From: Christiaan Baaij Date: Mon, 1 Mar 2010 11:09:22 +0000 (+0100) Subject: Proces jan's comment on the choice section X-Git-Url: https://git.stderr.nl/gitweb?a=commitdiff_plain;h=8905eead137ece683eaac04b4f7ab6711909a123;p=matthijs%2Fmaster-project%2Fdsd-paper.git Proces jan's comment on the choice section --- diff --git "a/c\316\273ash.lhs" "b/c\316\273ash.lhs" index 9f11e74..8452e4b 100644 --- "a/c\316\273ash.lhs" +++ "b/c\316\273ash.lhs" @@ -565,8 +565,8 @@ circuit~\cite{reductioncircuit} for floating point numbers. and \item function applications are translated to component instantiations. \end{inparaenum} - The output port can have a complex type (such as a tuple), so having just - a single output port does not pose any limitation. The arguments of a + The output port can have a structured type (such as a tuple), so having + just a single output port does not pose any limitation. The arguments of a function application are assigned to signals, which are then mapped to the corresponding input ports of the component. The output port of the function is also mapped to a signal, which is used as the result of the @@ -574,9 +574,10 @@ circuit~\cite{reductioncircuit} for floating point numbers. Since every top level function generates its own component, the hierarchy of function calls is reflected in the final netlist,% aswell, - creating a hierarchical description of the hardware. This separation in - different components makes the resulting \VHDL\ output easier to read and - debug. + creating a hierarchical description of the hardware. The separation in + different components makes it easier for a developer to understand and + possibly hand-optimize the resulting \VHDL\ output of the \CLaSH\ + compiler. As an example we can see the netlist of the |mac| function in \Cref{img:mac-comb}; the |mac| function applies both the |mul| and |add| @@ -592,7 +593,7 @@ circuit~\cite{reductioncircuit} for floating point numbers. \label{img:mac-comb} \end{figure} - The result of using a complex input type can be seen in + The result of using a structural input type can be seen in \cref{img:mac-comb-nocurry} where the |mac| function now uses a single input tuple for the |a|, |b|, and |c| arguments: @@ -609,7 +610,7 @@ circuit~\cite{reductioncircuit} for floating point numbers. \subsection{Choice} In Haskell, choice can be achieved by a large set of language constructs, consisting of: \hs{case} constructs, \hs{if-then-else} constructs, - pattern matching, and guards. The easiest of these are the \hs{case} + pattern matching, and guards. The most general of these are the \hs{case} constructs (\hs{if} expressions can be very directly translated to \hs{case} expressions). A \hs{case} construct is translated to a multiplexer, where the control value is linked to the selection port and @@ -619,17 +620,27 @@ circuit~\cite{reductioncircuit} for floating point numbers. % assignment in \VHDL, where the conditions use equality comparisons % against the constructors in the \hs{case} expressions. We can see two versions of a contrived example below, the first - using a \hs{case} construct and the other using a \hs{if-then-else} - constructs, in the code below. + using a \hs{case} construct and the other using an \hs{if-then-else} + construct, in the code below. The examples sums two values when they are + equal or non-equal (depending on the given predicate, the \hs{pred} + variable) and returns 0 otherwise. The \hs{pred} variable has the + following, user-defined, enumeration datatype: \begin{code} + data Pred = Equiv | NotEquiv + \end{code} + + The naive netlist corresponding to both versions of the example is + depicted in \Cref{img:choice}. + + \begin{code} sumif pred a b = case pred of - Eq -> case a == b of - True -> a + b - False -> 0 - Neq -> case a != b of - True -> a + b - False -> 0 + Equiv -> case a == b of + True -> a + b + False -> 0 + NotEquiv -> case a != b of + True -> a + b + False -> 0 \end{code} \begin{code} @@ -645,23 +656,24 @@ circuit~\cite{reductioncircuit} for floating point numbers. \caption{Choice - sumif} \label{img:choice} \end{figure} - - The example sums two values when they are equal or non-equal (depending on - the predicate given) and returns 0 otherwise. Both versions of the example - roughly correspond to the same netlist, which is depicted in - \Cref{img:choice}. - A slightly more complex (but very powerful) form of choice is pattern + A user-friendly and also very powerful form of choice is pattern matching. A function can be defined in multiple clauses, where each clause - specifies a pattern. When the arguments match the pattern, the + corresponds to a pattern. When an argument matches a pattern, the corresponding clause will be used. Expressions can also contain guards, - where the expression is only executed if the guard evaluates to true. Like + where the expression is only executed if the guard evaluates to true, and + continues with the next clause if the guard evaluates to false. Like \hs{if-then-else} constructs, pattern matching and guards have a (straightforward) translation to \hs{case} constructs and can as such be mapped to multiplexers. A third version of the earlier example, using both - pattern matching and guards, can be seen below. The version using pattern - matching and guards also has roughly the same netlist representation - (\Cref{img:choice}) as the earlier two versions of the example. + pattern matching and guards, can be seen below. The guard is the + expression that follows the vertical bar (\hs{|}) and precedes the + assignment operator (\hs{=}). The \hs{otherwise} guards always evaluate to + \hs{true}. + + The version using pattern matching and guards corresponds to the same + naive netlist representation (\Cref{img:choice}) as the earlier two + versions of the example. \begin{code} sumif Eq a b | a == b = a + b