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doc/src/references/gm-state-transitions.rst
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doc/src/references/gm-state-transitions.rst
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================================
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G-Machine State Transition Rules
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================================
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*********************
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Core Transition Rules
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*********************
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#. Lookup a global by name and push its value onto the stack
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.. math::
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\gmrule
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{ \mathtt{PushGlobal} \; f : i
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& s
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& d
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& h
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& m
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\begin{bmatrix}
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f : a
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\end{bmatrix}
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}
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{ i
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& a : s
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& d
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& h
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& m
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}
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#. Allocate an int node on the heap, and push the address of the newly created
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node onto the stack
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.. math::
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\gmrule
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{ \mathtt{PushInt} \; n : i
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& s
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& d
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& h
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& m
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}
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{ i
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& a : s
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& d
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& h
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\begin{bmatrix}
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a : \mathtt{NNum} \; n
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\end{bmatrix}
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& m
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}
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#. Allocate an application node on the heap, applying the top of the stack to
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the address directly below it. The address of the application node is pushed
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onto the stack.
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.. math::
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\gmrule
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{ \mathtt{MkAp} : i
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& f : x : s
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& d
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& h
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& m
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}
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{ i
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& a : s
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& d
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& h
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\begin{bmatrix}
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a : \mathtt{NAp} \; f \; x
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\end{bmatrix}
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& m
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}
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#. Push a function's argument onto the stack
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.. math::
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\gmrule
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{ \mathtt{Push} \; n : i
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& a_0 : \ldots : a_n : s
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& d
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& h
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& m
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}
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{ i
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& a_n : a_0 : \ldots : a_n : s
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& d
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& h
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& m
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}
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#. Tidy up the stack after instantiating a supercombinator
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.. math::
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\gmrule
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{ \mathtt{Slide} \; n : i
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& a_0 : \ldots : a_n : s
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& d
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& h
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& m
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}
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{ i
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& a_0 : s
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& d
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& h
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& m
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}
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#. If the top of the stack is in WHNF (currently this just means a number) is on
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top of the stack, :code:`Unwind` considers evaluation complete. In the case
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where the dump is **not** empty, the instruction queue and stack is restored
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from the top.
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.. math::
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\gmrule
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{ \mathtt{Unwind} : \nillist
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& a : s
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& \langle i', s' \rangle : d
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& h
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\begin{bmatrix}
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a : \mathtt{NNum} \; n
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\end{bmatrix}
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& m
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}
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{ i'
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& a : s'
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& d
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& h
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& m
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}
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#. Bulding on the previous rule, in the case where the dump **is** empty, leave
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the machine in a halt state (i.e. with an empty instruction queue).
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.. math::
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\gmrule
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{ \mathtt{Unwind} : \nillist
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& a : s
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& \nillist
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& h
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\begin{bmatrix}
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a : \mathtt{NNum} \; n
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\end{bmatrix}
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& m
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}
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{ \nillist
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& a : s
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& \nillist
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& h
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& m
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}
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#. Again, building on the previous rules, this rule makes the machine consider
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unapplied supercombinators to be in WHNF
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.. math::
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\gmrule
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{ \mathtt{Unwind} : \nillist
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& a_0 : \ldots : a_n : \nillist
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& \langle i, s \rangle : d
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& h
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\begin{bmatrix}
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a_0 : \mathtt{NGlobal} \; k \; c
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\end{bmatrix}
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& m
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}
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{ i
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& a_n : s
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& d
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& h
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& m \\
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\SetCell[c=2]{c}
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\text{when $n < k$}
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}
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#. If an application is on top of the stack, :code:`Unwind` continues unwinding
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.. math::
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\gmrule
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{ \mathtt{Unwind} : \nillist
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& a : s
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& d
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& h
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\begin{bmatrix}
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a : \mathtt{NAp} \; f \; x
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\end{bmatrix}
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& m
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}
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{ \mathtt{Unwind} : \nillist
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& f : a : s
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& d
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& h
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& m
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}
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#. When a supercombinator is on top of the stack (and the correct number of
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arguments have been provided), :code:`Unwind` sets up the stack and jumps to
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the supercombinator's code (:math:`\beta`-reduction)
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.. math::
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\gmrule
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{ \mathtt{Unwind} : \nillist
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& a_0 : \ldots : a_n : s
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& d
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& h
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\begin{bmatrix}
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a_0 : \mathtt{NGlobal} \; n \; c \\
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a_1 : \mathtt{NAp} \; a_0 \; e_1 \\
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\vdots \\
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a_n : \mathtt{NAp} \; a_{n-1} \; e_n \\
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\end{bmatrix}
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& m
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}
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{ c
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& e_1 : \ldots : e_n : a_n : s
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& d
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& h
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& m
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}
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#. Pop the stack, and update the nth node to point to the popped address
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.. math::
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\gmrule
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{ \mathtt{Update} \; n : i
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& e : f : a_1 : \ldots : a_n : s
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& d
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& h
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\begin{bmatrix}
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a_1 : \mathtt{NAp} \; f \; e \\
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\vdots \\
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a_n : \mathtt{NAp} \; a_{n-1} \; e_n
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\end{bmatrix}
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& m
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}
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{ i
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& f : a_1 : \ldots : a_n : s
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& d
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& h
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\begin{bmatrix}
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a_n : \mathtt{NInd} \; e
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\end{bmatrix}
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& m
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}
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#. Pop the stack.
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.. math::
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\gmrule
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{ \mathtt{Pop} \; n : i
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& a_1 : \ldots : a_n : s
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& d
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& h
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& m
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}
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{ i
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& s
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& d
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& h
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& m
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}
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#. Follow indirections while unwinding
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.. math::
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\gmrule
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{ \mathtt{Unwind} : \nillist
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& a : s
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& d
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& h
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\begin{bmatrix}
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a : \mathtt{NInd} \; a'
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\end{bmatrix}
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& m
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}
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{ \mathtt{Unwind} : \nillist
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& a' : s
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& d
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& h
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& m
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}
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#. Allocate uninitialised heap space
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.. math::
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\gmrule
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{ \mathtt{Alloc} \; n : i
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& s
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& d
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& h
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& m
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}
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{ i
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& a_1 : \ldots : a_n : s
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& d
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& h
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\begin{bmatrix}
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a_1 : \mathtt{NUninitialised} \\
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\vdots \\
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a_n : \mathtt{NUninitialised} \\
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\end{bmatrix}
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& m
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}
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#. Evaluate the top of the stack to WHNF
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.. math::
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\gmrule
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{ \mathtt{Eval} : i
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& a : s
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& d
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& h
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& m
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}
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{ \mathtt{Unwind} : \nillist
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& a : \nillist
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& \langle i, s \rangle : d
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& h
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& m
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}
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#. Reduce a primitive binary operator :math:`*`.
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.. math::
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\gmrule
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{ * : i
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& a_1 : a_2 : s
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& d
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& h
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\begin{bmatrix}
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a_1 : x \\
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a_2 : y
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\end{bmatrix}
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& m
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}
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{ i
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& a' : s
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& d
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& h
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\begin{bmatrix}
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a' : (x * y)
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\end{bmatrix}
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& m
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}
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#. Reduce a primitive unary operator :math:`\neg`.
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.. math::
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\gmrule
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{ \neg : i
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& a : s
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& d
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& h
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\begin{bmatrix}
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a : x
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\end{bmatrix}
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& m
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}
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{ i
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& a' : s
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& d
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& h
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\begin{bmatrix}
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a' : (\neg x)
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\end{bmatrix}
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& m
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}
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***************
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Extension Rules
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***************
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#. A sneaky trick to enable sharing of :code:`NNum` nodes. We note that the
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global environment is a mapping of plain old strings to heap addresses.
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Strings of digits are not considered valid identifiers, so putting them on
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the global environment will never conflict with a supercombinator! We abuse
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this by modifying Core Rule 2 to update the global environment with the new
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node's address. Consider how this rule might impact garbage collection
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(remember that the environment is intended for *globals*).
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.. math::
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\gmrule
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{ \mathtt{PushInt} \; n : i
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& s
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& d
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& h
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& m
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}
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{ i
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& a : s
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& d
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& h
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\begin{bmatrix}
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a : \mathtt{NNum} \; n
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\end{bmatrix}
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& m
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\begin{bmatrix}
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n' : a
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\end{bmatrix}
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\\
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\SetCell[c=5]{c}
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\text{where $n'$ is the base-10 string rep. of $n$}
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}
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#. In order for the previous rule to be effective, we are also required to take
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action when a number already exists in the environment:
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.. math::
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\gmrule
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{ \mathtt{PushInt} \; n : i
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& s
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& d
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& h
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& m
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\begin{bmatrix}
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n' : a
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\end{bmatrix}
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}
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{ i
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& a : s
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& d
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& h
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& m
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\\
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\SetCell[c=5]{c}
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\text{where $n'$ is the base-10 string rep. of $n$}
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}
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Block a user