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1 (* ========================================================================= *) |
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2 (* A POSSIBLY-INFINITE STREAM DATATYPE FOR ML *) |
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3 (* Copyright (c) 2001-2006 Joe Hurd, distributed under the GNU GPL version 2 *) |
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4 (* ========================================================================= *) |
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5 |
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6 structure Stream :> Stream = |
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7 struct |
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8 |
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9 val K = Useful.K; |
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10 |
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11 val pair = Useful.pair; |
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12 |
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13 val funpow = Useful.funpow; |
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14 |
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15 (* ------------------------------------------------------------------------- *) |
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16 (* The datatype declaration encapsulates all the primitive operations *) |
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17 (* ------------------------------------------------------------------------- *) |
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18 |
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19 datatype 'a stream = |
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20 NIL |
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21 | CONS of 'a * (unit -> 'a stream); |
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22 |
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23 (* ------------------------------------------------------------------------- *) |
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24 (* Stream constructors *) |
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25 (* ------------------------------------------------------------------------- *) |
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26 |
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27 fun repeat x = let fun rep () = CONS (x,rep) in rep () end; |
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28 |
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29 fun count n = CONS (n, fn () => count (n + 1)); |
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30 |
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31 fun funpows f x = CONS (x, fn () => funpows f (f x)); |
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32 |
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33 (* ------------------------------------------------------------------------- *) |
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34 (* Stream versions of standard list operations: these should all terminate *) |
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35 (* ------------------------------------------------------------------------- *) |
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36 |
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37 fun cons h t = CONS (h,t); |
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38 |
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39 fun null NIL = true | null (CONS _) = false; |
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40 |
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41 fun hd NIL = raise Empty |
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42 | hd (CONS (h,_)) = h; |
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43 |
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44 fun tl NIL = raise Empty |
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45 | tl (CONS (_,t)) = t (); |
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46 |
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47 fun hdTl s = (hd s, tl s); |
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48 |
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49 fun singleton s = CONS (s, K NIL); |
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50 |
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51 fun append NIL s = s () |
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52 | append (CONS (h,t)) s = CONS (h, fn () => append (t ()) s); |
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53 |
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54 fun map f = |
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55 let |
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56 fun m NIL = NIL |
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57 | m (CONS (h, t)) = CONS (f h, fn () => m (t ())) |
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58 in |
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59 m |
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60 end; |
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61 |
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62 fun maps f = |
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63 let |
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64 fun mm _ NIL = NIL |
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65 | mm s (CONS (x, xs)) = |
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66 let |
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67 val (y, s') = f x s |
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68 in |
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69 CONS (y, fn () => mm s' (xs ())) |
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70 end |
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71 in |
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72 mm |
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73 end; |
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74 |
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75 fun zipwith f = |
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76 let |
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77 fun z NIL _ = NIL |
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78 | z _ NIL = NIL |
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79 | z (CONS (x,xs)) (CONS (y,ys)) = |
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80 CONS (f x y, fn () => z (xs ()) (ys ())) |
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81 in |
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82 z |
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83 end; |
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84 |
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85 fun zip s t = zipwith pair s t; |
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86 |
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87 fun take 0 _ = NIL |
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88 | take n NIL = raise Subscript |
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89 | take 1 (CONS (x,_)) = CONS (x, K NIL) |
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90 | take n (CONS (x,xs)) = CONS (x, fn () => take (n - 1) (xs ())); |
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91 |
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92 fun drop n s = funpow n tl s handle Empty => raise Subscript; |
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93 |
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94 (* ------------------------------------------------------------------------- *) |
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95 (* Stream versions of standard list operations: these might not terminate *) |
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96 (* ------------------------------------------------------------------------- *) |
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97 |
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98 local |
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99 fun len n NIL = n |
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100 | len n (CONS (_,t)) = len (n + 1) (t ()); |
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101 in |
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102 fun length s = len 0 s; |
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103 end; |
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104 |
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105 fun exists pred = |
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106 let |
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107 fun f NIL = false |
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108 | f (CONS (h,t)) = pred h orelse f (t ()) |
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109 in |
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110 f |
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111 end; |
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112 |
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113 fun all pred = not o exists (not o pred); |
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114 |
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115 fun filter p NIL = NIL |
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116 | filter p (CONS (x,xs)) = |
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117 if p x then CONS (x, fn () => filter p (xs ())) else filter p (xs ()); |
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118 |
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119 fun foldl f = |
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120 let |
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121 fun fold b NIL = b |
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122 | fold b (CONS (h,t)) = fold (f (h,b)) (t ()) |
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123 in |
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124 fold |
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125 end; |
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126 |
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127 fun concat NIL = NIL |
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128 | concat (CONS (NIL, ss)) = concat (ss ()) |
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129 | concat (CONS (CONS (x, xs), ss)) = |
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130 CONS (x, fn () => concat (CONS (xs (), ss))); |
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131 |
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132 fun mapPartial f = |
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133 let |
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134 fun mp NIL = NIL |
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135 | mp (CONS (h,t)) = |
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136 case f h of |
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137 NONE => mp (t ()) |
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138 | SOME h' => CONS (h', fn () => mp (t ())) |
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139 in |
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140 mp |
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141 end; |
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142 |
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143 fun mapsPartial f = |
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144 let |
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145 fun mm _ NIL = NIL |
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146 | mm s (CONS (x, xs)) = |
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147 let |
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148 val (yo, s') = f x s |
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149 val t = mm s' o xs |
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150 in |
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151 case yo of NONE => t () | SOME y => CONS (y, t) |
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152 end |
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153 in |
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154 mm |
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155 end; |
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156 |
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157 (* ------------------------------------------------------------------------- *) |
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158 (* Stream operations *) |
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159 (* ------------------------------------------------------------------------- *) |
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160 |
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161 fun memoize NIL = NIL |
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162 | memoize (CONS (h,t)) = CONS (h, Lazy.memoize (fn () => memoize (t ()))); |
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163 |
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164 local |
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165 fun toLst res NIL = rev res |
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166 | toLst res (CONS (x, xs)) = toLst (x :: res) (xs ()); |
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167 in |
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168 fun toList s = toLst [] s; |
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169 end; |
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170 |
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171 fun fromList [] = NIL |
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172 | fromList (x :: xs) = CONS (x, fn () => fromList xs); |
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173 |
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174 fun toTextFile {filename = f} s = |
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175 let |
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176 val (h,close) = |
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177 if f = "-" then (TextIO.stdOut, K ()) |
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178 else (TextIO.openOut f, TextIO.closeOut) |
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179 |
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180 fun toFile NIL = () |
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181 | toFile (CONS (x,y)) = (TextIO.output (h,x); toFile (y ())) |
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182 |
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183 val () = toFile s |
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184 in |
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185 close h |
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186 end; |
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187 |
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188 fun fromTextFile {filename = f} = |
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189 let |
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190 val (h,close) = |
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191 if f = "-" then (TextIO.stdIn, K ()) |
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192 else (TextIO.openIn f, TextIO.closeIn) |
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193 |
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194 fun strm () = |
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195 case TextIO.inputLine h of |
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196 NONE => (close h; NIL) |
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197 | SOME s => CONS (s,strm) |
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198 in |
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199 memoize (strm ()) |
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200 end; |
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201 |
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202 end |