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Multiple types that make up a constructor are separated by asterisks The names of constructors within a type must be unique If several union types are defined, then the names of their constructors can overlap; however, you should.

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Simon, a take-off of the electronic toy with the same name, is a deceptively simple game in which the NXT generates a random ever-lengthening sequence that you must reproduce purely from memory. Moving the Four Ins and Outs project to a circuit board makes the game look more professional, and Figure 13-14 shows how the circuit was built on the PCB. We used the opposite end of the PCB because the other end was already used for a different project.

be careful when doing this, because it can be that further type annotations are required when constructing and consuming union types The following Volume type is a union type with three constructors, each composed of a single item of type float The syntax for constructing a new instance of a union type is the constructor name followed by the values for the types, with multiple values separated by commas Optionally, you can place the values in parentheses You use the three different Volume constructors to construct three different identifiers, vol1, vol2, and vol3 #light type Volume = | Liter of float | UsPint of float | ImperialPint of float let vol1 = Liter 25 let vol2 = UsPint 25 let vol3 = ImperialPint (25) To deconstruct the values of union types into their basic parts, you always use pattern matching.

Figure 7-2. Displaying the request and response (left) and the XMLHttpRequest object s 3565005953993bd3170c41194f12907b callback function (right)

Figure 13-14. Simon game The complete listing of the Simon NXC program is located in Appendix C, but let s take a quick look at how the program reads the buttons in Listing 13-4. I2CBytes sends WriteBuf[1] which equals all ones (hexadecimal FF). That turns off the LEDs with the high 4 bits, and establishes inputs for the low 4 bits. Then I2CBytes reads the port value into ReadBuf. Remember, 1 means the button is not pressed, so the do-loop repeats while the reply is all 1s or no buttons are pressed. When a button has been pressed, the for-loop checks each bit to figure out which button it was. Listing 13-4. GetButtons from Simon.NXC Program int GetButtons() { WriteBuf[1] = 0xff; do { I2CBytes(I2Cport, WriteBuf, RdCnt, ReadBuf); } while (ReadBuf[0] == 0xff) for(int i=0; i<4; i++) { if((ReadBuf[0] & 1) == 0) return i; ReadBuf[0]>>=1; } }

When pattern matching over a union type, the constructors make up the first half of the pattern matching rules You don t need a complete list of rules, but if you don t, there must be a default rule, using either an identifier or a wildcard to match all remaining rules The first part of a rule for a constructor consists of the constructor name followed by identifiers or wildcards to match the various values within it The following functions, convertVolumeToLiter, convertVolumeUsPint, and convertVolumeImperialPint, demonstrate this syntax: let convertVolumeToLiter x = match x with | Liter x -> x | UsPint x -> x * 0473 | ImperialPint x -> x * 0568 let convertVolumeUsPint x = match x with | Liter x -> x * 2113 | UsPint x -> x | ImperialPint x -> x * 1.

201 let convertVolumeImperialPint x = match x with | Liter x -> x * 1760 | UsPint x -> x * 0833 | ImperialPint x -> x.

You might want to change some threshold, set point, limit, or other constant number s value in your NXT program. Because the NXT has only three input buttons, entering numeric values means that you have to use the computer to edit the value in the program and then download it all over again. However, with the Keypad Input shown in Figure 13-15, you can enter numbers directly into the NXT without the computer.