua
                        ::stop_\(%var as lua identifier)::
                    end -- end of scope for stopping for-loop
        return %lua

# Dict iteration (lua's "pairs()")
immediately
    compile [..]
        for %key = %value in %iterable %body
        for (%key,%value) in %iterable %body
    ..to
        # This uses Lua's approach of only allowing loop-scoped variables in a loop
        assume (%key.type is "Var") or barf "Loop expected variable, not: \%key"
        assume (%value.type is "Var") or barf "Loop expected variable, not: \%value"
        %lua <-
            Lua ".."
                for \(%key as lua identifier),\(%value as lua identifier) in pairs(\(%iterable as lua expr)) do
                    \(%body as lua statements)
        if
            %body has subtree % where
                (%.type = "Action") and
                    (%.stub is "do next %") and
                        %.3.value = %key.value
        ..: to %lua write (Lua "\n    ::continue_\(%key as lua identifier)::")

        if
            %body has subtree % where
                (%.type = "Action") and
                    (%.stub is "do next %") and
                        %.3.value = %value.value
        ..: to %lua write (Lua "\n    ::continue_\(%value as lua identifier)::")
        to %lua write "\nend --foreach-loop"
        
        %stop_labels <- (Lua "")
        if
            %body has subtree % where
                (%.type = "Action") and
                    (%.stub is "stop %") and
                        %.2.value = %key.value
        ..: to %stop_labels write "\n::stop_\(%key as lua identifier)::"

        if
            %body has subtree % where
                (%.type = "Action") and
                    (%.stub is "stop %") and
                        %.2.value = %value.value
        ..: to %stop_labels write "\n::stop_\(%value as lua identifier)::"

        if: (length of %stop_labels) > 0
            %lua <-
                Lua ".."
                    do -- scope for stopping for % = % loop
                        \%lua
                        \%stop_labels
                    end
        return %lua

# Switch statement/multi-branch if
immediately
    compile [when %body] to
        %code <- (Lua "")
        %fallthroughs <- []
        %is_first <- (yes)
        %seen_else <- (no)
        %branches <-
            %body if (%body.type = "Block") else [%body]
        for %func_call in %branches
            assume (%func_call.type is "Action") or barf ".."
                Invalid format for 'when' statement. Only '*' blocks are allowed.
            with {..}
                %star: %func_call.1
                %condition: %func_call.2
                %action: %func_call.3
            ..
                assume (%star = "*") or barf ".."
                    Invalid format for 'when' statement. Lines must begin with '*'
                assume %condition or barf ".."
                    Invalid format for 'when' statement. Lines must begin with '*' and have a condition or the word "else"
                if: %action is (nil)
                    lua> "table.insert(\%fallthroughs, \(%condition as lua expr))"
                    do next %func_call

                if: %condition = "else"
                    assume (not %is_first) or barf "'else' clause cannot be first in 'when % = ?' block"
                    to %code write "\nelse\n    "
                    to %code write: %action as lua statements
                    %seen_else <- (yes)
                ..else
                    assume (not %seen_else) or barf "'else' clause needs to be last in 'when' block"
                    lua> "table.insert(\%fallthroughs, \(%condition as lua expr))"
                    to %code write "\("if" if %is_first else "\nelseif") "
                    for %i = %condition in %fallthroughs
                        if (%i > 1): to %code write " or "
                        to %code write %condition
                    to %code write " then\n    "
                    to %code write (%action as lua statements)

                %fallthroughs <- []
                %is_first <- (no)

        assume (%fallthroughs = []) or barf "Unfinished fallthrough conditions in 'when' block"
        assume ((length of %code) > 0) or barf "Empty body for 'when' block"
        to %code write "\nend --when"
        return %code

# Switch statement
immediately
    compile [when %branch_value = ? %body, when %branch_value is ? %body] to
        %code <- (Lua "")
        %fallthroughs <- []
        %is_first <- (yes)
        %seen_else <- (no)
        %branches <-
            %body if (%body.type = "Block") else [%body]
        for %func_call in %branches
            assume (%func_call.type is "Action") or barf ".."
                Invalid format for 'when' statement. Only '*' blocks are allowed.
            with {%star:%func_call.1, %condition:%func_call.2, %action:%func_call.3}
                assume (%star = "*") or barf ".."
                    Invalid format for 'when' statement. Lines must begin with '*'
                assume %condition or barf ".."
                    Invalid format for 'when' statement. Lines must begin with '*' and have a condition or the word "else"
                if: %action is (nil)
                    lua> "table.insert(\%fallthroughs, \(%condition as lua expr))"
                    do next %func_call

                if: %condition = "else"
                    assume (not %is_first) or barf "'else' clause cannot be first in 'when % = ?' block"
                    to %code write "\nelse\n    "
                    to %code write: %action as lua statements
                ..else
                    assume (not %seen_else) or barf "'else' clause needs to be last in 'when % = ?' block"
                    to %code write "\("if" if %is_first else "\nelseif") "
                    lua> "table.insert(\%fallthroughs, \(%condition as lua expr))"
                    for %i = % in %fallthroughs
                        if: %i > 1
                            to %code write " or "
                        if: (%.type is "Text") or (%.type is "Number")
                            to %code write "branch_value == \%"
                        ..else
                            to %code write "utils.equivalent(branch_value, \%)"
                    to %code write "then\n    "
                    to %code write (%action as lua statements)

            %fallthroughs <- []
            %is_first <- (no)
        
        assume (%fallthroughs = []) or barf "Unfinished fallthrough conditions in 'when' block"
        assume ((length of %code) > 0) or barf "No body for 'when % = ?' block!"
        to %code write "\nend"
        %code <-
            Lua ".."
                do --when % = ?
                    local branch_value = \(%branch_value as lua expr)
                    \%code
                end --when % = ?
        return %code

# Try/except
immediately
    compile [..]
        try %action and if it succeeds %success or if it barfs %fallback
        try %action and if it barfs %fallback or if it succeeds %success
    ..to
        Lua ".."
            do
                local fell_through = false
                local ok, ret = pcall(function()
                    \(%action as lua statements)
                    fell_through = true
                end)
                if ok then
                    \(%success as lua statements)
                    if not fell_through then
                        return ret
                    end
                else
                    \(%fallback as lua statements)
                end
            end
    parse [try %action] as
        try %action and if it succeeds: do nothing
        ..or if it barfs: do nothing
    parse [try %action and if it barfs %fallback] as
        try %action and if it succeeds: do nothing
        ..or if it barfs %fallback
    parse [try %action and if it succeeds %success] as
        try %action and if it succeeds %success or if it barfs: do nothing

# Do/finally
immediately
    compile [do %action] to
        Lua ".."
            do
                \(%action as lua statements)
            end --do

    compile [do %action then always %final_action] to
        Lua ".."
            do
                local fell_through = false
                local ok, ret = pcall(function()
                    \(%action as lua statements)
                    fell_through = true
                end)
                \(%final_action as lua statements)
                if not ok then error(ret) end
                if not fell_through then return ret end
            end

# Inline thunk:
immediately
    compile [result of %body] to
        Lua value ".."
            (function()
                \(%body as lua statements)
            end)()

# Coroutines:
immediately
    compile [values %body, coroutine %body, generator %body] to
        Lua value ".."
            (function()
                \(%body as lua statements)
            end)
    compile [->] to: Lua "coroutine.yield(true);"
    compile [-> %] to: Lua "coroutine.yield(true, \(% as lua expr));"
    compile [-> %k = %v] to: Lua "coroutine.yield(\(%k as lua expr), \(%v as lua expr));"